AQA Chemistry Unit 2

A compound is a substance made up of two or more elements joined together. When a compound is made up only of non-metals, they will be joined together covalently.

The aim of all atoms is to get a full outer shell of electrons to make them more stable. Non-metals (when bonding with other non-metals) do this by sharing pairs of electrons. Each shared pair of electrons is one covalent bond.

For example, nitrogen has five electrons in its outer shell. It can bond with three hydrogen atoms by sharing each one's single outer shell electron. This means that nitrogen has a full outer shell (there are now eight electrons in it- five from the nitrogen, three from the hydrogen atoms) and each of the hydrogen atoms also has a full outer shell with two electrons (because it is the first shell, and so only holds a maximum of two). 

Covalent bonds are shown using dot and cross diagrams, in which one element's electrons are symbolised by a dot, and the other's are shown by a cross, to easily see which elements originally belonged to which atoms. 

This is methane, in two different dot and cross diagrams (1 and 2) and its displayed formula (3). The lines in the displayed formula represent the covalent bonds between the atoms.

1)  2)3)

Some covalent substances are joined together in huge networks, called giant covalent structures, or macromolecules. For example, diamond is a form of carbon. Each carbon atom in diamond is covalently joined to four others. This is similar to silicon dioxide (sand). 

An ion is an electrically charged atom, caused either gaining electrons (making it negative) or losing electrons (making it positive). 

When a metal and a non-metal bond together, they don't share electrons like in covalent bonding. Instead, the atoms either donate electrons to another atoms or receive them. The non-metal atom gaining the electron will become negatively charged, and the metal atom that loses an electron will become positively charged. Because opposite charges attract to each other, the atoms will have a strong electrostatic force that bonds them together. 

Ionic bonds act in all directions and hold the ions in a regular pattern known as an ionic lattice. 

Groups in the periodic table contain elements all with the same number of electrons in their outer shell. All atoms want to become stable as quickly as possible, so group 1 elements react vigorously with group 7 elements because they stabilise each other's outer shells. This is particularly true if the metal (group 1) is heated prior to reacting it with the group 7 element.

When group 1 and 7 elements are reacted together, a metal halide is produced. For example:

• fluorine produces a metal fluoride
• chlorine produces a metal chloride
• iodine produces a metal iodide

The metal that the group 7 element is reacted with forms the 'metal' part (eg lithium + bromide makes lithium bromide). In these reactions, the metal always transfers an electron to the halogen. The metal is left with a positive ion, and the halogen is left as a negative ion. They attract each other. 

Alkali metals (group 1) can react with other non-metals as well. Sometimes more than one of an element will be needed, in order to balance out the charges (eg in sodium oxide, the sodium ion is Na+, and the oxygen ion is O2-, so two sodium ions are needed to balance it out)

Metals consist of giant structures of atoms held together in regular patterns by strong forces. 

Metallic bonding

In metal atoms, the electrons in the outer shell are not held very strongly by the nucleus. They often leave the nucleus and become delocalised, leaving behind a positively charged metal ion. The metal ions are held together by strong electrostatic forces because the delocalised electrons are free to move around between them. They are arranged in a regular pattern and form a giant metallic structure. 

The group number of the metal depends on how many delocalised electrons there will be in the metallc structure. For example, potassium is in group 1. When it bonds to other potassium atoms, each one loses the single electron in its outer shell. There is one delocalised electron for every positive potassium ion. Magnesium is in group 2. Its atoms have two electrons in the outer shell, so when these become delocalised, there are two electrons per positive magnesium ion. 

A simple molecule is made up of just a few atoms which are joined together by strong covalent bonds. Substances consisting of simple molecules have low melting and boiling points, particularly when compared to ionic, metal or giant covalent structures. 

Explaining melting/boiling points

In a simple molecule, the intermolecular forces between each atom are very weak. As it is not the bonds but these forces that have to be overcome when melting and boiling compounds, having weak intermolecular forces makes this very easy. 

