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PiXL KnowIT!

GCSE Chemistry

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AQA Topic – Chemical changes

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Overview � Chemical changes

Reactivity of metals

• Metal oxides
• The reactivity series
• Extraction of metals by reduction
• Oxidation and reduction in terms of electrons (HT)

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Reactions of acids

• Metals and acids
• Strong and weak acids (HT)
• pH scale and Neutralisation
• Salts
• Titration (chemistry only)

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Electrolysis

• Electrolysis of molten ionic compounds
• Electrolysis of an aqueous solution
• Using electrolysis to extract metals
• Representation at electrodes as half equations (HT)

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LearnIT!

KnowIT!

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Reactivity of metals

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• Metals oxides
• The reactivity series
• Extracting metals by

reduction

• Oxidation and reduction in terms of electrons (HT)

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Metal Oxides

A metal compound within a rock is an ore. Ores are mined and then purified.

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Whether it is worth extracting a particular metal depends on:

• How easy it is to extract it from its ore
• How much metal the ore contains
• The changing demands for a particular metal

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Most metals in ores are chemically bonded to other elements in compounds. Many of these metals have been oxidised (have oxygen added) by oxygen in the air to form their oxides.

Iron + oxygen 🡪 iron (III) oxide

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4Fe(s) + 3O₂(g) 🡪 2Fe₂O₃(s)

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To extract metals from their oxides, the metal oxides must be reduced (have oxygen removed).

Reactivity series

Metals can be arranged in order of reactivity in a reactivity series.

Reactivity series

Metals can be arranged in order of reactivity in a reactivity series.

Extracting metals

The reactivity of a metal determines the method of extraction.

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Gold and silver do not need to be extracted.

They occur native (naturally).

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Oxidation and reduction in terms of electrons (HT)

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A more reactive metal can displace a less reactive metal

from its compound in displacement reactions.

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Iron + copper(II) sulfate 🡪 iron sulfate + copper

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Fe(s) + CuSO₄(aq) 🡪 FeSO₄(aq) + Cu(s)

Higher:

An ionic equation shows only the atoms and ions that change in a reaction:

Fe(s) + Cu²⁺(aq) 🡪 Fe²⁺(aq) + Cu(s)

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Half equations show what happens to each reactant:

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Fe 🡪 Fe ²⁺ + 2e ^–

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Cu ²⁺ + 2e ^– 🡪 Cu

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The iron atoms are oxidised (lose 2 electrons) to form ions.

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The 2 electrons from the iron are gained (reduction) by copper ions as they become atoms.

Higher:

OILRIG

Oxidation Is Loss of electrons

Reduction Is Gain of electrons

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When reactions involve oxidation and reduction, they are known as redox reactions

QuestionIT!

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Extracting metals

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• The reactivity series
• Metals oxides
• Extracting metals by

reduction

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Extracting metals – QuestionIT

1. What is an ore?

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• What is produced when metals react with oxygen?

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• What is this process called and why?

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• What is reduction in terms of oxygen?

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• What type of ions do metals produce?

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• Which is more reactive potassium or iron?

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• Which two non-metals can be included in the reactivity series?

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Extracting metals – QuestionIT

8. Why is gold found as an element in the Earth?

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• How are metals, less reactive than carbon, extracted from their ores?

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• HT: Describe oxidation in terms of electrons.

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• HT: Describe reduction in terms of electrons.

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• Write the word equation for the reaction between lithium and water.

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AnswerIT!

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Extracting metals

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• The reactivity series
• Metals oxides
• Extracting metals by

reduction

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Extracting metals – QuestionIT

1. What is an ore?

Metal compound in a rock.

• What is produced when metals react with oxygen?

Metal oxide.

• What is this process called and why?

Oxidation, gain of oxygen.

• What is reduction in terms of oxygen?

Loss of oxygen.

• What type of ions do metals produce?

Positive.

• Which is more reactive potassium or iron?

Potassium.

• Which two non-metals can be included in the reactivity series?

Carbon and hydrogen.

Extracting metals – QuestionIT

8. Why is gold found as an element in the Earth?

Unreactive metal.

• How are metals, less reactive than carbon, extracted from their ores?

Reduction with carbon.

• HT: Describe oxidation in terms of electrons.

