AQA Chemistry GCSE- Unit 2

Unit 2:
Structures & Bonding
Structures & Properties
How Much?
Rates of Reaction
Energy & Reactions
Acids, Alkalis and Salts

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Structures & Bonding: Atomic Structure

An Atom is very very small. It is made up of Subatomic Particles, called Protons, Neutrons and Electrons.
Protons and Neutrons are bound tightly together in the Neucleus of the atom. Electrons circle around the neucleus in energy levels, or shells.


Each shell can be filled with a different number of electrons:

  • The first shell can hold 2 electrons
  • The second shell can hold 8 electrons
  • The third shell can also hold 8 electrons
  • The forth shell can hold 18 electrons (You don't need to know any further for GCSE)
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Structures & Bonding: Atomic Structure- Continued

The Atomic Number is the number of protons in the atom. This determines which element that atom belongs to. The Atomic Mass is the number of protons and neutrons in the atom. They each have a relative atomic mass of 1. Electrons weigh very little, so we don't count them.

Protons are positively charged. They are said to have a charge of +1. Electrons are negatively charged. They are said to have a charge of -1. Neutrons have no charge. They are neutral. Uncharged atoms have the same number of protons and neutrons.

Electrons are attracted to the positively charged nucleus because they have a negative charge. Energy is needed to overcome these attractive forces so electrons in shells further from the nucleus have more energy than those near to the nucleus.

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Atoms are usually drawn like this.


Atomic Structure can also be written in order of electrons in each shell. For example, this sodium atom's atomic structure would be written as [2,8,1], with a comma seperating the shells.

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Structures & Bonding: Ionic Bonding

Elements react so that their atoms can gain a full outer shell and become stable.They do this through either covalent bonding or by ionic bonding.

Ionic Bonding is when atoms gain or lose electrons. This is possible because electrons aren't tightly bound to the atom like protons. It is extremely difficult to seperate a proton from an atom. Ionic bonding usually occurs when one of the substances involved is metal and the other is a non-metal.


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Structures & Bonding: Ionic Bonding- Continued

Charged atoms are also called charged ions. Ionic compounds are very strong because the positive and negative ions are attracted to eachother. It is always the metal that becomes positively charged and the non-metal which becomes negatively charged. Metals can lose up to 3 ions in ionic bonding.

When an atom gains an electron, it becomes negatively charged, because there are more electrons (negative) than protons (positive). After become negatively charged, a ' - ' sign must be written after its chemical symbol and atomic structure.

When an atom loses an electron, it becomes positively charged, because there are more protons (positive) than electrons (negative). After becoming positively charged, a ' + ' sign must be written after its chemical symbol.

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Structures & Bonding: Ionic Bonding- Continued

For Example, if a Chlorine atom, which has the atomic structure [2,8,7], were to gain an electron, it would get a full outer shell of [2,8,8]. The negatively charged chlorine atom is now written as Cl-, and is unreactive. It has the same atomic structure as Argon. Similarly, it's atomic structure must be written out with a ' - ' sign as well, to distinguish it from Argon: [2,8,8]-

For example, if a sodium atom [2,8,1], were to lose an electron, it gains the atomic structure of the noble gas Neon, which is [2,8]. Now, the charged sodium ion is written as Na+ , and it's atomic structure is written as [2,8]+


This loss or gain of electrons occurs in reactions. Using these 2 examples, Chlorine would have to be reacted with Sodium for these ions to be created. This works because the electron which sodium loses to gain a stable atomic structure is donated to Chlorine, and becomes the electron that chlorine has to gain to achieve a stable atomic structure.

This is best represented in a diagram See Next Card

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Structures & Bonding: Ionic Bonding Diagrams

( bonded substances have high melting and boiling points, are soluable in water, and as solids are bad conductors.

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Structures & Bonding: Ionic Bonding Diagrams - Con

The atoms must also be drawn with brackets around and a ' + ' or ' - ' sign to represent the change in atomic structure.


In this picture, the electrons belonging to each atom are in a different colour, but they can also be represented using dots and x's.

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Structures & Bonding: Covalent Bonding

Covalent bonding is where 2 atoms share one or more electrons to gain a full outer shell. It is always a bond between 2 non-metals.

Water, for example, is made up of 2 hydrogen atoms and 1 oxygen atom. These atoms are covalently bonded. Oxygen shares an electron with each hydrogen atom, and in return, each hydrogen atom shares an electron with the oxygen atom. This way, all atoms gain a full outer shell. When 2 atoms have been covalently bonded together, they are known as molecules. Covalent bonded substances, such as water, have low melting points and the forces between the molecules, known as intermolecular forces will be small., even though foces between the atoms are very strong.

They usually exist as liquids or gasses at room temperature.

They are also poor electrical and thermal conductors.