Conducting electricity

Substances consisting of simple molecules do not conduct electricity. This is due to the fact that they do not have an overall electric charge. 

Ionic compounds:

• have high melting points 

A very large amount of energy is required to distrub the regular pattern and break the many strong bonds in ionic compounds, so they have very high melting and boiling points

• can conduct electricity when (in) liquid

As a solid, an ionic compound cannot conduct electricity because its ions are not free to move around. However, the ions are when it is in a liquid form or dissolved in a liquid, which allows them to carry the current through the compound. They are often dissolved rather than melted because, as mentioned above, a very high amount of energy is required to melt ionic compounds, so it is very expensive. It is not possible to do this with all ionic compounds, as they are not all soluble.

Diamond and graphite are both forms of carbon wih very different properties. This is due to their structure. 

Diamond 

In diamond, each carbon atom has strong covalent bonds joining it to four others to create a giant covalent structure. It is these strong bonds that give diamond its extreme hardness and high melting point (3550 degrees). As there are no charged particles free to move throughout the compound, diamond cannot conduct electricity 

Graphite 

In graphite, the carbon atoms are arranged very differently. Graphite has a layered structure, with each of its carbon atoms joining to three others by strong covalent bonds. However, the forces between the layers themselves are much weaker, allowing all the layers to slide over each other. This is what gives graphite its slippery property. Graphite still has a high melting point, so is a useful lubricant for machines. Unlike diamond, graphite can conduct electricity due to the delocalised electrons in it. For this reason graphite is often used to make the electrodes in electrolysis

Diamond and graphite are not the only forms of carbon. It can also exist as fullrenes, made up of hexagonal rings of carbon atoms. The most common fullrene is called buckminsterfullrene, and consists of molecules containing 60 carbon atoms joined together. They form a hollow sphere. The developing ways of placing drugs inside them and sending them to cancerous cells without damaging other cells on the way like chemo and radiotherapy do.

Fullrenes are examples of nanoparticles, which are tiny particles made up of a few hundred atoms. They measure between 1 and 100 nanometres across. Nanoscience is the study of nanoparticles. Nanoparticles have unique properties that make them very useful:

• Fullrene particles join together to form nanotubes. They have a huge surface area : volume, so make good catalysts. 
• Nanotubes are the strongest and stiffest materials ever discovered, so are used to reinforce things. 
• Nanoparticles are so small they can't reflect visible light, so they are used in sun cream to avoid making the person's skin looking white whilst still protecting it from UV light
• In the future they may be used in new types of computers, new coatings (self cleaning ovens and windows), sensors detecting substances in tiny amounts and light building materials

The structure of metals gives them their properties: the delocalised electrons allow them to conduct heat and electricity, and the metal ions packed in layers can slide over each other, making them malleable. 

Sometimes metals don't naturally have the properties we need. We make metals into alloys by creating a mixture consisting of the metal and one or more other elements. The properties of alloys are different to the properties of the elements that make it up. An example of an alloy is eighteen-carat gold. Pure gold is very soft, so eighteen-carat gold is made up of 75% gold and the remaining 25% is copper and silver. The copper and silver atoms distort the layers in the structure of pure gold, meaning they can't slide over each other as easily. This makes it harder. 

Shape memory alloys

Smart alloys, or shape memory alloys, remember their original shape, and if bent out of this shape, can return back to it when they are heated above a certain temperature. This principal is how braces work. The alloy making up the braces is constantly moving back into its original position, which is the shape that the teeth should be in. The wire exerts a constant force, shifting the teeth to where they should be. Shape memory alloys break if bent or twisted too much.

There are two types of poly(ethene): high-density poly(ethene) (HDPE) and low-density poly(ethene) (LDPE). LDPE is lighter, weaker, more transparant and more flexible than HDPE. These properties are caused by the polymer's starting conditions.