OIL – loss of electrons

• HT: Describe reduction in terms of electrons.

RIG – gain of electrons

• Write the word equation for the reaction between lithium and water.

lithium + water → lithium hydroxide + hydrogen

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Extracting metals – QuestionIT

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8. Write the balanced symbol equation for the reaction between lithium and water.

2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)

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• HT: Zinc can be extracted from zinc oxide by heating it with carbon in the blast furnace. Carbon monoxide is also produced. Which reactant is:

a) Oxidised? Carbon

b) Reduced? Zinc oxide

LearnIT!

KnowIT!

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Reactions of acids

PART 1

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• Acids and metals
• Neutralisation of acids
• Soluble salts

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Reactions of acids - PART 1

Acids react with some metals to produce salts and hydrogen.

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Metal + acid 🡪 salt + hydrogen

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Reactions between metals and acids only occur if the metal is more reactive than the hydrogen in the acid. If the metal is too reactive, the reaction with acid is violent.

The salt that is made depends on the metal and acid used.

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Salts made when metals react nitric acid are called nitrates.

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Zinc + Nitric acid 🡪 Zinc Nitrate + Hydrogen

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Salts made when metals react with sulfuric acids are called sulfates.

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Iron + Sulfuric Acid 🡪 Iron Sulfate + Hydrogen

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Salts made when metals react with hydrochloric acid are called chlorides.

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Magnesium + Hydrochloric acid 🡪 Magnesium Chloride + Hydrogen

Reactions of acids - PART 1 - HIGHER

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In the reaction between magnesium and hydrochloric acid, the hydrogen ions are displaced from the solution by magnesium as the magnesium is more reactive than hydrogen.

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The following ionic equation occurs:

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Mg(s) + 2H⁺(aq) 🡪 Mg²⁺(aq) + H₂(g)

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The chloride ions are not included as they do not change in the reaction. These are known as spectator ions.

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The reaction can be further represented by half equations, showing that the reaction between a metal and acid is a redox reaction.

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Mg 🡪 Mg²⁺ + 2e ^–

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The magnesium atoms lose two electrons, they have been oxidised.

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2H⁺ + 2e ^- 🡪 H₂

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The hydrogen ions have gained electrons, they have been reduced.

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Reactions of acids - PART 1

Acids are neutralised by alkalis (eg: soluble metal hydroxides) and bases (eg: insoluble metal hydroxides and metal oxides) to produce salts and water and by metal carbonates to produce salts, water and carbon dioxide.

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The salt name depends on the acid used and the positive ions in the alkali, base or carbonate.

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Salts are made of positive metal ions

(or ammonia ions - NH₄⁺) and a negative

ion from the acid. Like all ionic

compounds, salts have no overall charge,

so once you know the charges on the ions,

you can work out the formula.

Example: magnesium sulfate is MgSO₄

Making Soluble Salts from acids and alkalis

Salts can be made by reacting an acid with an alkali.

Acid + Alkali 🡪 Salt + Water

Making Soluble Salts from acids and bases

Salts can be made by reacting an acid with a insoluble base.

Acid + Bases 🡪 Salt + Water

Making Soluble Salts from acids and metal carbonates

Salts can be made by reacting an acid with a metal carbonate.

Acid + Metal carbonate 🡪 Salt + Water + Carbon dioxide

Reactions of acids - PART 1

Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates. The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt. Salt solutions can be crystallised to produce solid salts. You will complete this as a required practical.

1. Measure the required volume of acid with a measuring cylinder and add the weighed solid (insoluble metal, oxide, hydroxide or carbonate) in small portions with stirring.

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• Safety goggles required - the mixture may be heated to speed up the reaction. When no more of the solid dissolves it means ALL the acid is neutralised and there should be a little excess solid. You should see a residue of the solid (oxide, hydroxide, carbonate) left at the bottom of the beaker.

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• Filter the solution to remove the excess solid metal/oxide/carbonate, into an evaporating dish.  On filtration, only a solution of the salt is left.

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• Then hot concentrated solution is left to cool and crystallise. After crystallisation, you collect and dry the crystals with a filter paper.

If the solution is heated, the solvent will evaporate faster. Heating a

solution until all the solvent has evaporated is known as heating to dryness.

