IB Chemistry SLTopic 6 — Electron TransferPaper 1 & 2Core skill~11 min read
Acids with Reactive Metals
You met this reaction back in neutralisation, where it was flagged as the odd one out because no water is formed. Here is what it actually is: a proton collecting an electron from a metal and floating off as hydrogen gas.
📚 What you need to know
acid + metal → salt + hydrogen.
It is a redox reaction: the metal is oxidised and H+ is reduced.
The ionic equation is Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g); the anion is a spectator.
Only metals above hydrogen in the reactivity series react. Copper does not.
The rate depends on the reactivity of the metal and the concentration and strength of the acid.
Observations: effervescence, the metal dissolves, and the mixture warms up.
Hydrogen is tested with a lit splint, which gives a squeaky pop.
The salt is named the same way as in any acid reaction — the metal comes from the metal, and the rest comes from the acid. What is new is that no base is involved and no water is produced, which is why this sits in the redox topic rather than with neutralisation.
What is really happening
Strip out the spectator ions and the reaction is remarkably simple. Two hydrogen ions each collect an electron from a metal atom and pair up as a molecule of hydrogen gas.
Every bubble of gas represents two electrons that have left the metal. That is why the metal visibly dissolves as the reaction proceeds.
The chloride ions never appear in the ionic equation because nothing happens to them — they start as Cl–(aq) and finish as Cl–(aq). Exactly the same bookkeeping as in a neutralisation, except that here the surviving equation is a redox one rather than H+ + OH–.
Which metals bother
Only a metal that gives up its electrons more readily than hydrogen does can push H+ down to H2. That is the whole reason hydrogen is written into the reactivity series despite not being a metal: it is the reference point.
The top of the list is more a warning than a recommendation. Potassium, sodium and calcium react with dilute acids violently enough that the experiment is not done in a school laboratory.
Copper is the standard exam example of a metal that will not react with dilute acids. Explain it properly: copper is below hydrogen, so it is a weaker reducing agent than hydrogen and cannot give electrons to H+. “Copper is unreactive” alone rarely scores.
What changes the rate
Factor
Effect on the rate
Why
Metal further up the series
faster
it loses electrons more readily
More concentrated acid
faster
a higher [H+] means more frequent collisions
Strong acid rather than weak
faster
a strong acid of the same concentration supplies far more H+
Powdered rather than lump metal
faster
greater surface area exposed to the acid
Higher temperature
faster
more collisions, and more of them energetic enough
The observations follow from the same thing in every case: effervescence as hydrogen escapes, the metal disappearing as it becomes aqueous ions, and a temperature rise because the reaction is exothermic. Collect the gas and a lit splint gives the squeaky pop that confirms hydrogen.
WORKED EXAMPLE
Magnesium is added to dilute hydrochloric acid. Write the full equation, the ionic equation and both half-equations, and state which species is oxidised.
Full equationMg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)Ionic equation — drop the chlorideMg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)The two halvesMg → Mg²⁺ + 2e⁻2H⁺ + 2e⁻ → H₂magnesium is oxidised, 0 → +2And therefore magnesium is the reducing agent, while H⁺ is the oxidising agent.
WORKED EXAMPLE
Explain, in terms of electrons, why zinc reacts with dilute sulfuric acid but copper does not.
ZincZinc lies above hydrogen in the reactivity series, so it loses electrons more readily than hydrogen does.it can reduce H⁺ to H₂CopperCopper lies below hydrogen, so it is the weaker reducing agent of the two and holds its electrons more tightly.it cannot reduce H⁺, so no reactionThe position in the reactivity series is the reason, not the answer. Say what that position means about electrons.
WORKED EXAMPLE
0.240 g of magnesium is added to an excess of dilute hydrochloric acid. Calculate the volume of hydrogen produced at STP. (Ar Mg = 24.31; molar volume at STP = 22.7 dm3 mol–1)
Step 1 — moles of magnesiumn = 0.240 ÷ 24.31 = 9.87 × 10⁻³ molStep 2 — the ratio from the equationMg + 2HCl → MgCl₂ + H₂, so the ratio of Mg to H₂ is 1 : 1.n(H₂) = 9.87 × 10⁻³ molStep 3 — convert to a volumeV = 9.87 × 10⁻³ × 22.7 = 0.224 dm³224 cm³ of hydrogen“Excess acid” is a gift: it tells you the magnesium is limiting, so you never need the acid’s concentration.
💡 Exam tip
State clearly that this is a redox reaction, and say which species is oxidised and which reduced.
The anion of the acid is a spectator — leave it out of the ionic equation.
Explain non-reaction using position relative to hydrogen and what that means about losing electrons.
Give the three standard observations: effervescence, the metal dissolving, and the temperature rising.
Balance carefully with a diprotic acid: H2SO4 supplies two H+ per molecule.
Use 22.7 dm3 mol–1 for the molar volume at STP, and check whether the question wants cm3.
⚠️ Common mix-up
Calling this a neutralisation. No water is formed and no base is involved.
Writing H instead of H2 as the product. Hydrogen gas is diatomic.
Leaving the anion in the ionic equation.
Saying copper is “not reactive enough” without mentioning electrons or hydrogen.
Expecting a reaction with concentrated nitric acid to give hydrogen. That is a different reaction altogether.
Mixing up strength and concentration when explaining why one acid fizzes faster.
Up next: Primary Cells — so far the electrons have jumped straight from one species to another. Separate the two halves into different beakers and force the electrons to travel through a wire, and you have a battery.
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