LDPE: 100-300 degrees, 1500-3000 atmospheres pressure, oxygen or peroxide initiator

HDPE: 300 degrees, 1 atmospheres pressure, aluminium-based metal oxide catalyst

Thermosetting and thermosoftening

Some plastics shape easily when heated. They can be recycled more easily, and are called thermosoftening. These types of plastics consist of individual polymer chains tangled up, each with weak forces of attraction to the others. This makes them easy to mould. 

Thermosetting plastics cannot be recycled because they do not melt when heated. The polymer chains in thermosetting plastics have cross-links between them, and these cross-links are strong intermolecular forces between the polymer chains.

Atoms consist of a nucleus and energy levels, or shells, surrounding the nucleus. The nucleus contains positive protons with a relative mass of 1 and neutral neutrons, also with a relative mass of 1. The shells contain electrons, which have a negative charge and a relative mass of almost zero. 

Elements in the periodic table are written with two numbers as well as the symbol in the box. The lower number (usually below the symbol) represents the number of protons in the atom. Since there have to be an equal number of protons and electrons in order to make the atom have a neutral charge overall, this is also the number of electrons. This number is called the atomic number. The other number is the larger of the two. This is the sum of both the protons and neutrons in the atoms. It is called the mass number. The number of neutrons in an atom is the difference between the atomic and mass number. 

Isotopes

Atoms of the same element can have different numbers of neutrons. Since neutrons are not charged, this doesn't change the charge of the atom overall. Atoms of the same element but with different numbers of neutrons are called isotopes. 

Individual atoms are very tiny. They have such small masses that it makes more sense to use their relative atomic mass, rather than their mass in kilograms.

Relative atomic mass (also written as Ar) is the average mass number of all of an element's isotopes. For example, 75% of chlorine atoms have a mass number of 35. 25% of cholrine atoms have a mass number of 37. This means that the relative atomic mass for chlorine is 35.5, because we are taking an average of all the isotopes that chlorine has. We compare all atoms' relative atomic mass to carbon's relative atomic mass, which has a relative atomic mass of 12. 

Relative formula mass, or Mr, is the mass of a substance. It is worked out by adding together the relative atomic masses for each of the atoms in the formula. For example, water contains two hydrogen atoms and one oxygen atom. The Ar of hydrogen is 1, and there are two hydrogen atoms, so we have an Ar of 2 for hydrogen in water. The Ar of oxygen is 16. We add together the Ar of hydrogen and oxygen, which is 2 + 16. Water has an Mr of 18. 

The relative formula mass of a substance in grams is one mole of that substance. One mole of carbon is 12g because it has a relative atomic mass of 12. One mole of water is 18g because it has a relative formula mass of 18. This applies to all elements and substances.

Paper chromatography can be used to separate the mixtures of coloured compounds.

In chromatography, there is a mobile phase (the part that moves) and a stationary phase (this remains still). The compound being separated is placed onto the stationary phase. In paper chromatography this is the paper. The mobile phase then carries the substance through this stationary phase, which in the case of paper chromatography is water. Each compound moves through the stationary phase at a different speed, so they become separated. 

Gas Chromatography

• Step 1: The sample being tested is heated into a mixture of vapours
• Step 2: A carrier gas- usually helium- mixes with the vapours. The gas is the mobile phase
• Step 3: The carrier gas takes the vapours through a column packed with solid material. This solid material is the stationary phase
  
• Step 4: The different vapours travel through the solid material at different speeds. They become separated, and reach the detector at the end at different times 

Once substances have been separated in gas chromatography, they can be identified via their retention times. A substance's retention time is how long it takes for the substance to reach the detector. The first substance to reach the detector will have the shortest retention time; the last will have the longest. 

The detector sends a signal to a recorder when a substance reaches it, and the recorder draws a peak on a chromatogram, which is a visual graph of the substances. Each peak represents one substance, so the amount of substances in something can be identified easily. The area underneath the peak shows how much of that substance there was overall. 

Mass Spectrometer

The detector is usually a mass spectrometer. This is a very accurate detector. The chromatogram that it produces shows a molecular ion peak on the far right, which is the substance's relative formula mass.

