Chemical changes — GCSE Chemistry Revision
Everything you need to revise chemical changes for GCSE Chemistry: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Electrolysis: Breaking Down Compounds
Electrolysis uses electrical energy to break down ionic compounds. For this to happen, the ions must be free to move, either molten or dissolved in solution. At the negative electrode (cathode), positive ions gain electrons (reduction). At the positive electrode (anode), negative ions lose electrons (oxidation). This process is vital for extracting reactive metals like aluminium from their ores, and for purifying copper. Understanding the flow of ions and electrons is key to explaining the products formed at each electrode.
Reactivity Series: Predicting Chemical Changes
The reactivity series arranges metals in order of their reactivity, from most reactive to least reactive. More reactive metals readily lose electrons to form positive ions. A more reactive metal can displace a less reactive metal from a solution of its salt. This series helps predict the outcomes of displacement reactions and informs extraction methods. Metals above carbon in the series are extracted by electrolysis; those below carbon can be reduced by heating with carbon. Hydrogen and carbon are often included to show their relative reactivity.
Acids, Bases, and Neutralisation
Acids produce H+ ions in aqueous solutions, while bases are substances that neutralise acids. Alkalis are soluble bases that produce OH- ions in aqueous solutions. The pH scale measures the acidity or alkalinity of a solution, with 7 being neutral. Neutralisation is a reaction between an acid and a base, typically forming a salt and water. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water. This reaction is fundamental in many chemical processes and titrations.
Redox Reactions: Oxidation and Reduction
Oxidation and reduction always occur simultaneously in a redox reaction. Oxidation is the loss of electrons or gain of oxygen. Reduction is the gain of electrons or loss of oxygen. In terms of electrons: OIL RIG (Oxidation Is Loss, Reduction Is Gain). For example, in the reaction Mg + CuO → MgO + Cu, magnesium is oxidised (gains oxygen) and copper oxide is reduced (loses oxygen). Electron transfer can be shown using half equations, which represent what happens at each electrode during electrolysis or in redox reactions.
Key facts
- Electrolysis requires molten or dissolved ionic compounds.|Reactivity series helps predict displacement reactions.|Acids produce H+ ions, alkalis produce OH- ions.|Neutralisation forms salt and water.|Oxidation is loss of electrons; reduction is gain of electrons.|More reactive metals are extracted by electrolysis.
Key terms
- Electrolysis::The process of using electrical energy to break down an ionic compound into its elements.|Cathode::The negative electrode in an electrolytic cell where reduction (gain of electrons) occurs.|Anode::The positive electrode in an electrolytic cell where oxidation (loss of electrons) occurs.|Reactivity series::A list of metals arranged in order of decreasing reactivity, indicating their tendency to lose electrons.|Neutralisation::A chemical reaction between an acid and a base (or alkali), producing a salt and water.|Redox reaction::A reaction in which both oxidation and reduction occur simultaneously.
Common mistakes
- Confusing the cathode and anode, or which electrode is positive/negative.|Mixing up oxidation as 'gain of electrons' instead of loss, or vice versa for reduction.|Assuming all compounds conduct electricity, only ionic compounds can be electrolysed when molten or in solution.|Forgetting to balance half-equations when showing electron transfer in redox reactions.
Exam tips
- Remember 'PANIC' (Positive Anode, Negative Is Cathode) and 'OIL RIG' (Oxidation Is Loss, Reduction Is Gain) for redox and electrolysis.|Practise writing full and half ionic equations for electrolysis and displacement reactions, ensuring they are balanced.|Learn the common reactions of acids with metals, bases, and carbonates, along with their products.|Be able to justify the method of extraction for a given metal based on its position in the reactivity series.
Quick quiz
1. What is the pH of a neutral solution?
- 0
- 7
- 14
- Between 7 and 14
Show answer
7 — The pH scale ranges from 0-14. A pH of 7 indicates a neutral solution. Values below 7 are acidic, and values above 7 are alkaline.
2. Which of the following compounds can be electrolysed in its molten state?
- Glucose (C6H12O6)
- Sodium chloride (NaCl)
- Water (H2O)
- Diamond (C)
Show answer
Sodium chloride (NaCl) — Electrolysis requires an ionic compound with free-moving ions. Sodium chloride is an ionic compound that can be electrolysed when molten. Glucose is a covalent compound, water is covalent, and diamond is a giant covalent structure.
3. In the reaction Mg + CuO → MgO + Cu, which substance is oxidised?
- Magnesium
- Copper oxide
- Magnesium oxide
- Copper
Show answer
Magnesium — Oxidation is the gain of oxygen. Magnesium gains oxygen to form magnesium oxide, therefore magnesium is oxidised.
4. Which statement correctly describes the products of reacting a reactive metal with an acid?
- Salt and oxygen
- Salt and hydrogen
- Salt and water
- Salt and carbon dioxide
Show answer
Salt and hydrogen — A reactive metal reacting with an acid produces a salt and hydrogen gas. For example, magnesium + hydrochloric acid → magnesium chloride + hydrogen.
5. Which of these metals is extracted by heating its oxide with carbon?
- Potassium
- Aluminium
- Iron
- Calcium
Show answer
Iron — Metals below carbon in the reactivity series (like iron, zinc, lead, copper) can be extracted by reduction with carbon. Potassium, Aluminium, and Calcium are more reactive than carbon and require electrolysis.
Exam-style questions
A student sets up an electrolysis experiment using molten lead(II) bromide. Describe what happens at the positive electrode (anode) and write the half-equation for the reaction occurring. [3 marks]
Show mark scheme
- Bromide ions are attracted to the positive electrode.
- Bromide ions lose electrons (are oxidised) to form bromine molecules/gas.
- 2Br⁻ → Br₂ + 2e⁻
Explain why aluminium cannot be extracted from its ore, aluminium oxide, by heating with carbon, and suggest an alternative method. [3 marks]
Show mark scheme
- Aluminium is more reactive than carbon.
- So, carbon cannot displace aluminium from its oxide/reduce aluminium oxide.
- Aluminium is extracted by electrolysis (of molten aluminium oxide).
Magnesium reacts with hydrochloric acid. Write a balanced symbol equation for this reaction and state two observations you would expect to see. [4 marks]
Show mark scheme
- Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) (or correctly balanced equation with states)
- Magnesium solid disappears/gets smaller.
- Bubbles/effervescence/gas produced.
- Solution gets warmer/exothermic (allow 'heat given off').
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