Four metals are each added separately to equal volumes of the same dilute hydrochloric acid. The table shows the results. metal added to dilute hydrochloric...

This question tests the reactivity series through the reaction of metals with a dilute acid: the more reactive the metal, the faster it displaces hydrogen.

(a) The completed table, in order of decreasing reactivity, is:

zinc [1]; iron [1]; copper [1]; and the fourth mark is for showing the correct order of rate magnesium > zinc > iron > copper [1]. The pale green colour with iron is the Fe²⁺ ion, a useful identifying detail.

(b) Copper does not react because it is below hydrogen in the reactivity series [1]; being less reactive than hydrogen, it cannot displace hydrogen from the acid [1]. Only metals above hydrogen liberate hydrogen from dilute acids.

(c) The gas is hydrogen [1]; the test is to hold a lighted splint at the mouth of the tube, which gives a squeaky pop [1].

(d) Zinc is above hydrogen, so it displaces it:

\[\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\]

correct formulae [1]; balanced [1].

(e) Hydrogen ions gain electrons (reduction) to form hydrogen gas, so two electrons are needed to balance the two positive charges:

\[2\text{H}^+ + 2e^- \rightarrow \text{H}_2\]

2e^- on the left [1]; balanced [1].

(f) Any one change that gives the particles more frequent or more energetic collisions increases the rate: raise the temperature, use more concentrated acid, or use zinc powder instead of pieces to increase the surface area [1].