A-LevelChemistryElectrochemistry9701/42 · May/June 2020 · Q87 Marks

Define the term standard cell potential.

(a)(i)Define the term standard cell potential.[2]
(a)(ii)Complete the table with the substance used to make the electrode in each of these half-cells. half-cell Co³⁺/Co²⁺ Cl₂/Cl⁻ electrode[1]
(a)(iii)Use data from the Data Booklet to calculate the E°cell. E°cell = V [1][1]
(a)(iv)Write the equation for the overall cell reaction.[1]
(b)A fuel cell is an electrochemical cell that can be used to generate electrical energy. A methanol-oxygen fuel cell can be used as an alternative to a hydrogen-oxygen fuel cell. When the cell operates, the carbon atoms in the methanol molecules are converted into carbon dioxide. CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻ Calculate the volume of CO₂, in cm³, formed when a current of 2.5 A is delivered by the cell for 30 minutes. Assume the cell is operated at room conditions. volume of CO₂ = cm³ [2][2]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[2] M1 potential difference between two half-cells/two electrodes in a cell M2 under conditions of 1 atm., 298 K, (all) solutions being 1 mol dm⁻³
[1] both platinum
[1] Ecell = 1.82 - 1.36 = (+)0.46 V
[1] 2Co³⁺ + 2Cl⁻ → Cl₂ + 2Co²⁺
[2] M1 Q= 2.5 x 30 x 60 C = 4500 C AND 96500 OR 579000 seen moles of CO₂= 4500/579000 = 7.8 x10⁻³ or 7.77 x 10⁻³ M2 volume of CO₂ = 7.77 x 10⁻³ x 24000 = 187 cm³

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