Gas chromatography, particularly using a mass spectrometer, is very quick, accurate and sensitive. 

Percentage of an element in a compound

• Step 1: Write down the formula of the compound
• Step 2: Calculate the relative atomic mass (Mr) using the relative atomic masses
• Step 3: Divide the Ar of the element required by the Mr
• Step 4: Multiply by 100 to get the answer as a percentage

Empirical Formula

• Step 1: Write the acronym MADR down the side 
• Step 2: Split into columns- one for each element in the compound
• Step 3: Write all the information for MADR in and do the calculation

Mass of reactants and products from equations

• Step 1: Calculate the Mr of the reactant/product
• Step 2: Know that no mass is lost in a chemical reaction so both must be the same
• Step 3: Work out what the other number is be by using ratio and the numbers in the question

In a chemical reaction no atoms can be lost or gained. However, one product from a reaction may be less than calculated due to several reasons:

• some of the product may have been lost when separated from reaction mixture (eg stuck in filter paper)
• reactants may act in different ways to how it was expected (eg burning something in air to oxidise it may cause it to react with nitrogen instead)
• the reaction may be reversible so the products reform the reactants before they are measured

yield- amount of product after a reaction | theoretical yield- expected mass calculated from the masses of the reactants | actual yield- the mass of the products after the reaction has happened

Percentage yield

• Step 1: Divide actual yield by theoretical yield
• Step 2: Multiply by 100 to get answer as a percentage

• In covalent bonding, atoms share pairs of electrons and form molecules or giant covalent structures of non-metals
• In ionic bonding atoms transfer electrons, creating oppositely charged ions in giant lattices made of metals and non-metals
• Dot and cross diagrams show the arrangement of electrons in bonding
• In metallic bonding, closely packed positive ions are surrounded by delocalised electrons
• Simple molecules have low melting and boiling points and do not conduct electricity
• Ionic compounds are solid at room temperature, have high melting and boiling points, are soluble and conduct electricity when liquid or dissolved
• Diamond, graphite anf fullrene are forms of carbon with different structures
• The properties of metals can be changed by forming alloys with other metals
• Thermosetting polymers have cross-links between chains. They cannot be melted
• The relative formula of a substance in grams is called one mole of that substance
• Gas chromatography and mass spectrometry are instrumental techniques used to separate and identify the substances in a compound
• An empirical formula shows the simplest ratio of elements in a compound
• Percentage yields may be low due to product lost, reacting differently or reforming reactants

Rate of reaction- how quickly a reaction occurs 

Factors affecting rates of reaction

• Temperature
• Concentration
• Pressure
• Surface area
• Presence of catalysts

Calculating rate of reaction

Option 1: amount of reactant used / time

Option 2: amount of product formed / time

Temperature affects the rate at which all reactions occur. This is because reactions happen when two particles collide, and in order to collide with each other, they need energy to move around. They also need a minimum amount of energy for the collision to be successful, called the activation energy. This energy that is needed comes from heat. 

• At low temperatures the particles move slowly so are less likely to collide, and when they do collide, they are less likely to transfer enough energy for a reaction
• At high temperatures, the particles move faster. They are more likely to collide with each other, and when they do collide, they are more likely to have enough energy for a successful reaction

The rate of reaction increases as the concentration increases. This is because there are more particles in a smaller space, so they are more likely to collide with each other and successfully react. 

Similarly, a higher pressure will increase the rate of a reaction. This is for the same reasons as increasing concentration, as seen above. 

Increasing the concentration or pressure of reactants increases the frequency of collisions because they are closer together. This therefore increases the rate of reaction, because in order for a reaction to occur, two particles must collide with enough activation energy.

A reaction in which one of the products is in powder form will take place much faster than a reaction in which one of the products is in the form of a solid block or lump. Powder has a larger surface area than a lump of a reactant, which increases the frequency of collisions because there is a higher chance that a particle from one reactant will collide with a particle from another. 