QuestionIT!

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Reactions of acids

PART 1

​

• Acids and metals
• Neutralisation of acids
• Soluble salts

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Reactions of acids – QuestionIT

1. What is produced when acids react with metals?

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• HT: What is a redox reaction?

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• What is produced when an acid reacts with a carbonate?

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• What salt is produced by the following acids?
1. Hydrochloric acid
2. Sulfuric acid
3. Nitric acid

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• How are soluble salts made from acids and insoluble substances?

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• Name the process of producing solid salts from salt solution.

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Reactions of acids – QuestionIT

7. Complete the following equations:

a) magnesium + sulfuric acid 🡪

b) sodium hydroxide + hydrochloric acid 🡪

c) lithium carbonate + nitric acid 🡪

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8. Write a balanced symbol equation, with state symbols, for the reaction between zinc and hydrochloric acid.

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9. Higher:

Write an ionic equation, with state symbols, to show magnesium

reacting with hydrochloric acid.

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AnswerIT!

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Reactions of acids

PART 1

​

• Acids and metals
• Neutralisation of acids
• Soluble salts

​

Reactions of acids – QuestionIT

1. What is produced when acids react with metals?

Salt + water.

• HT: What is a redox reaction?

Oxidation and reduction.

• What is produced when an acid reacts with a carbonate?

Salt + water + carbon dioxide.

• What salt is produced by the following acids?
1. Hydrochloric acid Chloride
2. Sulfuric acid Sulfate
3. Nitric acid Nitrate
• How are soluble salts made from acids and insoluble substances?

Solid added to acid until no more reacts; excess solid filtered off.

• Name the process of producing solid salts from salt solution.

Crystallisation.

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Reactions of acids – QuestionIT

7. Complete the following equations:

a) magnesium + sulfuric acid 🡪 magnesium sulfate + hydrogen

b) sodium hydroxide + hydrochloric acid 🡪 sodium chloride + water

c) lithium carbonate + nitric acid 🡪 lithium nitrate + water +

carbon dioxide

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8. Write a balanced symbol equation, with state symbols, for the reaction between zinc and hydrochloric acid.

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

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9. Higher:

Write an ionic equation, with state symbols, to show magnesium

reacting with hydrochloric acid.

Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)

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LearnIT!

KnowIT!

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Reactions of acids

PART 2

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• pH and neutralisation
• Titration (chemistry only)
• Strong and weak acids (HT)

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Reactions of acids - PART 2

Indicators are substances which change colour when you add them to acids and alkali.

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Litmus goes red in acid and blue in alkali.

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Universal indicator, made from many dyes is used to tell you pH. The scale runs from 0 (most acidic) to 14 (most alkaline). Aqueous solutions of acids have a pH value less than 7, and for alkalis greater than 7 and anything in the middle is neutral (pH 7). You can use a pH meter to record the change of a pH over time.

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Acids produce hydrogen ions (H⁺) in aqueous solutions and alkalis produce hydroxide ions (OH^–).

In neutralisation reactions between an acid and alkali, hydrogen ions react with hydroxide ions to produce water.

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Neutralisation symbol equation:

H⁺(aq) + OH^–(aq) ➞ H₂O(l)

Reactions of acids - PART 2 - CHEMISTRY ONLY

The volumes of acid and alkali solutions that react with each other

can be measured by titration using a suitable indictor.

3. Stop adding the acid when the end-point is reached (the appropriate colour change in the indicator happens). Note the final volume reading. Repeat steps 1 to 3 until you get consistent readings.

2. Fill the burette with acid and note the starting volume. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.

1. Use the pipette to add 25 cm³ of alkali to a conical flask and add a few drops of indicator.

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Reactions of acids – PART 2 – CHEMISTRY ONLY�Higher

The concentration of a solution is the amount of solute per volume of solution.

Chemists measure concentration in moles per cubic decimetre (mol/dm³).

​

Where:

n is the number of moles (mol) or the mass of the solute (g)

c is the concentration (mol/dm³or g/dm³)

v is the volume (dm³)

What is the concentration of a solution that has 35.0g of solute in 0.5dm³of solution?