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Structures & Bonding: Covalent Bonding


Here, Oxygen's electrons are represented by red dots, and Hydrogens' electrons are represented by blue crosses. This is how a covalent bond should be drawn in an exam.

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Structures & Bonding: Metallic Bonding

When 2 metal atoms bond together, the electrons from the outer shell of both of the atoms become free to move throughout the whole structure. The electrons are said to be 'delocalised'. You could say that the metal atoms are positively charged because they've donated their electrons to the 'sea of electrons'. Metallic bonding can be described as a 'sea of electrons' moving through a structure of positive metal ions'.


Metallically bonded substances have very high melting and boiling points, are good thermal and electrical conductors and aren't very soluable.

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Structures & Properties: Ionic Compounds

An ionic compound consists of a giant structure of ions arranged in a lattice. The oppositely charged ions experience electrostatic forces between eachother in all directions which makes them very strong. This holds the ions in the lattice together very tightly.

Because these electrostatic forces are so strong, it takes a lot of energy to break them apart. This is why ionic compounds have high melting and boiling points. Solid ionic compounds can't conduct electricity because the ions are held in a fixed position in the lattice by the electrostatic forces.

When enough energy is supplied to the lattice to break the bonds apart, the compound melts, allowing the ions to move freely throughout the lattice. Many ionic compounds can also be dissolved in water, so the water molecules break the lattic apart, having the same effect, allowing the ions to move freely. This means they are able to carry electrical charges. 

Melted liquid ionic compounds and solutions of ionic compounds can conduct electricity.

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Structures & Properties: Simple Molecules

Molecules are made up of covalently bonded atoms. They have low melting and boiling points, so they're generally liquids and gasses at room temperature.

An example of a covalently bonded substance is water (H20).

The covalent bonds themselves are very strong, but the intermolecular bonds, (the bonds between the molecules), are weak, and overcoming these forces does not require much energy. This is why covalently bonded substances have low melting and boiling points.

Covalently bonded substances do not conduct electricity because the molecules are uncharged, so they cannot carry electrical charge.


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Structures & Bonding: Giant Covalent Structures

A giant covalent structure is a structure in which covalent bonds join many many atoms together to form large networks of covalent bonds. They are also referred to as macromolecules or giant molecular structures. Examples: Diamond, Graphite and Silicon Dioxide

In Diamond and Silicon Dioxide, the giant lattices are held in place by the strong covalent bonds between the atoms. It makes them very hard and chemically unreactive, with high melting and boiling points.

In graphite, the carbon atoms are arranged in layers with weak forces between the layers. This means the layers can slide over eachother easily.

Graphite is also made up of delocalised electrons, much like those seen in metallic structures, so graphite can conduct electricity, unlike diamond and silicon dioxide. Graphite contains free electrons because although carbon usually bonds with 4 other carbon atoms, it only bonds with 3 in graphite, leaving 1 spare electron, which becomes delocalised.

In Fullerenes, carbon atoms join together to make large cages which can have a variety of shapes, including balls, onions and tubes, which can hold other substances. Scientists are trying to find ways of using them in nanoscience.

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Structures & Bonding: Giant Metallic Structures

In pure metals, all the atoms are the same size and are arranged in layers, meaning they can slide over eachother. This gives metals the ability to bend and be drawn into wires.

Metals are also good conductors of electricity because metallic structures are held together by a 'sea' of delocalsied electrons that can carry electrical charge. This structure also aids pure metal's ability to change shape.

In alloyed metals, not all the atoms are the same size, which inhibits the movement of layers of atoms, preventing the metals from being so ductile and maleable.



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How Much?: Mass Numbers

Revision of Atomic Structure:
Atoms are made up of a nucleus, containing protons and neutrons, and energy levels, or shells, in which electrons orbit the nucleus. The relative mass of a proton or a neutron is 1. Electrons weigh so little that they have a relative mass of 0. This means almost all of the mass of an atom is found in the nucleus.
The atomic number of an atom is how many protons it holds within its nucleus.
The atomic mass of an atom is how many protons and neutrons it contains.
Atomic mass = atomic number + number of neutrons.

Atoms of the same element always have the same number of protons, but the number of neutrons in an atom can vary. Atoms which have the same number of protons but differing numbers of neutrons are known as isotopes.

For example, carbon has 2 main isotopes: Carbon-12, which contains 6 protons and 6 neutrons, and Carbon-14, which contains 6 protons and 8 neutrons. (Sometimes, extra neutrons can make the nucleus of an atom unstable, which makes it radioactive.)

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How Much?: Mass Numbers (continued)

Different isotopes of the same element have different physical properties, but the same chemical properties.
For Example, hydrogen has 3 main isotopes: hydrogen (1, proton, no neutrons), deuterium (1 proton, 1 neutron) and tritium (1 proton, 2 neutrons).
They each have a different mass number due to the varying number of neutrons. The extra neutron in Tritium makes this isotope radioactive: this is a different physical property to hydrogen and deutrium.
They can all dissolve in water: this shows that they have the same chemical properties.


