Reactivity series: order and tests
Reactivity series, most to least reactive: potassium, sodium, lithium, calcium, magnesium, aluminium, zinc, iron, copper, silver, gold. Set experimentally by: (1) cold water — only K, Na, Li, Ca react vigorously; (2) dilute HCl or — metals above copper fizz, faster = more reactive; (3) displacement — a more reactive metal displaces a less reactive one from its oxide or salt. A more reactive metal loses electrons more easily.
Redox: oxygen and electron transfer
Oxidation = loss of electrons (or gain of oxygen); reduction = gain of electrons (or loss of oxygen). Mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons). Redox = oxidation and reduction together. The oxidising agent is the species reduced; the reducing agent is the one oxidised. In , zinc is oxidised (reducing agent) and is reduced (oxidising agent).
Rusting: needs water + oxygen; prevention
Iron rusts only when BOTH water and oxygen are present — remove either and it cannot rust (salt speeds it up but is not required); the product is hydrated iron(III) oxide. Prevention: (1) barrier methods — paint, oil or plastic keep water and oxygen off; (2) galvanising — a zinc coating, a barrier also sacrificial if scratched (zinc more reactive than iron); (3) sacrificial protection — a block of a more reactive metal (zinc, magnesium) is oxidised instead of the iron.
Drawn from real examiner reports.
Oxidising agent is the species reduced
The oxidising agent is the species reduced (gains electrons / gives oxygen); the reducing agent is the species oxidised. In the thermite reaction , iron(III) oxide is the oxidising agent and aluminium the reducing agent. As aluminium is oxidised, it cannot also be the oxidising agent.
June 2023 Paper 1CR Q5(c)(ii): many did not name iron(III) oxide as the oxidising agent; a few wrongly called aluminium the oxidising agent even while saying it lost electrons.
Rusting needs oxygen, not carbon dioxide
Rusting needs water AND oxygen. A minority write carbon dioxide as a cause — that scores zero. Salt water speeds rusting up but is not required; remove either water or oxygen and iron cannot rust. The corrosion product is hydrated iron(III) oxide, not an iron carbonate.
June 2023 Paper 1CR Q5(a)(i): a small minority wrote carbon dioxide instead of oxygen as a cause of rusting.
Name the barrier method for paint
When asked how paint (or plastic/oil) stops rusting, name the barrier method: the coating forms a barrier that keeps water and oxygen away from the iron surface. Saying only that paint "stops the reaction with oxygen and water" misses the marking point — you must state that it acts as a barrier.
June 2023 Paper 1CR Q5(b)(i): many knew paint stops oxygen/water but failed to name the barrier method.
Use the word "displaces"
When a more reactive metal removes a less reactive metal from its oxide or salt, you must use the word displaces (or displacement). Vague phrases such as "snatches the oxygen" or "takes the place" are not enough. Also compare the reactivities explicitly: the more reactive metal displaces the less reactive one because it is higher in the series.
June 2023 Paper 1CR Q5(c)(i): "snatches the oxygen from the iron" was insufficient; the word displaces was required for the second mark.
Galvanising ≠ sacrificial protection
Galvanising and sacrificial protection are not interchangeable. Galvanising is specifically a zinc coating on iron (a barrier that is also sacrificial if scratched). Sacrificial protection is attaching a separate block of a more reactive metal (zinc or magnesium). Answering "sacrificial protection" when galvanising is asked, or vice versa, loses the mark.
June 2023 Paper 1CR Q5(b)(ii): galvanising was well known, but some wrote sacrificial protection, which was not credited here.
More reactive means loses electrons easily
"More reactive" means the metal loses electrons more easily (forms its positive ion more readily) and sits higher in the reactivity series — it does NOT mean the metal is physically "stronger" or "harder". Reactivity is about how readily the metal reacts, e.g. how fast it fizzes in acid or how easily it displaces another metal.
(common Edexcel chemistry error)
Metal-reaction equations: product type first
Decide the product type first, then balance by inspection: metal + acid -> salt + hydrogen; metal + water -> metal hydroxide + hydrogen (K/Na/Li/Ca only); displacement -> the more reactive metal takes the oxide or salt. Check every element balances.
Rusting answer: name method + compare
In rusting-prevention answers, name the method AND compare the reactivities: "zinc is more reactive than iron". The comparison is a separate marking point — protecting-metal answers that never state the metal is more reactive than iron lose that mark.
Metal + acid practical: excess and gas
In a metal + acid rate practical, be clear which reactant is in excess (the other is used up and limits the gas made). Use a gas syringe with a bung — the bung stops gas escaping. The reaction is over when no more gas is collected / no more bubbles form.
Redox role: agent does the opposite
To identify redox roles, the agent does the OPPOSITE of its name: the oxidising agent is itself reduced, the reducing agent is itself oxidised. In a displacement, the metal that dissolves is oxidised (reducing agent); the ion that becomes a metal is reduced (oxidising agent).
The reactivity series orders metals by how readily they react (how easily they lose electrons to form positive ions). The 4CH1 order, most to least reactive, is:
potassium sodium lithium calcium magnesium aluminium zinc iron copper silver gold
Mark-scheme definitions (each needs two elements):
How the order is established (state symbols: solid, liquid, gas, aqueous):
| Test | What you see | Order it reveals |
|---|---|---|
| Add metal to cold water | Only K, Na, Li, Ca react (fizz, float, give ) | very reactive metals at the top |
| Add metal to dilute HCl or | Faster fizzing (more ) = more reactive; Cu, Ag, Au do not react | middle of the series |
| Metal + metal-oxide / metal + salt solution | More reactive metal displaces the less reactive one | direct pairwise comparison |
Key equation types:
List the 4CH1 reactivity series from most to least reactive.
Magnesium reacts with excess dilute hydrochloric acid: . Calculate the mass of magnesium chloride formed from of magnesium. (: Mg ; of .)
