O-LevelChemistryStoichiometry5070/21 · Oct/Nov 2022 · Q910 Marks

Zinc powder reacts with dilute hydrochloric acid. Zn + 2HCl → ZnCl2 + H2 When 20.0cm³ of dilute hydrochloric acid is added to excess zinc, the volume of hydrogen gas produced at room temperature and pressure is 60.0cm³.

(a)(i)Calculate the concentration, in mol/dm³, of the dilute hydrochloric acid. concentration mol/dm³[3]
(a)(ii)The reaction is repeated using large pieces of zinc instead of zinc powder. All other conditions stay the same. Describe how the rate of reaction changes. Explain your answer using ideas about collisions between particles.[2]
(b)Dilute hydrochloric acid is electrolysed. Construct an ionic equation for the reaction at the cathode.[1]
(c)(i)Zinc is used to prevent iron from rusting. State the essential conditions needed for rusting.[1]
(c)(ii)Explain how zinc prevents iron from rusting by the method of sacrificial protection.[2]
(c)(iii)Give one example of the use of sacrificial protection.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[3] mol H2 = 60/24 000 OR 2.5 × 10⁻³ mol (1)
[3] mol HCl = M1 × 2 OR 5 × 10⁻³ mol (1)
[3] concentration of HCl = 0.25 mol / dm³ (1)
[2] rate decreases (no mark)
[2] fewer particles exposed on surface (1)
[2] frequency of collisions decreases (1)
[1] 2H⁺ + 2e⁻ → H2
[1] water AND oxygen
[2] zinc is more reactive than iron / zinc is higher in the reactivity series than iron (1)
[2] zinc corrodes in preference to iron (1)
[1] (stops) underground pipes (corroding) / (stop) ships hulls (corroding)

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