Iron

Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.

Blast furnace in a modern steel works

Carbon is more reactive than iron, so it can push out or displace the iron from iron oxide. Here are the equations for the reaction:

iron oxide + carbon → iron + carbon dioxide

2Fe₂O₃ + 3C → 4Fe + 3CO₂

In this reaction, the iron oxide is reduced to iron, and the carbon is oxidised to carbon dioxide.

In the blast furnace, it is so hot that carbon monoxide will also reduce iron oxide:

iron oxide + carbon monoxide → iron + carbon dioxide

Fe₂O₃ + 3CO → 2Fe + 3CO₂

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Crystallising salt solutions

You may be asked to describe how to make a soluble salt.

If the base dissolves in water, you need to add just enough acid to make a neutral solution. Check a small sample with universal indicator paper. If ammonia solution is used, you can add a little more than needed to get a neutral solution.

Warm the salt solution to evaporate the water. You get larger crystals if you evaporate the water slowly.

Copper oxide, and other transition metal oxides or hydroxides, do not dissolve in water. If the base does not dissolve in water, you need an extra step. You add the base to the acid until no more will dissolve and you have some base left over (called an excess). You filter the mixture to remove the excess base, then evaporate the water in the filtrate to leave the salt behind.

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Naming salts

The name of the salt produced in a neutralisation reaction can be predicted. The first part of the name is 'ammonium' if the base used is ammonia. Otherwise, it is the name of the metal in the base. The second part of the name comes from theacid used:

• Chloride (if hydrochloric acid is used)
• Nitrate (if nitric acid is used)
• Sulfate (if sulfuric acid is used)

The table shows some examples:

 

Acid	+	Base	→	Salt + Water
Hydrochloric acid	+	Copper oxide	→	Copper chloride + water
Sulfuric acid	+	Sodium hydroxide	→	Sodium sulfate + water
Nitric acid	+	Calcium hydroxide	→	Calcium nitrate + water

Ammonium salts

Many artificial fertilisers are ammonium salts, made by the reaction of an acid with ammonia solution. For example:

 

Acid	Alkali	Fertiliser
Nitric acid	Ammonia solution	Ammonium nitrate
Phosphoric acid	Ammonia solution	Ammonium phosphate
Sulfuric acid	Ammonia solution	Ammonium sulfate

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Making soluble salts

You need to be able to describe the reactions of acids with bases and metals. You should be able to work out the particular salt formed in the reaction.

Acids and bases

When acids react with bases, a salt and water are made:

• acid + metal oxide → salt + water
• acid + metal hydroxide → salt + water

Remember that most bases do not dissolve in water. But if a base can dissolve in water, it is also called an alkali.

Reactive metals

Acids will react with reactive metals, such as magnesium and zinc, to make a salt and hydrogen:

• acid + metal → salt + hydrogen

The hydrogen causes bubbling during the reaction, and can be detected using a lighted splint.

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Neutralisation reactions

Ions are charged particles which are formed when atoms, or groups of atoms, lose or gain electrons. For the examination, you need to know which ions are produced by acids, and which are produced by alkalis. You will also need to know the ionic equation for neutralisation.

State symbols

State symbols are used in symbol equations:

• (s) means solid
• (l) means liquid (not the same as dissolved in water - see below)
• (g) means gas
• (aq) means aqueous (dissolved in water)

Acids

When acids dissolve in water they produce aqueous hydrogen ions, H⁺(aq). For example, looking at hydrochloric acid:

HCl(aq) → H⁺(aq) + Cl^–(aq)

Alkalis

When alkalis dissolve in water they produce aqueous hydroxide ions, OH^–(aq). For example, looking at sodium hydroxide:

NaOH(aq) → Na⁺(aq) + OH^–(aq)

Ammonia is slightly different. This is the equation for ammonia in solution:

NH₃(aq) + H₂O(l) → NH₄⁺(aq) + OH^–(aq)

Be careful to write OH^– and not Oh^– or oh^–.

Neutralisation reaction

When the H⁺(aq) ions from an acid react with the OH^–(aq) ions from an alkali, a neutralisation reaction occurs to form water. This is the equation for the reaction:

H⁺(aq) + OH^–(aq) → H²O(l)

For example, hydrochloric acid and sodium hydroxide solution react together to form water and sodium chloride solution. The acid contains H⁺ ions and Cl^– ions, and the alkali contains Na⁺ ions and OH^– ions. The H⁺ ions and OH^– ions produce the water, and the Na⁺ ions and Cl^– ions produce the sodium chloride, NaCl(aq).

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Acids, bases and salts

Acids have a pH of less than 7. Bases have a pH of more than 7. When bases are dissolved in water, they are known as alkalis. Salts are made when an acid reacts with a base, carbonate or metal. The name of the salt formed depends on the metal in the base and the acid used. For example, salts made using hydrochloric acid are called chlorides.