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How Much?: Masses of Atoms and Moles

Chemical equations tell us the ratio of atoms in each reactant needed to make the products.

e.g. ' Mg + 2HCl -> MgCl2 + H2 ' tells us that we need twice as many Hydrogen and Chlorine atoms as Magnesium atoms to make the products: Magnesium Chloride and Hydrogen.

However, when carrying out experiments, we need to know the amounts needed in a suitable scale for us to measure, for example, in cm3 or grams. Different elements don't weigh the same thing, so when carrying out experiments, we first need to work out how much of each substance is needed in grams or cm3. We do this using the relative atomic masses of elements, which can be found in the top left hand corner of each element in the periodic table.

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How Much?: Masses and Moles

Some elements in the periodic table have a mass number with a decimal place (e.g. Chlorine has the mass number 35.5) This is becuase it's based on Relative Atomic Mass (Ar) which consideres their mass, but also the ratio of isotopes. Ar's are the mass of an atom compared with an atom of carbon12 (which is defined to have an Ar of exactly 12.)
For example, Chlorine has 2 main isotopes: Chlorine 35 and Chlorine 37, however, they have different proportions. 75% of chlorine atoms are Chlorine 35, and only 25% are Chlorine 37. To work out the Relative Atomic Mass, you use the weighted mean mass, in this case:
(  (  75  X  35  )  +  (  25  X  37  )   )  /  100  = 35.5

In lots of Chemical Calculations, you need to know the relative formular mass of a chemical compound. This is found by adding relative atomic masses of each element in the compound together.
For example, the Mr of Sulphuric Acid (H2SO4) uses the calculation:
(  2  x  1  )(  32  )  +  (  4  x  16  ) 98 

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How Much?: Masses and Moles

The Mole is a unit of measurement. It is worth 6 X 10 to the 23. It means that if you has 6 X 1023 atoms of an element, then you would get its relative atomic mass in grams. For example, if you has 6 X 1023 Carbon atoms, then you would have 12g of Carbon, or 1 Mole of Carbon, because its relative atomic mass is 12. This is the same for relative formular masses.

One mole is the relative atomic mass or relative formular mass of any substance expressed in grams.

If you were given a mass of an element or molecule, you could work out how many moles are in that mass by dividing it by the molecule's relative atomic/relative molecular mass.
 For example, if you had 32g of Oxygen atoms, then you could do 32 / 16 = 2, so there are 2 moles of oxygen in 32g of oxygen atoms.

A mole of any substance always contains the same number of particles.

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How Much?: Percentages & Formulae

Percentage Composition Calculations are used to work out how much of an compound each element represents. To do this, you must:

  • 1. Work out the relative formula mass of the compound
  • 2. Divide the Ar you want by the Mr of the compound
  • 3. Multiply the answer by 100
  • (4. If there's more than 1 of the element that you want, times it by the number that there are)

For Example:
Find the Percentage by Mass of Oxygen in Sodium Hydroxide (NaOH)

1. Mr = (  23  )  +  (  16  )  +  (  1  )  =  40
2. Ar = 16,   (  16  /  40  )
3. (  16  /  40  )  X  100  =  40 %

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How Much?: Percentages & Formulae

Finding an Emprical formula is about working out how many atoms of a certain element are in a compound. For example, in H2SO4, we can see that there are 2 hydrogen atoms, 1 sulphur atom and 4 oxygen atoms.

A question on emprical formulae will state how much (in grams) of each element is in a compound, and you have to work out it's emprical formula based on that.

We do this in 3 steps.

1. Divide the masses of each element by the element's Ar- this will work out how many moles of each element there is
2. Divide all of these answer by the smallest of the answer (this will give a ratio)
3. Write each answer after the appropriate chemical symbol (don't bother if the answer is 1)

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How Much?: Percentages & Formulae

Empirical Formular example question:

'A sample of a chemical compound contains 25.2g of Carbon, 8.5g of Hydrogen and 33.7g of Oxygen. Work out its emprical formula.'

25.2g / 12 = 2.1 (smallest)
8.5g / 1  = 8.5
33.7g / 16 = 2.10625

2.1 / 2.1 = 1
8.5 / 2.1 = 4.0476... (4)
2.10625 / 2.1 = 1.00309... (1)

3.    CH4O

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How Much?: Equations & Calculations

Balancing Formulas

Balanced formulas tell us how many moles of each substance in the reactants react and how many moles of each substance is produced in the products.

Balanced equations have equal numbers of atoms on each side. For example:

2H2 + O2 -> 2H2O
(4 atoms of hydrogen and 2 atoms of oxygen on each side)
If asked to balance an equation, remember that you can only change the big numbers in front of the elements or compounds, never the little numbers.