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35/0.5 = 70 g/dm³

Example 1:

Example 2:

How many moles of magnesium nitrate are there in 0.50 dm³ of a 2 mol/dm³ solution?

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2 x 0.50 = 1 mol

Reactions of acids – PART 2 - CHEMISTRY ONLY�Higher

If the volumes of two solutions that react completely are known and the concentrations of one solution is known, the concentration of the other solution can be calculated.

2NaOH(aq) + H₂SO₄(aq)→ Na₂S0₄(aq) + 2H₂O(l)

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It takes 12.20cm³ of sulfuric acid to neutralise 24.00cm³ of sodium hydroxide solution, which has a concentration of 0.50mol/dm³.

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Calculate the concentration of the sulfuric acid in g/dm³

0.5 mol/dm³ x (24/1000) dm³ = 0.012 mol of NaOH

Example:

The equation shows that 2 mol of NaOH reacts with 1 mol of H₂SO₄, so the number of moles in 12.20cm³ of sulfuric acid is (0.012/2) = 0.006 mol of sulfuric acid

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Calculate the concentration of sulfuric acid in mol/ dm³

0.006 mol x (1000/12.2) dm³ =0.49mol/dm³

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Calculate the concentration of sulfuric acid in g/ dm³

H₂SO₄ = (2x1) + 32 + (4x16) = 98g

0.49 x 98g = 48.2g/dm³

Reactions of acids - PART 2 - HIGHER

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The molecules split up to form hydrogen ions.

A weak acid (aq) has a lower pH than a strong acid (aq) of the same concentration.

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This is because a weak acid has a lower concentration of hydrogen ions.

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As the pH decrease by one unit, the hydrogen ion concentration of the solution increase by a factor of 10.

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Acids must dissolve in water to show their acidic properties.

A concentrated acid has a relatively large amount of solute dissolved in the solvent. A dilute acid has a relatively smaller amount of solute dissolved in the solvent

A strong acid is completely ionised in aqueous solution. E.g. Hydrochloric, nitric and sulfuric acid.

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A weak acid is only partially ionised in aqueous solution. E.g. Ethanoic, citric and carbonic.

QuestionIT!

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Reactions of acids

PART 2

​

• pH and neutralisation
• Titration (chemistry only)
• Strong and weak acids (HT)

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Reactions of acids – QuestionIT

1. What ions do aqueous acids contain?

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• What ions do aqueous alkalis contain?

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• What is the pH scale?

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• How can pH be measured?

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• What pH is a neutral solution?

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• What pH do aqueous acid solutions have?

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• What pH do aqueous alkali solutions have?

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Reactions of acids – QuestionIT

8. Write a balanced symbol equation for the reaction between hydrogen ions and hydroxide ions.

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• CHEMISTRY ONLY: What are the units for the concentration of a solution?

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• CHEMSITRY ONLY: What is the concentration of a solution that has 40g of solute in 2dm³ of solution?

Reactions of acids – QuestionIT

11. HT: What is a strong acid?

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• HT: Name 3 strong acids.

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• HT: What is a weak acid?

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• HT: Name 3 weak acids.

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• HT: What is a dilute acid?

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• HT: What happens to the hydrogen ion concentration as the pH decreases by 1?

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AnswerIT!

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Reactions of acids

PART 2

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• pH and neutralisation
• Titration (chemistry only)
• Strong and weak acids (HT)

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Reactions of acids – QuestionIT

1. What ions do aqueous acids contain?

H⁺

• What ions do aqueous alkalis contain?

OH^-

• What is the pH scale?

Measure of the acidity or alkalinity of a solution.

• How can pH be measured?

Universal indicator, pH probe.

• What pH is a neutral solution?

7

• What pH do aqueous acid solutions have?

Less than 7.

• What pH do aqueous alkali solutions have?

More than 7.

Reactions of acids – QuestionIT

8. Write a balanced symbol equation for the reaction between hydrogen ions and hydroxide ions.

H⁺(aq) + OH^-(aq) 🡪 H₂O(l)

• CHEMISTRY ONLY: What are the units for the concentration of a solution?

mol/dm³ or g/dm³

• CHEMSITRY ONLY: What is the concentration of a solution that has 40g of solute in 2dm³ of solution?