Acids and bases

Diagram of pH scale and universal indicator colours

Acids

Substances with a pH of less than 7 are acids. The more strongly acidic the solution, the lower its pH number. Acidic solutions turn blue litmus paper red. They turn universal indicator paper red if they are strongly acidic, and orange or yellow if they are weakly acidic.

Bases

Substances that can react with acids and neutralise them to make a salt and water are called bases. They are usually metal oxides or metal hydroxides. For example, copper oxide and sodium hydroxide are bases.

Alkalis

Bases that dissolve in water are called alkalis. Copper oxide is not an alkali because it does not dissolve in water. Sodium hydroxide is an alkali because it does dissolve in water.

Alkaline solutions have a pH of more than 7. The stronger the alkali, the higher the pH number. Alkalis turn red litmus paper blue. They turn universal indicator paper dark blue or purple if they are strongly alkaline, and blue-green if they are weakly alkaline.

Neutral solutions

Neutral solutions have a pH of 7. They do not change the colour of litmus paper, but they turn universal indicator paper green. Water is neutral.

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Reversible reactions

In reversible reactions, the reaction in one direction will be exothermic and the reaction in the other direction will be endothermic.

The decomposition of ammonium chloride is a reversible reaction:

ammonium chlorideammonia + hydrogen chloride

Ammonium chloride decomposes when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens.

Copper sulfate

The reaction between anhydrous copper sulfate and water is reversible:

hydrated copper sulfate (blue)anhydrous copper sulfate (white) + water

Water is driven off from hydrated copper sulfate when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens. This is easily observed. When water is added to anhydrous copper sulfate, enough heat is released to make the water bubble and boil.

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Endothermic reactions

These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer.

Some examples of endothermic reactions are:

• Electrolysis
• The reaction between ethanoic acid and sodium carbonate
• The thermal decomposition of calcium carbonate in a blast furnace

Endothermic reactions can be used for everyday purposes. For example, certain sports injury cold packs use endothermic reactions.

The animation shows an exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid.

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Energy changes and reversible reactions

Exothermic reactions transfer energy to the surroundings. Endothermic reactions take in energy from the surroundings.

Reversible reactions are where the products can react to remake the original reactants. If the forward reaction is exothermic, the reverse reaction is endothermic.

Exothermic reactions

When a chemical reaction occurs, energy is transferred to or from the surroundings - and there is often a temperature change.

Exothermic reactions transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:

• Combustion (burning)
• Many oxidation reactions, for example rusting
• Neutralisation reactions between acids and alkalis

When a flame burns it transfers heat to its surroundings.

Exothermic reactions can be used for everyday purposes. For example, hand warmers and self-heating cans for drinks (such as coffee) use exothermic reactions.

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Collisions and reactions

You will be expected to explain, in terms of particles and their collisions, why changing the conditions of a reaction changes its rate.

Collisions

For a chemical reaction to occur, the reactant particles must collide. Collisions with too little energy do not produce a reaction.

The collision must have enough energy for the particles to react. The minimum energy needed for particles to react is called the activation energy.

Changing concentration or pressure

If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:

• There are more reactant particles in the same volume
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing particle size

If a solid reactant is broken into small pieces or ground into a powder:

• Its surface area is increased
• More particles are exposed to the other reactant
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing the temperature

If the temperature is increased:

• The reactant particles move more quickly
• More particles have the activation energy or greater
• The particles collide more often, and more of the collisions result in a reaction
• The rate of reaction increases

Using a catalyst

Catalysts increase the rate of reaction without being used up. They do this by lowering the activation energy needed. With a catalyst, more collisions result in a reaction, so the rate of reaction increases. Different reactions need different catalysts.

Catalysts are important in industry because they reduce costs.

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Factors affecting the rate

You will be expected to remember the factors that affect the rate of reactions, and to plot or interpret graphs from rate experiments.

How to increase the rate of a reaction

The rate of a reaction increases if:

• The temperature is increased
• The concentration of a dissolved reactant is increased
• The pressure of a reacting gas is increased
• Solid reactants are broken into smaller pieces
• A catalyst is used

Rate of reaction and changing conditions

The graph above summarises the differences in the rate of reaction at different temperatures, concentrations and size of pieces. The steeper the line, the greater the rate of reaction. Reactions are usually fastest at the beginning, when the concentration of reactants is greatest. When the line becomes horizontal, the reaction has stopped.

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Rates of reaction

The rate of a reaction can be measured by the rate at which a reactant is used up, or the rate at which a product is formed.

The temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts, are all factors which affect the rate of a reaction.

Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.

Measuring rates

Different reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. For example, the chemical weathering of rocks is a very slow reaction: it has a low rate of reaction. Explosions are very fast reactions: they have a high rate of reaction.

Reactants and products

There are two ways to measure the rate of a reaction:

1. Measure the rate at which a reactant is used up
2. Measure the rate at which a product is formed

The method chosen depends on the reaction being studied. Sometimes it is easier to measure the change in the amount of a reactant that has been used up; sometimes it is easier to measure the change in the amount of product that has been produced.