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How Much?: Equations & Calculations

This balanced equation tells us that 2 moles of Hydrogen react with 1 mole of oxygen to form 2 moles of water. We can use this to work out many much of each reactant is needed in grams to make a certain amount of the product. To do this, we need to know that the Ar for hydrogen is 1 and the Ar for Oxygen is 16.

Ar of Hydrogen = 1                  So the mass of 1 mole of H2 = 2 x 1 = 2g
Ar of Oxygen = 16                   So the mass of 1 mole of O2 = 2 x 16 = 32g
Mr of Water (H20) = (16 + 2) = 18     So the mass of 1 mole of water = 18g

Our balanced equation tells us that 2 moles of hydrogen react with 1 mole of oxygen to give 2 moles of water, so

2 moles of hydrogen  - 2 x 2g = 4g
1 mole of oxygen       - 1 x 32g = 32g
2 moles of water        - 2 x 18g = 36g

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How Much?: Equations & Calculations

These calculations can be used to work out how much of two chemicals react together (in moles or grams).
Example question:
2NaOH + Cl2 -> NaOCl + NaCl + H2O
'If a solution contains 100g of Sodium Hydroxide, how much Chlorine gas should be passed through the solution to make NaOCl (bleach), so all the NaOH reacts and so there is no excess Cl being wasted?'
1. Work out how many moles of each substance there are:
Moles of NaOH = 100g / Mr of NaOH = 100 / (23 + 16 + 1) = 100 / 40 = 2.5 moles
From the reaction, we know that 2 moles of NaOH reacts with 1 mole of Cl2, so there are 2.5 / 2 = 1.25 moles of Cl2 needed
2. Use the number of moles of Cl2 to find how much Cl is needed in grams
The Mr of Cl2 = (35.5 x 2) = 71
1.25 moles x 71 = 88.75g
So 88.75g of Chlorine is needed to react with exactly 100g of Sodium Hydroxide.

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How Much?: Making as much as we want

No experiment works perfectly, so the actual mass of products is always less than the mass calculated using equations.

We calculate the percentage yield of this experiment by dividing the actual mass of the product by the calculated mass of product, and then multiplying this answer by 100.

For example, 'In an experiment, 88g of propane was burned and 120g of water was produced.'

In a perfect experiment, 144g of water would have been produced.

So here, we'd do

( 120 / 144 ) X 100 = 83.3 %
The percentage yield of this reaction is 83.3%

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How Much?: Making as much as we want

Very few chemical reactions have a yield of 100% because:

  • The reaction may be reversible, (so as products form they react to form the reactants again)
  • Some reactants may react to give unexpected products
  • Some of the product may be left behind in the apparatus
  • The reactants may not be completely pure
  • Some chemical reactions produce more than one product, and it may be difficult to separate the product that we want from the reaction mixture.
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How Much?: Making as much as we want

The amount of the reactants that turn into useful products in reactions is called the atom economy.
Companies making chemical products want high atom economies to reduce waste products so less money is wasted and less waste goes into the environment
The percentage atom economy is found using this equation:
% atom economy = ( Mr of useful product / Mr of all productsx 100

Ethanol (C2H5OH) can be converted into ethene (C2H4) (to make poly(ethene))

C2H5OH -> C2H4 + H20
Mr of C2H4 = ( 12 x 2 ) + ( 1 x 4 ) = 28
Mr of H20 = ( 1 x 2 ) + ( 16 x 1 ) = 18
( Mr of C2H5OH = ( 28 + 18 ) = 46 )

% atom economy = ( 28 / 46 ) x 100 = 61%

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How Much?: Reversible Reactions

Many reactions, such as combustion, are irreversible. This means the products cannot react to form the reactants.
Some reactions, however, are reversible. Their products can react to form the reactants again.
When writing chemical equations for reversible reactions, we do not use the usual one-way arrow. Instead, we use two arrows, each with just half an arrowhead - the top one pointing right, and the bottom one pointing left. For example,

hydrated copper(II) sulfate Equilibrium symbol ( Anhydrous copper(II) sulfate + water

An example of a reversibe reaction is when hydrated copper (II) sulphate (blue solid) is heated to produce anhydrous copper(II) sulfate (white solid) and water (clear liquid). When the water is added to the anhydrous copper (II) sulfate, it produces the hydrated copper (II) sulphate again.
The reaction between anhydrous copper(II) sulfate and water is used as a test for water. The white solid turns blue in the presence of water.

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How Much?: Reversible Reactions

If you start with just the reactants, the forward reaction will only be present, with the reactants forming the products.
Soon, the products will start forming the reactants again in a backwards reaction, but at a slower rate than the forward reaction.
Eventually, these rates will reach dynamic equilibrium, where they are the same.

