Dilute hydrochloric acid is added to excess copper oxide. The black oxide becomes a blue-green copper chloride solution, with black solid remaining.
.1
Write the balanced equation.
[2 marks]
.2
Explain why excess copper oxide is used and how it is removed.
[2 marks]
.3
Explain why the mass of wet crystals would overestimate the mass of salt recovered.
[2 marks]
7 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A student measures a beaker as 42.36 g empty and 47.82 g with a dry solid. Each balance reading has uncertainty ±0.01 g. Add absolute uncertainties for a mass found by subtraction.
.1
Calculate the mass of solid.
[2 marks]
.2
Calculate the percentage uncertainty in the solid mass.
[3 marks]
.3
Suggest how a larger mass of the same solid affects percentage uncertainty with this balance.
[2 marks]
4 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
Three noble gases have boiling points: helium −269 °C, neon −246 °C and argon −186 °C.
.1
Calculate the rise in boiling point from helium to argon.
[2 marks]
.2
Describe the trend and predict whether krypton, below argon, boils above or below −186 °C.
[2 marks]
6 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
A factory uses electrolysis to extract aluminium. One operating mixture needs 1200 energy units per batch; adding cryolite reduces the requirement to 900 units per equal batch.
.1
Calculate the percentage reduction in energy use.
[3 marks]
.2
Explain how cryolite reduces energy use and why the positive electrodes need replacement.
[3 marks]
4 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A pupil estimates the energy change for CH4+Cl2→CH3Cl+HCl. Bond energies are C-H 413, Cl-Cl 243, C-Cl 338 and H-Cl 431 kJ/mol.
.1
Calculate the energy change using only bonds that change.
[3 marks]
.2
Evaluate the claim that breaking the C-H bond supplies the energy released.
