3.2.4—Acids with reactive metals

A metal above hydrogen in the activity series can donate electrons to acid and release hydrogen gas; a metal below hydrogen, such as copper, does not react with dilute hydrochloric acid.

$metal + acid → salt + H2(g)$

Balance the electron transfer behind the molecular equation: metal atoms are oxidized and 2H⁺ + 2e⁻ → H₂ is the reduction. Use the metal charge and acid anion to construct the salt rather than assuming every metal forms a 2+ ion.

Representative question

[Maximum number: 2]

Outline, using an ionic equation, what is observed when magnesium powder is added to a solution of ammonium chloride.

bubbles

OR

gas

OR

magnesium disappears
$2 \mathrm{NH}_{4}^{+}(\mathrm{aq})+\mathrm{Mg}(\mathrm{~s}) \rightarrow \mathrm{Mg}^{2+}(\mathrm{aq})+2 \mathrm{NH}_{3}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})$

Marking guidance:

Do not accept "hydrogen" without reference to observed changes. Accept "smell of ammonia".

Accept $2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Mg}(\mathrm{s}) \rightarrow \mathrm{Mg}^{2+}(\mathrm{aq}) +\mathrm{H}_{2}(\mathrm{~g})$

Equation must be ionic.

Retrieve the route: assign oxidation states, balance half-equations, predict displacement, label cells, trace electrons and ions, follow organic redox pathways, calculate potentials and choose electrolysis products.

Check electron loss/gain, anode/cathode versus polarity, spontaneous sign, salt-bridge direction, ions present, organic functional-group direction and object-cathode placement.

3.2.4—Acids with reactive metals

Metals with Dilute Acids

Predicting Hydrogen Release

Assessment in practice

Electron Transfer Reactions Summary

Question 1