O-LevelChemistryMetals & Reactivity5070/21 · Oct/Nov 2018 · Q810 Marks

Chromium is a transition element. Sodium is an element in Group I of the Periodic Table.

(a)Explain how both chromium and sodium conduct electricity.[1]
(b)Describe two differences in the physical properties of chromium and sodium.[2]
(c)An alloy of iron and chromium is stronger than pure iron. Use ideas about the structure of metals to explain why the alloy is stronger.[2]
(d)Molten chromium reacts with steam to form chromium(III) oxide, Cr2O3, and hydrogen. Construct the equation for this reaction.[1]
(e)The structure of an ion containing chromium is shown. [Figure showing structure of -O-Cr-O-O-Cr-O- with O-O branches] Write the formula for this ion.[1]
(f)Adding dilute acid to yellow CrO42-(aq) forms orange Cr2O72-(aq). The ionic equation for this reaction is shown. 2CrO42-(aq) + 2H+(aq) <=> Cr2O72-(aq) + H2O(l) yellow orange
(f)(i)Describe and explain what you would observe when a few drops of concentrated aqueous sodium hydroxide are added to the orange solution.[2]
(f)(ii)Suggest why a change in pressure has no effect on the reaction shown.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[1] delocalised electrons move (throughout the metal structure) / sea of electrons move (from place to place) (1)
[2] Any two from: chromium has a high(er) melting point or boiling point / sodium has low(er) melting point or boiling point (1) chromium dense / sodium not very dense (1) chromium is hard / sodium is soft (1) chromium less malleable / sodium very malleable(1)
[2] Any two from: chromium particles different size to iron particles (1) regular arrangement of layers (of particles) in iron disrupted (1) layers of iron (particles) can slide (more easily) / layers of alloy cannot slide (so easily) (1)
[1] 2Cr + 3H2O → Cr2O3 + 3H2
[1] Cr2O7²⁻
[2] goes yellow (1) OH⁻ ions react with H⁺ ions / equilibrium shifts to the left / more CrO4²⁻ formed / less Cr2O7²⁻ / less H⁺ (1)
[1] there are no gaseous reactants or products / there are no gases in the equation

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