Chemistry GCSE


  • Created by: Krissie
  • Created on: 17-04-11 15:23

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.

( are usually drawn like this. Atomic Structure can also be written in order of electrons in each shell. For example, this carbon atom's atomic structure would be written as [2,4], with a comma seperating the shells.

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Structures & Bonding: The Periodic Table

In The Periodic Table, elements are arranged in order of their atomic number. Elements also have groups and periods.

The period that the element is in, is in accordance with the number of occupied shells that atom has. The group that the element is in depends on how many electrons are in the outer most shell of that atom. This means they all share similar chemical properties.

In the periodic table, periods are arranged horizontally, and groups are arranged vertically.

Elements, which of whom's atoms have a full outer shell are unreactice, or stable. They're in group 0 of the periodic table, and are also called the Noble Gasses. As the group numbers decrease from group 8 (which is actually group 0), the elements become MORE REACTIVE. Elements in group 1 are extremely reactive. (e.g. Potassium)


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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.

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.

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 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


Ionically 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.

(, 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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Energy & Reactions- Bond Energies

During a chemical reaction, bonds between atoms are broken in the reactants and new bonds are made in the products. Energy is always transferred when chemical bonds are broken and formed.

Energy changes in reactions is all about bond energy.

Bond energy is the energy that is held in chemical bonds between atoms, which can  be represented by a single or double line between chemical symbols. Different atoms have different bond energies. For example, the bond energy between an atom of hydrogen and an atom of oxygen (written H - O ) is 464 kJ/mol, whereas a bond between oxygen and oxygen (written O - O ) is 144 kJ/mol.

Double bonds are stronger than single bonds, so they hold more energy. For example, a double bond between 2 atoms of oxygen (written O = O ) hold 498 kJ/mol.  

It takes energy to break bonds. At the start of the reaction, some of the bonds are broken so that new bonds can be formed. The energy that it takes to break these bonds is called the activation energy, which is the sum of all of the bonds in the reactants. Because energy is absorbed by the reaction from the surroundings to break the bonds, this is an endothermic process.

Energy is released when new bonds are formed. This makes it an exothermic process.

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Energy & Reactions- Exothermic & Endothermic

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.
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.
Exothermic and Endothermic reactions can be represented using Energy Level Diargrams.

 ( is an example of an exothermic reaction. The products are on a lower level than the reactants because they have given out energy to their surroundings.

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Energy & Reactions- Exothermic & Endothermic- Cont


This is an example of an endothermic reaction. The products are higher than the reactants because they have absorbed more heat from their surroundings.

Delta H ( ( represents the difference in energy levels of the reactants and produces, and is measured in kJ/mol. If delta H is positive, then more energy has been absorbed during the reaction than released. If it's negative, than less energy has been absorbed during the reaction than released. This means that all exothermic reactions have a negative delta H, and all endothermic reactions have a positive delta H

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Energy & Reactions- Calculating Energy Change

A good example of calculating energy change is using a reaction between hydrogen and chlorine,   H2 + Cl2  --> 2HCl

The ' 2 ' in subscript shows that there is a bond between the 2 hydrogen and the 2 chlorine atoms. This reaction could also be represented like this, with the ' - 's representing the bonds:

 H - H       +      Cl - Cl     -->     H - Cl      +      H - Cl 

Using a bond energy table, we can see that a bond between 2 hydrogen atoms holds 436 kJ/mol, and a bond between 2 chlorine atoms holds 242 kJ/mol.

BOND                              BOND ENERGY (kJ/mol)

H – H                                  436

Cl – Cl                               242

H Cl                               431

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Energy & Reactions- Energy Change- Continued

We know that the first thing that happens in a reaction is that the bonds in the reactants are broken, and that this is endothermic process, so the answer is positive, so we add up the energy needed to break these bonds:

To break a bond between 2 hydrogen atoms, we need 436 kJ/mol, and to break a bond between 2 chlorine atoms we need 242 kJ/mol. Added, these make 678 kJ/mol, which is our activation energy. It's positive, so we can put a ' + ' sign in front. ( + 678 kJ/mol )

Now we have to work out the energy that is released when the bonds between the hydrogen and chlorine atoms are formed. We know from our table that this bond holds 431 kJ/mol, and from the symbol equation that there are 2 H - Cl bonds, so we times 431 by 2, to get 862 kJ/mol. We also know that bond formation is an exothermic process, which gives a negative answer, so we can put a ' - ' sign in front to show that: ( - 862 kJ/mol )

Now we have to add the 2 bond energies from each process together:

+ 678 + ( - 862 ) = -184

The answer is negative, showing that it is an exothermic reaction. The answer is the overall Delta H, showing that the energy change between the reactants and the products is -184 kJ/mol.