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How Much?: Reversible Reactions

We can change the relative proportions of the reactants and products in a reaction mixture by changing the reaction conditions. By doing this, we can optimize the amount of products we produce. The conditions we can change are the pressure,the temperature and the concentration of the reactants and products.

Le Chatelier (a French Chemist)'s principle states:
'If a chemical system at equilibrium experiences a change in concentration, temperature, or total pressure; the equilibrium will shift in order to oppose that change.'


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How Much?: Reversible Reactions


Optimizing the production of products requires counting the number of molecules of gas on either side of the equation.

If there are different numbers of has molecules on either side of the equation, increasing the pressure encourages the reaction to move in the direction that produces the lowest number of molecules.

For example, in the reaction 2SO2 (g)  + O2 (g)  Equilibrium symbol ( 2SO3 , we can see that there are 3 molecules of gas on the left hand side of the equation and only 2 molecules of gas on the right hand side.
By increasing the pressure, the forward reaction is favoured because there are less gas molecules in the products of the forward reaction than there are in the products of the backward reaction.

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How Much?: Reversible Reactions


If a forward reaction is exothermic, decreasing the temperature will take away energy from the reaction. To oppose this change, the reaction will generate more energy for itself, so the forward reaction is favoured, increasing the yield of the products.

If a forward reaction is endothermic, increasing the temperature will supply extra energy to the reaction and favour more product being formed.

Increasing the concentration of reactants or removing the products favour the production of more product and drives the forward reaction.
Decreasing the concentration of the reactants favours the backward reaction.

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How Much?: Reversible Reactions

Basic physics tells us that:

  • Increasing the temperature of a reaction increases the rate of the reaction.
    This is because the particles in the reaction are given more energy, so they move more quickly and are more likely to collide with enough energy to bond to form the products.
  • Increasing the pressure of a reaction increases the rate of the reaction.
    This is because the particles in space provided are closer together, so they are more likely to collide.
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How Much?: Reversible Reactions

This creates a problem in some cases in optimizing the conditions of reversible reactions;

  • if the forward reaction is exothermic, decreasing the temperature increases the yield, but it also slows down the reaction, and
  • if the  forward reaction produces the highest number of molecules, decreasing the pressure increases the yield, but it also slows down the reaction.


This means that manufacturers using these principals must find a comprimise between faster reactions or increased yields to provide the optimum efficiency for the production of the useful product.

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How Much?: Making Ammonia - the Habour Process

Plants need nitrogen to survive. They absorb it into their systems through their roots from the soil. When crops are then removed during farming, the soil is left with a much lower concentration of nitrogen, making it hard for more plants to grow there.

Fortunately, 100 years ago, a German chemist called Fritz Haber discovered a way to produce large amounts of Ammonia, (NH3), from Nitrogen gas, which is abundant in the atmosphere, and Hydrogen which he got from natural gas. Ammonia is used as a plant fertiliser.

It's very important to understand the significance of the development of plant fertilisers. It allowed for much quicker, more efficient farming methods. This meant less people were needed to produce enough food for everyone, so people could work in cities and towns instead, and urbanisation and industrialisation was possible.

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How Much?: Making Ammonia - the Habour Process

Making Ammonia involves:

  • purifying the Nitrogen and Hydrogen
  • passing them over an iron catalyst ( to speed up the reaction )
  • high temeperatures (about 450 degrees Celsius) and
  • high pressure (about 200 atmospheres)

This reaction is reversible, so the Ammonia constantly breaks down to Hydrogen and Nitrogen again. To reduce this, we remove the ammonia by cooling and liquefying it as soon as it is formed. The left over Hydrogen and Nitrogen is used again.

N2  +  2H2  Equilibrium symbol (  2NH3

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Rates of Reactions: How Fast?

The rate of a reaction is the speed at which the reactants are turned into the products. We can measure the rate of a reaction by either timing how long it takes for the reactants to get used up or by measuring the time it takes for the products to be produced.
Rate of Reaction = Amount of reactant used or amount of pruduct formed / time
We can measure this in three ways:

  • 1.  If a gas is given off, the weight of the products will be less than that of the reactants. Using this, we could measure the time it takes for the mass to decrease by a certain amount.
  • 2.  If a gas is given off, we could time how long it takes for a certain volumn of mass to be produced, using a gas syringe.
  • 3.  Some reactions make an insoluble solid, which makes the solution go cloudy. We could measure the time it takes for the solution to turn a certain cloudiness, using either a light meter to measure the amount of light that the solution transmits or the 'disappearing cross' method, where you place the reactants in a glass container over a piece of paper marked 'X' and time how long it is before you the products mean that you can no longer see the 'X'.
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Rates of Reactions: Collision Theory

There are 4 main factors which affect the rate of chemical reactions:

  • Temperature
  • Surface Area
  • Concentration or Pressure
  • Presence of a Catalyst

Collision theory states that in a reaction, for the reactants to produce the products, a certain amount of energy must be present, so that the atoms have enough energy to colide and react.