Concentration = mass ÷ volume = 40 g ÷ 2 dm³ = 20 g/dm³

Reactions of acids – QuestionIT

11. HT: What is a strong acid?

Completely ionised in aqueous solution.

• HT: Name 3 strong acids.

Hydrochloric, nitric, sulfuric.

• HT: What is a weak acid?

Partially ionised in aqueous solution.

• HT: Name 3 weak acids.

Ethanoic, citric, carbonic

• HT: What is a dilute acid?

Contains less solute in the same volume

• HT: What happens to the hydrogen ion concentration as the pH decreases by 1?

Increases by a factor of 10

LearnIT!

KnowIT!

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Electrolysis

PART 1

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• The process of electrolysis
• Electrolysis of a molten ionic compounds
• Electrolysis of aqueous solutions

Electrolysis - PART 1

When an ionic compound is melted or dissolved in water, the ions are free to move about the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes. Passing an electric current though electrolytes causes the ions to move to the electrodes.

Positive ions go to negative electrode (cathode) and are reduced (gain of electrons).

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Negative ions go to the positive electrode (anode) and are oxidised (loss of electrons).

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Ions are discharged at the electrodes producing elements. This is called electrolysis.

Electrolysis - PART 1

When an ionic compound is electrolysed in a molten state using inert electrodes, the metal is produced at the cathode and the non-metal is produced at the anode.

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lead bromide 🡪 lead + bromine

Higher: You can represent what is happening at each electrode using half equations.

Higher:

At the anode

2Br ^- 🡪 Br₂ + 2e ^– or 2Br ^- - 2e ^- 🡪 Br₂

Higher:

At the cathode

Pb ²⁺ + 2e ^– 🡪 Pb

Molten lead (II) bromide

Bromide ions Br ^-

Lead ions Pb ⁺

The positively charged lead ions Pb + (cations) are attracted to cathode and the negatively charged bromide ions Br ^- are attracted to the anode.

Electrolysis - PART 1

The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative reactivity of the elements involved.

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sodium chloride 🡪 hydrogen + chlorine

+ sodium hydroxide

Higher:

At the cathode

2H⁺ + 2e ^– 🡪 H₂

Higher:

At the anode

2Cl ^- 🡪 Cl₂ + 2e ^– or 2Cl ^- - 2e ^- 🡪 Cl₂

Uses of the products:

Chlorine: Bleach and PVC

Hydrogen: Margarine Sodium hydroxide: Bleach and soap

At the negative electrode:

Metal will be produced on the electrode if it is less reactive than hydrogen.

Hydrogen will be produced if the metal is more reactive than hydrogen.

At the positive electrode:

Oxygen is formed at positive electrode.

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If you have a halide ion (Cl^-, I^-, Br^-) then you will get chlorine, bromine or iodine formed at that electrode.

This happens because in the aqueous solution, water molecules break down producing hydrogen ions and hydroxide ions that are discharged.

H₂O(l) 🡪 H⁺(aq) + OH^-(aq)

Higher: At the anode

4OH^- → O₂ + 2H₂O + 4e^-

or 4OH^- – 4e^- → O₂ + 2H₂O

Hydrogen H⁺

Chloride Cl ^-

Sodium chloride solution

QuestionIT!

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Electrolysis

PART 1

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• The process of electrolysis
• Electrolysis of a molten ionic compounds
• Electrolysis of aqueous solutions

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Electrolysis – QuestionIT

1. Why can a molten or dissolved ionic compound conduct electricity?

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• What is electrolysis?

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• What is the name of the electrode that positive ions move to?

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• What is the name of the electrode that the negative ions move to?

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• What is produced at the cathode when lead bromide is electrolysed?

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• What is produced at the anode when lead bromide is electrolysed?

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Electrolysis – QuestionIT

7. What is produced at the cathode is the metal in the solution is more reactive than hydrogen?

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• What is produced at the anode if the solution does not contain halide ions?

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• HT Only: Write half equations for the reactions that happen at the

electrodes during the electrolysis of molten copper chloride.

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10. Predict the products of electrolysis of copper sulfate solution

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11. HT Only: Write a half equation for the reactions that happen at the

electrodes during the electrolysis of copper bromide solution.

AnswerIT!