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Energy & Reactions- Reversible Reactions

Many reaction, 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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Energy & Reactions - Energy from Fuels

When we burn a fuel we are using an exothermic reaction as a source of energy. We usually use this energy to provide us with heat or light for transport.

Not all fuels provide the same amount of energy when they burn, and although it is difficult to measure how much energy each fuel produces, we can compare different amounts of energy produced in reactions, using a caloriemeter.

The test would involve burning different fuels under a caloriemeter of water and measuring the time it took for the water to reach a certain temperature.

To work out how much energy is transferred to the water, we use this equation:

Energy = mass of water x specific heat capacity of water* x temperature rise

(Joules)  = (grams) x (Joules per grams per degrees centigrade) x (degrees centigrade)

*Each substance has a different 'specific heat capacity', and water's is 4.2J/g/degrees centigrade, meaning that it takes 4.2J of energy to raise the temperature of 1g of water by 1degree C.

Energy per gram (J/g) = energy released (J) / mass of fuel burned (g).

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Energy & Reactions - Energy from Fuels - Con.

So, for example:

If we had an alcohol (X), and we heated 100 cubic cm of water with it, until the temperature of the water rose by 25 degrees, and then we measured the alcohol to find that 0.2g of it had burned, the energy, in Joules used would be:

100 x 4.2 x 25 = 10,500 Joules

Then we find the energy per gram, and since we used up 0.2g of fuel, we do :

(,500 / 0.2 = 52,500 Joules per Gram

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The Periodic Table- Arrangement

In The Periodic Table, elements are arranged in order of their atomic number. Elements also have groups and periods.

The period that the element is in, is in accordance with the number of occupied shells that atom has.

The group that the element is in depends on how many electrons are in the outer most shell of that atom. This means they all share similar chemical properties.

In the periodic table, periods are arranged horizontally, and groups are arranged vertically.

Elements, which of whom's atoms have a full outer shell are unreactice, or stable. They're in group 0 of the periodic table, and are also called the Noble Gasses. As the group numbers decrease from group 8 (which is actually group 0), the elements become MORE REACTIVE. Elements in group 1 are extremely reactive. (e.g. Potassium)

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The Periodic Table- Group 1

Elements in group 1 of the periodic table are also known as the Alkali Metals.

  • Alkali metals have similar behavior because they all have 1 electron in their outer shell. When they undergo a chemical reaction they lose this outer electron to form an ion with a single positive charge.

  • They are very reactive, and get more reactive as you do down the group: this is because it gets easier to lose the outer electron and become a positive ion because it's further away from the nucleus and the attraction is therefore weaker
  • They react with cold water, producing hydrogen and metal hydroxides, which are strong alkaline solutions, e.g.

lithium + water → lithium hydroxide + hydrogen

2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)

  • Their reactions are very exothermic, and when added to water they can ignite the hydrogen that they produce, causing a flame. Potassium burns with a purple flame

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The Periodic Table- Group 1 - Continued


  • They can be cut with a knife
  • Purely (unoxidised), they are silvery and shiny, but they oxidise quickly. They are stored in oil to stop this from happening.
  • Low melting and boiling points: lithium has the highest melting point, and they decrease as you go down the group

They are less dense than water, so they float

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The Periodic Table- Transition Metals

The elements in the central block of the periodic table are called the Transition Metals. They don't have groups, and lie between groups 2 and 3 in the periodic table.


    • they are good conductors of heat and electricity
    • they can be hammered or bent into shape easily
    • they are less reactive than alkali metals such as sodium
    • they have high melting points - but mercury is a liquid at room temperature
    • they are usually hard and tough
    • they have high densities
    • they form coloured compounds
    • they form ions with different charges, e.g. Fe2+ ( Iron II oxide) and Fe3+ (Iron III oxide)
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The Periodic Table- Transition Metals- Continued


  • Iron can be made into steel, which is a useful building material.
  • Copper is used in wires because it's a good conductor of electricity.
  • Gold and Silver don't corrode in water or air, and are good conductors of electricity, so ithey're used in jewelery and electrical systems.
  • Some can be used as catalysts by reducing the activation energy needed for reactions- e.g, Nickel speeds up the rate of hydrogenation of alkenes and Iron speeds up the rate of production of ammonia in the Haber process


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The Periodic Table- Group 7- The Halogens

The Halogens are the non-metals, found on the right of the periodic table.