The minimal amount of energy is the activation energy. Giving the reactants more energy or making the chance of particles coliding bigger will make it more likely that reactions will happen, and make reactions happen faster.

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Rates Of Reactions: Collision Theory

When a big solid reactant reacts with a solution, the inside of the large piece is not in contact with the solution it is reacting with, so it can't react. It has to wait for the outside to react first, but in smaller lumps, or as a powder, each tiny piece is surrounded by solution, so the reaction can take place much more easily.
For example, a 2cm x 2cm x 2cm cube has a surface area of 24cm squared, but if that cube was cut up into 4 1cm x 1cm x 1cm cubes, its surface area would be 36cm squared, so more particles of the solid are in contact with the other reactant, so the reaction takes less time.

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Rates of Reactions: The effects of Temperature and

When we heat up a reaction, the particles are given more energy, so they move faster and collide more often, and colide with more energy, so it's more likely that the reactants will react so the reaction happens faster.

Increasing the concentration of reactants in a solution increases the reaction rate because there are more particles of the reactants moving around in the same volume. The more 'crowded' together the reactant particles are, the more likely it is that they will bump into each other and a reaction will take place. This is similar for increasing the pressure.


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Rates of Reactions: The effect of catalysts

A catalyst is a substance which increases the rate of a chemical reaction but is not changed chemically during the reaction, so it can be used over and over again.

Catalysts lower the activation energy needed to start a reaction. This means that more of the collision between particles result in a reaction taking place. Some catalysts work by providing a surface for the reacting particles to come together.


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Rates of Reactions: Graph


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Energy & Reactions: Exothermic & Endothermic react

Exothermic reactions release energy as heat into their surroundings, so they feel hot. This is because more energy is released when bonds are formed than the energy taken in to break the bonds.

Examples of exothermic reactions include combustion and respiration.


Endothermic reactions take in heat from their surrounds, and feel cold. This is because the process of breaking the bonds takes more energy than the process of releasing energy by forming bonds.

Examples of endothermic reactions include photosynthesis and thermal decomposition.


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Energy & Reactions: Energy & Reversible reactions

Energy changes are involved in reversible reactions.

If one reaction (either the forward or backward reaction) is exothermic, it releases energy. The other reaction will be endothermic, so it will absorb energy

In reversible reactions, the amount of energy released by the exothermic reaction is exactly the same as the amount of energy absorbed by the endothermic reaction.

But remember, by changing the temperature of the reaction conditions, we can alter the yields of each reaction.

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Energy & Reactions: More about the Haber Process

By using what we know about changing yields in reversible reactions, we can understand why the conditions that the production of Ammonia are used.

(1)N2 (g)2H2 (g)  Equilibrium symbol (  2NH3 (g)

Here, we can see that there are 2 moles of gas on the right-hand side of the reaction, and 3 moles of gas on the left hand side. Because of this, the reaction is carried out at high pressures, so the forward reaction will be favoured and more Ammonia will be formed.
However, carrying out reactions on an industrial scale with high pressures is expensive, because the equipment used must withstand the strong forces, and there are risks of explosions, so a compromise is made.
This reaction is usually carried out at pressures of around 200 - 350 atmospheres pressure.

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Energy & Reactions: More about the Haber Process

Ammonia production is usually carried out at temperatures around 450 degrees Celsius.

The reaction for the production of Ammonia is exothermic, however, although lower temperatures would increase the amount of ammonia at equilibrium, the ammonia would be produced too slowly for industrial efficiency.


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Electrolysis: The Basics

The word 'electrolysis' means 'splitting up using electricity'. In electrolysis, an electric current is used to decompose a substance (in liquid form), called the electrolyte, made up of ions, into simpler substances.
Two conducting rods, called the 'electrodes' are connected to a power supply; one electrode connected to the positive terminal and the other, the negative terminal. These are dipped in the electrolyte.
The electrodes are normally made of inert, (unreactive), substances such as graphite or platinum, so they don't react with the electrolyte or the products of the electrolysis reaction.
The electrode connected to the positive terminal is called the anode. During electrolysis, negative ions move to the anode.
The electrode connectetd to the negative terminal is the cathode. During electrolysis, positive ions move to the cathode.
Once they reach the electrodes, the ions can lose their charge and be deposited as elements. Depending on the compoud being electrolysed, gases are given off or metals are deposited at the electrodes.

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Electrolysis: The Basics

An example of an electrolysis reaction: Lead Bromide
PbBr2  (l)  ->  Pb  (l)  +  Br2  (g)

Lead Bromide is an ionic substance so it does not conduct electricity when solid. It is melted into molton lead bromide so the ions can move freely and it can conduct electricity.
The positive lead ions move towards the cathode where they are discharged and deposited as molten lead.
The negative bromide ions move towards the anode, where they are discharged and given off as Bromine gas.