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Electrolysis

PART 1

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• The process of electrolysis
• Electrolysis of a molten ionic compounds
• Electrolysis of aqueous solutions

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Electrolysis – QuestionIT

1. Why can a molten or dissolved ionic compound conduct electricity?

Free moving ions.

• What is electrolysis?

When an electric current is passed through a molten or aqueous ionic solution and the salt breaks down into simpler substances.

• What is the name of the electrode that positive ions move to?

Cathode.

• What is the name of the electrode that the negative ions move to?

Anode.

• What is produced at the cathode when lead bromide is electrolysed?

Lead.

• What is produced at the anode when lead bromide is electrolysed?

Bromine.

Electrolysis – QuestionIT

7. What is produced at the cathode is the metal in the solution is more reactive than hydrogen?

Hydrogen.

• What is produced at the anode if the solution does not contain halide ions?

Oxygen.

• HT Only: Write half equations for the reactions that happen at the

electrodes during the electrolysis of molten copper chloride.

Negative electrode: Cu²⁺(aq) + 2e⁻ → Cu(s)

Positive electrode: 2Cl⁻(aq) → Cl₂(g) + 2e⁻

10. Predict the products of electrolysis of copper sulfate solution

Positive electrode: Oxygen gas; Negative electrode: Copper.

11. HT Only: Write a half equation for the reactions that happen at the

electrodes during the electrolysis of copper bromide solution.

Negative electrode: Cu²⁺(aq) + 2e⁻ → Cu(s)

Positive electrode: 2Br⁻(aq) → Br₂(g) + 2e⁻

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LearnIT!

KnowIT!

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Electrolysis

PART 2

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• Using electrolysis to extract metals

Electrolysis - PART 2

Metals can be extracted from molten compounds using electrolysis.

It is used if the metal is too reactive to be extracted by reduction with carbon or if the metal reacts with carbon.

Large amounts of energy are used in the extraction process to melt the compounds and to produce the electrical current.

Aluminum is manufactured by electrolysis of molten aluminum oxide.

Aluminium oxide 🡪 aluminium + oxygen

Aluminium oxide has a very high melting point so is mixed with molten cryolite to lower the temperature required to carry out the electrolysis.

Aluminium goes to the negative electrode and sinks to bottom.

Higher: Al ³⁺ + 3e ^– 🡪 Al

Oxygen forms at positive electrodes.

Higher: 2O ^2- 🡪 O₂ + 4e ^–

The oxygen reacts with the carbon electrode making carbon dioxide causing damage. The electrode needs replacing due to this reaction.

C + O₂ 🡪 CO₂

QuestionIT!

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Electrolysis

PART 2

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• Using electrolysis to extract metals

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Electrolysis – QuestionIT

1. Why is electrolysis used to extract aluminium form its ore?

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• Why is electrolysis an expensive way to extract metal from its ore?

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• Name the compound from which aluminium is extracted.

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• What is this compound dissolved in before electrolysis?

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• What is the anode made of?

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Electrolysis – QuestionIT

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6. Describe what happens at the positive electrode during the electrolysis of aluminium oxide.

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• HT Only: Write half equations for the reactions that occur at the positive and negative electrodes during the production of aluminium.

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AnswerIT!

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Electrolysis

PART 2

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• Using electrolysis to extract metals

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Electrolysis – QuestionIT

1. Why is electrolysis used to extract aluminium form its ore?

Aluminium is more reactive than carbon.

• Why is electrolysis an expensive way to extract metal from its ore?

Large amounts of energy needed.

• Name the compound from which aluminium is extracted.

Aluminium oxide/ bauxite.

• What is this compound dissolved in before electrolysis?

Cryolite.

• What is the anode made of?

Carbon.

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Electrolysis – QuestionIT

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6. Describe what happens at the positive electrode during the electrolysis of aluminium oxide.

Oxide ions give up their electrons to form oxygen atoms, these join together in pairs to form oxygen gas, the oxygen reacts with the carbon electrode to make carbon dioxide gas.

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6. HT Only: Write half equations for the reactions that occur at the positive and negative electrodes during the production of aluminium.

Positive electrode: 2O²⁻(aq) → O₂(g) + 4e⁻

Negative electrode: Al³⁺(aq) + 3e⁻ → Al(l)

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