Properties & Trends:

  • Low melting and boiling points: increasing as you go down the group
  • Colourwise, they get darker as you go down the group. Fluorine is very pale yellow, chlorine is yellow-green, and bromine is red-brown. Iodine crystals are shiny purple.
  • They're diatomic; they always travel as 2 atoms bonded covalently.
  • Typical properties of non-metals: insulators of electricity & heat, low density etc.
  • They become less reactive as you go down the group. Flourine is the most reactive, and is very dangerous. It can cause serious chemical burns to the skin and eyes
  • At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. There is therefore a trend in state from gas to liquid to solid down the group.

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The Periodic Table- Group 7- The Halogens- Continu

  • More reactive halogens displace less reactive ones (e.g. Chlorine will displace the Bromine in a displacement reaction between chlorine and sodium bromide)

chlorine + sodium bromide → sodium chloride + bromine

  • - The halogens react with metals to make salts called metal halides.
    • metal + halogen → metal halide

      For example, sodium reacts with chlorine to make sodium chloride (common salt).

      sodium + chlorinesodium chloride

    • Halogens can kill bacteria- Chlorine sterilizes drinking water at low concentratiions and keeps swimming pools clear of bacteria.
    • They are used as bleaching agents- Chlorine is used to bleach paper white.
  • Uses:

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The Periodic Table- Group 0- The Noble Gasses

What would be group 8 of the periodic table is actually group 0, also known as the noble gasses.

These are the most unreactive (inert) elements in the periodic table because they already have a full outer shell of electrons. This means that they travel as single atoms, so they are monatomic

They are non-metals, and exist as colourless gases at room temperature.

They have low boiling points, with the boiling points increasing as you go down the group.

They have low densities (helium's density is less than that of air) because the atoms are far apart in a gaseous state. The densities increase as you go down the group.

(<- Helium Balloons (Up)

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Oils & Feuls- Hydrocarbons

Hydrocarbons are elements that only contain hydrogen and carbon atoms.

Alkanes are hydrocarbons that only have single bonds between those atoms. The molecular formula of any alkane applies to CnH2n+2. They include Methane (CH4), Ethane (C2H6), Propane (C3H8) and Butane (C4H10).We say that they are saturated because you can't add any more hydrogen atoms on. They all have a structure that resembles this:


Alkenes have a double bond between 2 of the carbon atoms.The molecular formula of any alkene applies to CnH2n. They include Methene (CH2), Ethene (C2H4), Propene (C3H6) and Butene (C4H8). We say that they're unsaturated because more hydrogen atoms could be joined to them. Their structure resembles this:


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Oils & Feuls- Hydrocarbons- Continued

When hydrocarbons burn in oxygen they form Carbon Dioxide and Water:

Methane Oxygen  →  Carbon Dioxide + Water

The products can be tested by:

a) Bubbling the carbon dioxide through lime water to see if it turns cloudy

b) Adding the water to anhydrous copper sulphate to see if the white powder turns blue ( This is also an exothermic reaction )

However, when hydrocarbons burn in an insuffiscient amount of oxygen, they produce carbon monoxide, which is poisonous and very dangerous.

Because they produce carbon dioxide, which is a greenhouse gas, when they burn, the combustion of hydrocarbons is bad for the environment. Impurities in some fuels, such as sulphur petrol, are also harmful for the enviroment. Cars now have to be fitted with catalytic converters, which reduce harmful impurities from being released into the atmosphere. Power stations are also fitted with systems that stop impurities from being released, e.g. smoke precipitator.

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Oils & Feuls- Properties of Hydrocarbons

Small Hydrocarbons with only a few carbon atoms:

  • exist as gases
  • low boiling points
  • volitile
  • flow easily
  • easily ignited

Hydrocarbons with about 5-12 carbon atoms:

  • exist as liquids

Large hydrocarbons with 12+ carbon atoms:

  • exist as solids
  • not volitile
  • don't flow easily
  • hard to ignite
  • high boiling points
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Oils & Feuls- Fractional Distillation

Crude Oil is a mixture of hydrocarbons. It's not very useful as a mixture, so we have to seperate out different groups of hydrocarbons that are similar sizes and therefore have similar properties so we can use them. We seperate them using their different boiling points.Substances with lower boiling points evaporate from a heated mixture of substances before those with high boiling points. This means that smaller hydrocarbons would evaporate first.

