IB Chemistry SL R3.2 Electron transfer reactions questions

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Question 1

Multiple choiceMedium1 mark

A voltaic cell operates by the reaction Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\mathrm{Mg(s)+2HCl(aq)\rightarrow MgCl_2(aq)+H_2(g)} Initially, 0.0040 mol0.0040\ \mathrm{mol} Mg and 0.0060 mol0.0060\ \mathrm{mol} HCl are available. What amount of H2\mathrm{H_2} forms, and what is the sign of the electrode where it forms?

Show answer and explanation

Answer: D. The equation requires 2 mol2\ \mathrm{mol} HCl per 1 mol1\ \mathrm{mol} H₂, so HCl limits the reaction: n(H2)=0.0060/2=0.0030 moln(\mathrm{H_2})=0.0060/2=0.0030\ \mathrm{mol} H⁺ is reduced at the hydrogen electrode, making it the cathode; the cathode is positive in a voltaic cell.
A: gives the correct amount but assigns the hydrogen electrode the negative sign.
B: uses the amount of Mg as though it all reacts, despite insufficient HCl.
C: uses the Mg amount without checking the limiting reagent and also assigns the cathode a negative sign.
Remember: in a voltaic cell, reduction occurs at the positive cathode.

Question 2

Paper 2Medium6 marks

A student investigates displacement reactions between halogens and halide ions in aqueous solution. The results are shown below. Chlorine, bromine and iodine occur in this order from top to bottom in Group 17.

Halogen addedHalide ions presentResult
Cl₂(aq)Br⁻(aq)Reaction
Br₂(aq)I⁻(aq)Reaction
I₂(aq)Br⁻(aq)No reaction
(a)

Write the balanced ionic equation for the reaction between Br₂(aq) and I⁻(aq).

[2]
(b)

Using your equation in (a), identify the species oxidized.

[1]
(c)

A student predicts that I₂(aq) reacts with Cl⁻(aq). Predict whether this reaction occurs. Use the results and the positions of the elements in Group 17 to justify your answer.

[3]

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