Types of reactions (B): swapping partners and electrons
Week 2 · Lesson 1 Types of Chemical Reactions (B): double replacement, neutralisation and the universal indicator, acid with a salt, precipitation, hard water, oxidation and reduction as electron transfer · 3 lessons
3
Losing and gaining electrons: oxidation and reduction
Science Prep 3 Term 1, Unit 1: pp. 9–14
GoalDefine oxidation and reduction as loss and gain of electrons, and name the oxidising and reducing agents in a reaction.
New words
oxidation · loss of electrons by an atom, ion or moleculereduction · gain of electrons by an atom, ion or moleculeoxidising agent · the substance that gains electrons, so it is itself reduced
Explanation
In many reactions electrons move from one substance to another. Oxidation is the loss of electrons. Reduction is the gain of electrons. The two always happen together, which is why such reactions are called redox reactions. The substance that gains electrons is the oxidising agent, and it is reduced. The substance that loses electrons is the reducing agent, and it is oxidised. Mind the trap: the agent is the one that causes the change in the other substance. Example: Mg + Cu²⁺ → Mg²⁺ + Cu. A magnesium atom loses two electrons, Mg → Mg²⁺ + 2e⁻, so magnesium is oxidised and is the reducing agent. A copper ion gains the two electrons, Cu²⁺ + 2e⁻ → Cu, so the copper ion is reduced and is the oxidising agent. Electrons are not made or destroyed: what one loses the other gains. A positive ion is an atom that lost electrons. The older definitions, gaining oxygen or losing hydrogen, are special cases of the electron idea.
Term
Electrons
The substance is
oxidation
lost
oxidised; reducing agent
reduction
gained
reduced; oxidising agent
Worked examples
Na→oxidation: electron lostNa+ + e−
In 2Na + Cl₂ → 2NaCl, each Na atom loses one electron: Na → Na⁺ + e⁻. Two atoms lose 2 electrons in all. The chlorine molecule gains those 2 electrons: Cl₂ + 2e⁻ → 2Cl⁻. Sodium is the reducing agent and chlorine the oxidising agent; 2 electrons lost = 2 gained.
Ag+ + e−→reduction: electron gainedAg
In Zn + 2Ag⁺ → Zn²⁺ + 2Ag, one zinc atom loses 2 electrons, and each of two silver ions gains 1. Zinc is oxidised and is the reducing agent. Silver ions are reduced and are the oxidising agent. The totals match: 2 lost, 2 × 1 = 2 gained.
In Mg + 2HCl → MgCl₂ + H₂, magnesium is oxidised and the hydrogen ions of the acid are reduced to hydrogen gas. So this single replacement from Lesson 2 of Week 1 is also a redox reaction.
Class activity
Coins as electrons. Make cards for atoms and ions: Zn (zinc atom), Cu²⁺ (copper ion), Zn²⁺, Cu. Give the zinc card two coins as its two outer electrons. Move both coins from the zinc card to the copper-ion card and swap the cards to show Zn²⁺ and Cu. Say which card was oxidised and which was reduced. Repeat with 4 coins and two copper-ion cards. Count the coins before and after: they must match.
Practice
1
What is oxidation, and what is reduction?
Oxidation is loss of electrons; reduction is gain of electrons.
2
In Mg → Mg²⁺ + 2e⁻, is magnesium oxidised or reduced? Is it an oxidising or a reducing agent?
Oxidised (it loses electrons); it is a reducing agent.
3
Four aluminium atoms each become Al³⁺. How many electrons are lost in all?
4 × 3 = 12 electrons.
4
A student writes: “the solution changes into copper”. Correct it using electrons.
Copper ions in the solution gain electrons and become copper atoms: Cu²⁺ + 2e⁻ → Cu.