Things to measure

The measurement itself depends on the nature of the reactant or product:

• The mass of a substance - solid, liquid or gas - is measured with a balance
• The volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette

It is usual to record the mass or total volume at regular intervals and plot a graph. The readings go on the vertical axis, and the time goes on the horizontal axis.

For example, if 24 cm³ of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12 cm³ hydrogen / min.

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Empirical formula - Higher tier

You can use information about reacting masses to calculate the formula of a compound. Here is an example:

Question

• Suppose 3.2 g of sulfur reacts with oxygen to produce 6.4 g of sulfur oxide. What is the formula of the oxide?
•  
• Use the fact that the A_r of sulfur is 32 and the A_r of oxygen is 16

Here is the calculation again in tabular form to help you remember the steps:

Steps to calculation the formula of a compound

Step	Action	S	O
1	Find masses	3.2	3.2
2	Look up given Ar values	32	16
3	Divide masses by Ar	0.1	0.2
4	Find the ratior	1	2

Result: the formula for the oxide = SO₂

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Reacting masses calculations - Higher tier

If you have a balanced equation for a reaction, you can calculate the masses of reactants and products.

Sample question

Look at this equation: CaCO₃(s)    →    CaO(s) + CO₂(g)

If we have 50g of CaCO₃, how much CaO can we make?

First, work out the M_r values for the two compounds:

M_r of CaCO₃ is 40 + 12 + 16 + 16 + 16 = 100

M_r of CaO is 40 + 16 = 56

This means that 100 g of CaCO₃ would yield 56 g of CaO in this reaction. In the question we are told we have only half of that amount of CaCO₃, 50 g. So we will get half the amount of CaO, 28 g.

So the mass of CaO we can make = 28 g

Notice in this that 22 g of CO₂ would also be produced, as 50 - 28 = 22

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Reversible reactions

Many reactions, such as burning fuel, are irreversible - they go to completion and cannot be reversed easily. Reversible reactions are different. In a reversible reaction, the products can react to produce the original 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:

ammonium chlorideammonia + hydrogen chloride

The equation shows that ammonium chloride (a white solid) can break down to form ammonia and hydrogen chloride. It also shows that ammonia and hydrogen chloride (colourless gases) can react to form ammonium chloride again.

The animation below shows a reversible reaction involving white anhydrous copper(II) sulfate and blue hydrated copper(II) sulfate, the equation for which is:

anhydrous copper(II) sulfate + water hydrated copper(II) sulfate

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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Percentage yield

The principle of conservation of mass lets you calculate the theoretical mass of product expected in a chemical reaction. However, it is not always possible in practice to get the entire calculated amount of product. This is because:

• Reversible reactions may not go to completion
• Some product may be lost when it is removed from the reaction mixture
• Some of the reactants may react in an unexpected way

Yield

The yield of a reaction is the mass of product obtained:

• The theoretical yield is the maximum theoretical mass of product in a reaction (calculated using the idea of conservation of mass)
• The actual yield is the mass of product you get when you actually do the reaction

The percentage yield is the ratio of actual mass of products obtained compared with the maximum theoretical mass.

Percentage yield - Higher tier

The percentage yield of a reaction is calculated using this equation:

percentage yield = (actual mass of product) ÷ (theoretical mass of product) × 100

For example, the maximum theoretical mass of product in a certain reaction is 20 g, but only 15 g is actually obtained.

Percentage yield = 15 ÷ 20 × 100 = 75%

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Conservation of mass

Mass is never lost or gained in chemical reactions. We say that mass isconserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning.

This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example:

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Quantitative chemistry

You should be able to calculate the masses of reactants and products from balanced equations, and the percentage composition by mass of an element in a compound.

Higher tier students should also be able to calculate the percentage yield of a reaction, and the empirical formula of a compound from information about reacting masses.

Percentage composition

Percentage composition is just a way to describe what proportions of the different elements there are in a compound.

If you have the formula of a compound, you should be able to work out thepercentage by mass of an element in it.

Example

The formula for sodium hydroxide is NaOH. It contains three different elements:Na, O and H. But the percentage by mass of each element is not simply 33.3 per cent, because each element has a different relative atomic mass. You need to use the A_r values to work out the percentages. Here is how to do it:

Question

What is the percentage by mass of oxygen (O) in sodium hydroxide (NaOH)?

• First, work out the relative formula mass of the compound, using the A_r values for each element. In the case of sodium hydroxide, these are Na = 23, O = 16, H = 1. (You will be given these numbers in the exam.)
•  
• Next, divide the A_r of oxygen by the M_r of NaOH, and multiply by 100 to get a percentage.

Remember, if there is more than one atom of the element in the compound, you need to multiply your answer by the number of atoms. If your answer is more than 100 per cent, you have gone wrong!

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