A student electrolyses concentrated aqueous potassium chloride using inert electrodes in a teacher-controlled demonstration with suitable ventilation. Potassium is more reactive than hydrogen.
.1
Predict the electrode products and explain the difference from molten potassium chloride.
[3 marks]
.2
Explain how charge is carried through the aqueous electrolyte and through the external metal wires.
[2 marks]
4 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
An exothermic synthesis gives the following measured data. At 250 °C: equilibrium yield 80%, time to equilibrium 40 min. At 350 °C: yield 60%, time 8 min. A plant must reach equilibrium within 10 min and obtain at least 55% yield.
.1
Calculate the factor by which equilibration is quicker at 350 °C.
[2 marks]
.2
Justify a temperature choice from these two settings and explain why the highest yield alone is not the best criterion.
[2 marks]
6 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A tank initially holds 2.0 dm³ of solution at 18 g/dm³. A 0.50 dm³ portion is removed after mixing, then 0.50 dm³ of water is added.
.1
Calculate the final concentration in the tank.
[4 marks]
.2
Explain why restoring the initial volume does not restore the initial concentration.
[2 marks]
5 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A bottle contains carbon dioxide molecules. A sample of magnesium chloride contains Mg2+ and Cl− ions.
.1
Compare how bonds form in the two substances.
[3 marks]
.2
Explain why one magnesium atom needs two chlorine atoms to form the compound.
[2 marks]
6 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A sample contains very long covalent molecules. It softens when heated, does not conduct electricity and becomes solid again on cooling.