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Electrolysis: The Basics

When ionic substances have very high melting points, they can be dissolved in water instead so that they conduct electricity.
However, water also forms ions (H+ and OH- ions), so it's harder to predict what will be formed, because the products at the anode and cathode may be different to what is expected.

When copper bromide is electrolysed, solid copper forms at the cathode and bromine solution is formed at the anode.

CuBr2  (aq)  ->  Cu  (s)  +  Br2  (aq)

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Electrolysis: Changes at the Electrodes

When ions are discharged at the electrodes, they gain or lose electrons, depending on their charge. Negative ions lose electrons and positive ions gain electrons. This makes them neutral.
In the electrolysis of molton lead bromide, positively charged lead ions (Pb2+) move towards the cathode, where each ion gains 2 electrons to become a neutral lead atomGaining electrons is called reduction. The lead ions are said to be reduced.
The negatively charged bromide ions (Br-) move towards the anode, where each ion loses one electron to become a neutral bromide atom. Two bromide atoms then form a covalent bond to make a bromine molecule (Br2). Losing electrons is called oxidation. The bromine ions are said to be oxidised.

Because both REDuction and OXidation take place at the same time during electrolysis (at opposite electrodes) the reaction is sometimes called a REDOX reaction.

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Electrolysis: Changes at the Electrodes

We represent what is happening at the electrodes using half equations. They are called 'half equations' because they only show half of the whole reaction.

Using the Lead Bromide reaction again:

At the cathode:

Pb2+  +  2e-  ->  Pb          (the e- represents an electron)

At the anode:

2Br-  ->  Br2  +  2e-      OR

2Br-  -  2e-  ->  Br2

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Electrolysis: Changes at the Electrodes

Because water also form ions when an ionic substance is dissolved for electrolysis, we have to work out which ions will be discharged at the electrodes.
The general rule for this is that the less reactive element will usually be formed.

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Electrolysis: Electrolysing Brine

When sodium chloride solution (brine), is electrolysed, 3 very important products are formed:

  • Chlorine gas at the positive electrode
  • Hydrogen gas at the negative electrode
  • A solution of Sodium Hydroxide


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Electrolysis: Electrolysing Brine


  • Chlorine is a very poisonous green gas which causes great damage to our bodies if it is inhaled.
  • However, it is used to kill bacteria in drinking water and swimming pools.
  • When chlorine is reacted with sodium hydroxide, it makes bleach, which is a strong oxidising agent used to kill bacteria. It is used in homes, hospitals and industry to maintain good hygiene.
  • Chlorine in used in many different disinfectants.
  • It is also used in the plastic polymer, PVC.
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Electrolysis: Purifying copper

Electrolysis is used to remove impurities from copper before it can be used in electrical wiring, etc.

A bar of impure copper is used as the anode and a thin sheet of pure copper is the cathode. The electrolysis takes place in a solution containing copper ions (e.g. copper sulphate solution)
At the anode, copper atoms are oxidised. They form copper ions and go into the solution.
At the cathode, copper ions are reduced to form copper atoms which are deposited on the cathode as pure copper.
The solid impurities sink to the bottom of the electrolysis cell forming 'anode sludge'

Half equations:
ANODECu (s)  ->  Cu2+  +  2e-
CATHODECu2+  +  2e ->  Cu (s)

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Electrolysis: Electrolysing Brine


  • The hydrogen produced by electrolysing brine is pure, so it's very useful in the food industry. It is used during hydrogenation (turning oils to margerines).
  • When reacted with chlorine, hydrogen chlorine gas can be formed. This gas can be dissolved in water to make pure hydrochloric acid, which is used widely by the food, cosmetic, cleaning and pharmaceutical industries.


  • Used in making soap
  • Used in making paper
  • Used to control pH in many industrial processes
  • Combined with chlorine it produces bleach
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Electrolysis: Changes at the Electrodes

When we carry out electrolysis in water the situation is made more complicated by the fact that water contains ions (H+ and OH- ions).
The general rule for working out what will happen is to remember that if two elements can be produced at an electrode, the less reactive element will usually be formed.

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Acids, Alkalis and Salts: Acids and Alkalis

When a substance is dissolved in water it becomes an aqueous solution, shown through the state symbol (aq). This solution may be acidic (e.g. citric acid, sulfuric acid, ethanoic acid), alkaline or neutral (e.g. pure water), depending on the chemical dissolved. Bases are chemical which can neutralise acids.
Alkalis are bases which dissolve in water.


