Pearson Edexcel IGCSE Chemistry: Reactivity and Group Trends (4CH1)

This guide covers sub-topics (a)–(d), the reactivity and periodicity cluster of Topic 2 Inorganic Chemistry, Pearson Edexcel International GCSE Chemistry (4CH1), Issue 3, September 2024. Topic 2 runs to eight named sub-topics from (a) Group 1 through (h) Chemical Tests; this guide covers the four that establish how reactivity trends are identified and compared across the periodic table.

Where this fits in 4CH1

Topic 1 (Principles of Chemistry) establishes atomic structure and bonding; Topic 2 applies that foundation to real groups and reactions, starting with the two most clearly patterned groups in the periodic table (Groups 1 and 7) before moving to the broader concept of a reactivity series that spans metals generally. Both papers draw on all four topic areas, with Paper 2 additionally assessing content marked ‘C’ (bold in the official specification).

Syllabus coverage

PEARSON EDEXCEL INTERNATIONAL GCSE CHEMISTRY (4CH1) — SUB-TOPICS (A)–(D)

• (a) Group 1 (alkali metals): lithium, sodium and potassium — how similarities in their reactions with water provide evidence for recognising them as a family of elements; how differences in their reactions with air and water provide evidence for the trend in reactivity down Group 1; using known trends to predict the properties of other alkali metals; (C, bold content) explaining the reactivity trend in terms of electronic configuration
• (b) Group 7 (halogens): chlorine, bromine and iodine — colours, physical states at room temperature and trends in physical properties; using known trends to predict the properties of other halogens; how displacement reactions between halogens and halides provide evidence for the reactivity trend down Group 7; (C) explaining the reactivity trend in terms of electronic configuration
• (c) Gases in the atmosphere — approximate percentages by volume of the four most abundant gases in dry air; determining the percentage of oxygen in air experimentally, using reactions of metals (e.g. iron) and non-metals (e.g. phosphorus); combustion of elements in oxygen, including magnesium, hydrogen and sulfur; formation of carbon dioxide from thermal decomposition of metal carbonates, including copper(II) carbonate; carbon dioxide as a greenhouse gas and its link to climate change
• (d) Reactivity series — arranging metals in a reactivity series based on reactions with water and with dilute hydrochloric or sulfuric acid, and based on displacement reactions between metals and metal oxides, and between metals and aqueous metal salt solutions; the order of reactivity of potassium, sodium, lithium, calcium, magnesium, aluminium, zinc, iron, copper, silver and gold; the conditions under which iron rusts, and rust prevention by barrier methods, galvanising and sacrificial protection; the terms oxidation, reduction, redox, oxidising agent and reducing agent, defined in terms of gain or loss of oxygen and loss or gain of electrons

How to approach it

Groups 1 and 7 are best revised as mirror-image trends: reactivity increases down Group 1 (the alkali metals) but decreases down Group 7 (the halogens) — a common source of confusion under exam pressure is applying the wrong direction to the wrong group. Anchoring each trend to its underlying cause (easier loss of the single outer electron further down Group 1; harder gain of one outer electron further down Group 7, as the outer shell is more shielded from the nucleus) makes the direction easier to reconstruct even if the memorised fact briefly slips.

The reactivity series (d) is one of the most exam-productive pieces of content in this specification because it predicts outcomes across many different reaction types — whether a displacement reaction will occur, which metal will be extracted more easily, and why some metals corrode faster than others. Learn the eleven-metal order (potassium down to gold) as a single memorised sequence, since nearly every question in this sub-topic assumes it as background knowledge.

Redox definitions (d) can be stated in two equivalent ways — in terms of oxygen (oxidation is gain of oxygen, reduction is loss of oxygen) or in terms of electrons (oxidation is loss of electrons, reduction is gain of electrons) — and exam questions may use either. Practise translating between the two definitions for the same reaction rather than learning only one.

Worked example: predicting a displacement reaction

Using the reactivity series, predict whether zinc will react with copper sulfate solution.

Reactivity order (relevant section): ... zinc, iron, copper ...
Zinc is more reactive than copper (higher in the series).
Prediction: zinc displaces copper from copper sulfate solution,
            forming zinc sulfate and depositing copper metal.
Redox view: zinc is oxidised (loses electrons, forms Zn2+);
            copper ions are reduced (gain electrons, form Cu atoms).

This single reaction can be described correctly using either the oxygen-based or electron-based definition of oxidation and reduction — practising both descriptions for the same example is a reliable way to avoid confusing the two conventions in an exam answer.

Common mistakes

Applying Group 1’s “reactivity increases down the group” rule to Group 7, where reactivity decreases down the group, or vice versa. Reciting the eleven-metal reactivity order with a metal out of place, especially around the middle of the sequence (aluminium, zinc, iron). Defining oxidation and reduction correctly in one convention (oxygen or electrons) but applying the other convention’s rule by mistake. Forgetting that rust prevention methods (barrier, galvanising, sacrificial protection) work by different mechanisms and are not interchangeable explanations.

Quick revision checklist

• Learn Group 1 and Group 7 reactivity trends as opposite directions, each linked to its electron-configuration cause.
• Memorise the eleven-metal reactivity series in order.
• Practise both the oxygen-based and electron-based definitions of oxidation and reduction on the same example reaction.
• Be able to explain barrier methods, galvanising and sacrificial protection as three distinct rust-prevention mechanisms.

Official syllabus

Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 — qualifications.pearson.com.