A catalyst is a substance that increases the rate of a reaction. It is not a reactant, so does not get used up in the reaction itself. Each catalyst is specific to particular reactions, so not all catalysts will wokr for all reactions. 

Catalysts are very useful in industry. Without them, many reactions would be too slow to be profitable. They greatly reduce the costs of producing some products. 

Exothermic reactions- a reaction in which energy is transferred to surroundings.

All combustion reactions are exothermic as they transfer heat and light energy for their surroundings.

Neutralisation is also an exothermic reaction, as heat energy is transferred so the overall temperature of the substances decreases following a neutralisation reaction. 

Another example of an exothermic reaction is oxidation (however, not all oxidation reactions are exothermic). This is seen when burning jelly babies, as they release sound energy. Displacement reactions are another example of exothermic reaction. 

Uses of exothermic reactions

• Handwarmers- in some handwarmers, oxidation of iron occurs very quickly, releasing heat energy
• Celf-heating coffee- there are two compartments in self-heating coffee cans; one contains the coffee, the other contains reactants that cause an exothermic reaction and release heat

Endothermic reaction- a reaction that takes in energy from its surroundings

Examples of endothermic reactions:

• sherbert fizzing in the mouth
• ammonium nitrate dissolving in water 

Uses of endothermic reactions:

• sports injury packs

If a reversible reaction is endothermic in one direction, it will be exothermic in the other (and vice versa). The same amount of energy is transferred in each direction, it just changes if it is going in to or out of the surroundings. 

The pH scale measures the acidity or alkalinity of any substance

• A pH of 7 means the solution is neutral
• A pH of less than 7 means it is acidic
• A pH of more than 7 means it is a base

What makes something acidic?

Hydrogen ions make a solution acidic if they are dissolved in water. 

What is a base?

A base is any substance that can react with an acid to neutralise it, containing metal oxide, hydroxide or carbonate

What makes something alkali?

Hydroxide ions made something alkaline. Alkalis are soluble bases. For example, copper oxide is a base, but is not soluble, so is not an alkali.

Salt- the remaining ions from a neutralisation reaction have opposite charges and attract each other. This forms a solid precipitate, which crystallises when dried out, forming a salt. A salt contains metal ions.

• Hydrochloric acid makes chloride salts (eg sodium chloride)
• Sulfuric acid makes sulfate salts (eg copper sulfate)
• Nitric acid makes nitrate salts (eg zinc nitrate)

The metal ions in the salt can come from three sources: metals, insoluble bases and alkalis

Copper sulfate is a soluble salt. It is made by reacting copper oxide and sulfuric acid, which produces copper sulfate and water. The method for making it is as follows:

• 1) Add copper oxide to dilute sulfuric acid until it no longer reacts
• 2) Filter to remove the unreacted copper oxide 
• 3) Heat the solution until about half the water in it has evaporated
• 4) Leave to stand. It will crystallise, forming the salt copper sulfate

Soluble salts can be formed in three ways

• adding acid to a metal oxide to form a salt and water
• adding acid to a metal hydroxide to form a salt and water
• adding acid to a reactive metal to form a salt and hydrogen 

The method for making a soluble salt from a metal oxide is on card C2.26. The method differs when using metal hydroxide:

• 1) Dilute acid is added to universal indicator
• 2) The metal hydroxide is added to this mixture to neutralise it 
• 3) Charcoal powder is then added to remove the colour
• 4) Heat over a water bath to leave you with the salt 

Precipitate- suspension of small solid particles in a liquid/solution that makes it look cloudy

A precipitation reaction is one in which a precipitate is formed as one of the products. The precipitation formed in a precipitation reaction is an insoluble salt, because it doesn't dissolve in the solution that it is suspended in.

Precipitation reactions are useful for removing unwanted ions from solutions. The reaction has to be between ionic compounds, which means it can be summarised using an ionic equation, which shows the parts of the reaction that have changed. For example:

lead nitrate + potassium iodide -> lead iodide + potassium nitrate

The potassium and nitrate have not changed at all, so the ionic equation shows only the changing part:

Pb2+ (aq) + 2I (aq) -> PbI2 (s) 

There has to be two iodine atoms because this balances out the charges of the lead atom. 