Hydrochloric acid is formed when hydrogen chloride gas dissolves in water.
HCl  (g)  --(water)-->  H+  (aq)  +  Cl-  (aq)
All acids form H+ ions when added to water because hydrogen ions make solutions acidic. Chloride ions are also formed.
A H+ ion is a hydrogen atom which has lost an electron, leaving only a proton. For this reason, acids are also called 'proton donors'

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Acids, Alkalis and Salts: Acids and Alkalis


are opposite to acids in the way that they react. Alkalis are the most commonly used bases because they are dissolved in water.
Sodium Hydroxide solution is formed when solid sodium hydroxide is dissolved in water.
NaOH  (s)  --(water)-->  Na+  (aq)  +  OH-  (aq)
Hydroxide ions (OH-) are formed when any alkali is added to water. Hydroxide ions make a solution alkaline.

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Acids, Alkalis and Salts: Acids and Alkalis

Indicators (such as Litmus paper) are chemicals which change colour when added to acids or alkalis.
The pH scale is used to measure acidity. The scale runs from 0 (most acidic) to 14 (most alkaline).
Universal indicator is an indicator made from a number of dyes. It turns different colours at different values of pH.
Anything in the middle of the pH scale (pH 7) is neutral.

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Acids, Alkalis and Salts: Making Salts from Metals

( e.g. 2HCl  (aq)     +  Mg  (s)         --> MgCl2  (aq)                                +  H2  (g) )
Hydrochloric acidMagnesium --> Magnesium Chloride Solution +  Hydrogen
NOTE: This is only possible if the metal is above hydrogen in the reactivity series.


  • The salts formed when hydrochloric acid is neutralised are always chlorides
  • The salts formed when sulphuric acid is neutralised are always sulphates
  • The salts formed when nitric acid is neutralised are always nitrates

The oxidation of a transition metal (such as iron(III) oxide) is an example of a base that can be used to make salts in this way.
6HCl (aq) Fe2O3  (s)  -->  2FeCl2  (aq)  +  3H2O  (l)
Hydrochloric acid  +  solid iron(III) oxide  --> iron(III) chloride solution + water

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Acids, Alkalis and Salts: Making Salts from Soluti

There are 2 other important ways of making salts from solutions:

1. Neutralisation reactions

HCl  (aq) NaOH  (aq)  -->  NaCl  (aq)  +  H2O  (l)
Hydrochloric acid + sodium hydroxide solution --> sodium chloride solution +  water

This is what's happening with the ions:   H+  (aq)  +  OH  (aq)  -->  H2O  (l)

To test when the acid and alkali are finished reacting and the solution has become neutral, an indicator can be used.
For example, Litmus paper turns red in the presence of a strong acid, blue in the presence of a strong alkali, and green in a neutral solution.

This type of reaction is used to make Ammonium Salts which are used as plant fertilisers.

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Acids, Alkalis and Salts: Making Salts from Soluti

2. Precipitation Reactions

Sometimes, by combining two solutions, an insoluable salt, called the precipitate can be made.

Pb(NO3)2  (aq) 2NaCl  (aq)  -->  PbCl2  (s) 2NaNO3  (aq)
Lead nitrate solution + sodium chloride solution -->  solid lead chloride + sodium nitrate solution.

Precipitation reactions can be used to remove pollutants from wastewater from factories and industrial parks before it's discharged into rivers and the sea.
An important precipitation reaction is the removal of metal ions from water that has been used in industrial processes. By raising the pH of the water, insoluble metal hydroxides precipitate out of the solution, which produces a sludge that can easily be removed from the solution. The clean water can then be discharged into rivers or the sea.

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Thank u this has been extremely helpful for my revision!!!


You're welcome :) Thanks for the rating!


could you please give me a link ta download this powerpoint?


Samreen wrote:

could you please give me a link ta download this powerpoint?

These are revision cards- not a powerpoint?

Azza Essakhi

really useful revision cards but i downloaded the mp3 and there are a few typos that it picks up on and there arent enough full stops so there are no gaps between some sentences. otherwise its perfect :)



Muzz :P

Great Notes... they really help!!!!  :)

jess devlin

Thank you so much for these notes I have my exam in 5 days and I was freaking out but thanks to ur notes I know what i'm doing, thankyou :):):):)


i take it this is for higher?


yes it is callum


So so helpful! Thankyou!


aww thanks everyone :) you're all very welcome- glad i could help **


this amazing thankyou sooo much :D **

Vicky Hendry - Team GR

Thanks so much! Helped loads with revision!**x


OMG YOU ARE A LIFE SAVER I LOVE YOU! these are amazing:')



Amazing! Makes it soooo easy to learn! Thank you so much.


This is really good, will you be making a biology (AQA B2) one? :)


Thanks so much for this!! Please make a Biology and Physics unit 2 one and I will love you forever!! :D


what grade did you lot achieve with these revision cards?

anna meakin

thanks are you making biology anf physics ones?

anna meakin

thanks are you making biology anf physics ones?

Kevin Fang

Thanks, this is really helpful. Helping me in my mock, and hopefully this will continue to help me in the real exams.

- Kevin.


Thanks fellas

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