In Industry they use a fractional distillation tower. A large tower is fitted over a heated mixture of hydrocarbons. As the temperature increases, the different hydrocarbons evaporate and condense at different levels. Hydrocarbons with smaller chains and lower boiling points condense at the top of the tower, whereas hydrocarbons with long chains and high boiling points condense nearer the bottom of the tower. Condensed oil is collected at different levels in the tower, seperating the oil into hydrocarbons of different sizes.

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Oils & Feuls- Fractional Distillation Tower


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Oils & Feuls- Good & Bad- Petrol

With growing concerns about global warming and decrease of fossil fuels, alternative fuels have to be examined to see if we can provide alternative renewable resources for future generations.

Most cars run on petrol in the UK, but in Brazil, where sugar cane production is high, lots of cars have been adapted to run on ethanol.


  • Available from the ground
  • Most cars are built for petrol
  • Cheaper than Ethanol
  • Well trusted and experienced by the population
  • Not CO
  • 2 neutral
  • A non-renewable resource
  • Cars can produce impurities when burning petrol (e.g. sulphur dioxide)
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Oils & Feuls- Good & Bad- Ethanol


  • Can be renewable
  • Can be CO2 neutral
  • Can be made using 2 different methods
  • Doesn't contain impurities, so it doesn't produce sulphur dioxide when it burns
  • People may not trust a new fuel
  • Plants needed to grow it take up valuable land
  • More expensive than petrol
  • Hot climate needed to produce the sugar cane for production
  • Costs of adapting cars to run on ethanol

Ethanol can be produce by fermenting sugar cane (a plant). This means that the plants take in carbon dioxide as they grow, so they are CO2 neutral.

It can also be produced by reacting ethene with steam when both are heated together at 300 degrees centigrade and compressed to 70 atmospheres pressure and passed over a catalyst of phosphoric acid. This produces large quantities of ethanol, it's quicker than growing the sugar cane and less land and labour is needed, however it's not CO2 neutral, and it's non-renewable.

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Oils & Fuels- Cracking

Crude oil has too many longer chains of hydrocarbons than needed, and not enough short chains of hydrocarbons, which have high demand, so a process called cracking, where long hydrocarbons are passed over a catalyst in a gaseous state is used to break the longer hydrocarbons down into smaller, more useful hydrocarbons.

Cracking produces alkenes called monomers. When reacted together, they form polymers, which are hydrocarbon chains which can be tens of thousands of carbon atoms long. During polymerisation, the double bond of one molecule breaks and joins onto another molecule, forming a new bond. The second molecule then joins onto another, and so on. Polymerisation is an addition reaction because molecules are being added together to make 1 product.

( in short form.

The name of the polymer is the name of the monomer with the prefix 'poly'                              (e.g. Ethene produces polyethen) 

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Products from Rock- Limestone

Limestone is mainly made out of Calcium Carbonate (CaCO3). When heated, it thermally decomposes to form calcium oxide (quicklime) and carbon dioxide. (All metal carbonates break down to metal oxides and carbon dioxide.) Metals higher up in the reactivity series have carbonates which need more heat to decompose. Calcium oxide reacts with water to produce calcium hydroxide (slaked lime).


  • Limestone can be used as a building material and in the manufacturing of iron.
  • Glass - heated with sand and soda (sodium carbonate).
  • Cement - heated with clay in a kiln.
    • Concrete - mixed with sand, water and crushed rock
    • Mortar - mixed with sand and water
  • Quicklime - heated.
    • Slaked lime- mixed with water
      • Lime motar - mixed with water
  • Limestone, quicklime and slaked lime are used in the neutralisation of acidic lakes and soils
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Products from Rock- Limestone- Continued

There are advantages and disadvantages of the limestone industry.


  • Provides employment opportunities to people, supporting small town economies
  • Useful natural resource
  • Cement is strong when squashed and straight


  • Disfigures the natural environment
  • Causes a lot of noise and traffic
  • Cement needs to be reinforced when it's not straight
  • Walls made out of limestone can wear away in the rain and damage the buildings that they are a part of, especially when the rain is more acidic due to pollution
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Relative Atomic Mass & Relative Molecular Mass

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

The Ar of an element is its weight compared with an atom of carbon

In lots of Chemical Calculations, you need to know the relative molecular mass of a molecule. This is found by adding relative atomic masses together.

