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A-Level Redox, electrode potentials and electrolysis: worked solution
2 marks. Full working, one step per line.
Question
After being absorbed into the blood, ethanol is carried to the body's organs. Because the amount of alcohol vapour in the lungs tracks the blood alcohol level, a breathalyser can measure it. In an early type of breathalyser, the person exhales into the unit and the breath passes through potassium dichromate solution, which oxidises ethanol to acetic acid and turns from orange to green. Write the balanced ionic equation for this oxidation of ethanol by dichromate ions under acidic conditions.
Worked answer
Ethanol is oxidised to ethanoic (acetic) acid, losing 4 electrons: CH3CH2OH + H2O -> CH3COOH + 4H+ + 4e-. Dichromate is reduced: Cr2O7²- + 14H+ + 6e- -> 2Cr³+ + 7H2O. Balancing electrons (x3 and x2 for 12 e-) and combining: 3CH3CH2OH + 2Cr2O7²- + 16H+ -> 3CH3COOH + 4Cr³+ + 11H2O. Check: H 34=34, O 17=17, Cr 4=4, charge +12=+12.
Practise this topic
This question is part of A-Level Redox, electrode potentials and electrolysis, in A-Level H2 Chemistry.
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