Electrolysis is the breaking down of a substance using electricity, used usually when an element is too reative to naturally exist on its own, yet we want it on its own.

It only works when using ionic compounds, because they are able to conduct electricity when dissolved or in molten form. The solution of the substance being broken down is called the electrolyte. 

There are two electrodes used in electrolysis. These are stick-like structures made of a non-reactive element that are placed into the electrolyte. They have a current running through them. One is negatively charged (called the cathode) and the other is negatively charged (called the anode). 

• At the cathode- positively charged ions gain electrons (called reduction)
• At the anode- negatively charged ions lose electrons (called oxidation)
  

Half equations show what is taking place during electrolysis. The electrons are represented by e- and just like in a normal equation, the atoms must be balanced on each side. However, on the right hand side, the charges must also be balanced, because the element formed at the electrode never has a charge.  

Electrolysis can be used to coat an object with a thin layer of a more expensive metal, either for protection or for aesthetic. This is called electroplating. 

In electroplating, one of the electrodes is replaced with the object that needs to be covered with the expensive metal. The metal that is going to cover the object is in the electrolyte, and whether this metal forms a positive or negative ion will determine which electrode the object must replace. 

Electrolysis then occurs as it normally would, but with the desired metal forming over the object instead of at the eletrode. 

Extracting Aluminium

Aluminium is relatively high in the reactivity series. It is too reactive to naturally exist on its own, so instead is found in the earth's crust as aluminium oxide, in a bauxite ore. Because it is above carbon in the reactivity series, we cannot react it with carbon to remove the oxygen, and must instead use electrolysis. 

• Step 1) Remove impurities from the ore to get pure aluminium oxide
• Step 2) Dissolve the aluminium oxide in cryolite (this lowers the melting point, which for aluminium is much higher than when it is in cryolite)
• Step 3) Pour the aluminium and cryolite mixture into the electrolysis cell 
• Step 4) Pass a current through the electrodes
  • Aluminium ions move to the negative electrode and form aluminium in liquid form
  • Oxide ions move to the positive electrode and react with the carbon anode to form carbon dioxide
• The half equation at the negative electrode is Al3+ + 3e- -> Al
• The half equation at the positive electrode is 2O2- - 4e- -> O2

When an ionic compound has been melted for electrolysis, it is easy to tell what will be formed at each electrode because there are only two options. However, if the compound has been dissolved, it is more difficult because the water it is dissolved in takes part in the reaction as well. The following rules will always apply:

At the negative electrode

• The metal is produced if it is low in the reactivity series
• Hydrogen gas (from the water) is formed if the metal is above copper in the reactivity series

At the positive electrode 

• A halide ion, if present, will form a halogen 
• If carbonate, sulfate or nitrate ions are present, oxygen is produced

• Rate of reaction is the change in the amount of product per second
• Reactions are faster at higher temperatures, pressures, concentrations and surface areas
• Activation energy is the minimum amount of energy required for two particles to react
• Catalysts increase the rate of reaction but are unchanged at the end of it 
• Some reactions (eg combustion) are exothermic, so release energy to their surroundings
• Some reactions (eg thermal decomposition) are endothermic, and take in energy from surroundings
• Reversible reactions are endothermic in one direction and exothermic in the other
• Acids have a pH of less than 7 and are neutralised by bases
• Metal oxides or hydroxides are bases
• Alkalis are soluble bases. They have a pH of more than 7
• Acids contain H+ ions and alkalis contain OH- ions. They react to form water
• Salts are formed when acids react with bases, alkalis or reactive metals
• Soluble salts are formed by neutralisation followed by crystallisation 
• Insoluble salts are made by precipitation reactions, which are often used to treat water
• In electrolysis, an electric current is passed through an ionic compound, causing it to break down into elements at the positve and negative electrodes