For example, the Mr of Sulphuric Acid (H2SO4) uses the calculation:

(  2  x  1  )(  32  )  +  (  4  x  16  ) 98 

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Percentage Composition Calculations

Percentage Composition Calculations are used to work out how much of an compount 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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Emprical Formula

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 formular 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 all of the masses of each element by the element's Ar

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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Emprical Formula- Example

'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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The Mole

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 molecular masses.

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.


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Theoretical Yields

Theoretical Yields involve the relationship between 2 substances in an equation.

To do this we must:

1. Calculate the moles of one substance, by dividing the mass of that substance by it's Mr

2. Finding the ratio from the eauation to calculate the moles of the other substance (ratio from the big numbers before the emprical formula)

3. Multiply the number of moles found in 1 by the other substance's ratio number

4. Multiply the answer by the wanted substance's relative molecular mass

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Theoretical Yields- Example

'What mass of NaOH is needed in order to make 100g of sodium sulphate?'

2NaOH + H2SO4  → Na2SO4 + 2H2O

1. We only have the mass of sodium sulphate, so we work out how many moles there are of that substance, so 100g / Mr of sodium sulphate ( ( 23 x 2 ) + ( 32 ) +  ( 16 x 4 )  =  142) = 100 / 142  =  0.7042

2. The ratio of the 2 substances we're linking is from the big numbers in front of the formulas. In this case, there are 2 NaOHs for every Na2SO4's, so the ratio of sodium hydroxide to sodium sulphate is 2 : 1.

3. The number we found before was 0.7042. Because we're trying to work out the mass of sodium hydroxide, and there are twice as moles of sodium hydroxide than sodium sulphate, we multiply 0.7042 by 2, to get 1.4084.

4. The Mr of sodium hydroxide is ( 23 + 16 + 1 = 40 ), so we multiply 1.4084 by 40 to get 56.3g.

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Percentage Yields

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

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.

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. So here, we'd do

( 120 / 144 ) X 100 = 83.3 %

Atom economy is similar. In atom economy, we divide the mass of useful product by the total mass of the product, and multiply the answer by 100.

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Rates Of Reactions

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 products to get used up or by measuring the time it takes for the products to be produced. 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'.


Rate of Reaction = Amount of reactant used or amount of pruduct formed / time

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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: Surface Area

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: Temperature, Concentration

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: 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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Chemical Vocab

Atom: Tiny particles of which everything is made of, consisting of protons, neutrons and electrons.

Molecule: Two or more atoms chemically joined together.

Element: A substance made of only 1 type of atom.

Compound: A substance made up of molecules with different types of atoms


Mixture: Different atoms and molecules not chemically joined together.

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The Early Periodic Table

During the 19th century, chemists were finding new element almost every year, while trying to find patterns in the behaviour of the elements. This would allow them to organise the elements and understand more about them.

Idea 1: John Dalton arranged the elements in order of their mass, measured in various chemical reactions. In 1808 he published a table of elements in a book.

Idea 2: John Newlands built of Dalton's ideas with his law of octaves. Newlands based this on the observation that the properties of every eighth element seemed similar. Unfortunatly, because not all of the elements had been found, it didn't work.

Idea 3: Alexandre-Emile Beguyer de Chancourtois from France came up with Newlands ideas before him, and produced a clear diagram to demonstrate this, but when his book was published, the diagram was left out.

Idea 4: Russian scientist Dmitri Mendeleev arranged the 50 elements that had been discovered by atomic mass. He arranged them so that a periodic pattern in their physical and chemical properties could be seen.He left gaps where elements had not been discovered, but predicted their properties. He is remembered as 'the father of the modern periodic table'. Some of his ideas didn't work because some elements are slightly different, so argon and pottassium were mixed up.

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This is good notes.

Good Job!

leah murray


These notes are really good. Also i have been wondering how do you get pictures on revision notes?



Oh it's easy, you can just copy and paste from google :) it's a bit annoying though, you can't move them around properly :P

Thanks, by the way :)



Great notes :) Thankss

Shannon Tennant-Smith - Team GR


great notes! :)

Gless Fuentebella


is this AQA GCSE Chemistry Unit 2?



Meow damn kris when dyu do all this? x



i know that this is old but still helpful

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