The Student Room Group

Measuring Enthalpy Change

1) a) ΔH for the reaction between zinc and copper(II) sulfate solution is −219 kJ mol−1.
Construct a balanced equation for this reaction. (1 mark)

b) When the two are mixed together would you expect the temperature to rise or fall? Why? (2 marks)

c) 50 cm3 of a 1.0 mol dm−3 solution of copper(II) sulfate is reacted with an excess of zinc powder.
i) Calculate n, the moles of copper(II) sulfate. (1 mark)

ii)Now calculate q, the expected energy change in this reaction. (1 mark)

iii)Rearrange q = mcΔT to give ΔT. (1 mark)

iv) Given that c = 4.18 J K−1 g−1 calculate the maximum expected temperature rise for this reaction. (1 mark)

d) Calculate the maximum expected temperature rise if the experiment is repeated with:
i) 50 cm3 of 0.5 mol dm−3 copper sulfate solution (2 marks)

ii) 100 cm3 of 0.5 mol dm−3 copper sulfate solution. (2 marks)
Original post by Naomiaj
1) a) ΔH for the reaction between zinc and copper(II) sulfate solution is −219 kJ mol−1.
Construct a balanced equation for this reaction. (1 mark)
b) When the two are mixed together would you expect the temperature to rise or fall? Why? (2 marks)
c) 50 cm3 of a 1.0 mol dm−3 solution of copper(II) sulfate is reacted with an excess of zinc powder.
i) Calculate n, the moles of copper(II) sulfate. (1 mark)
ii)Now calculate q, the expected energy change in this reaction. (1 mark)
iii)Rearrange q = mcΔT to give ΔT. (1 mark)
iv) Given that c = 4.18 J K−1 g−1 calculate the maximum expected temperature rise for this reaction. (1 mark)
d) Calculate the maximum expected temperature rise if the experiment is repeated with:
i) 50 cm3 of 0.5 mol dm−3 copper sulfate solution (2 marks)
ii) 100 cm3 of 0.5 mol dm−3 copper sulfate solution. (2 marks)

What have you tried so far and where are you stuck?

(a) Is a typical metal displacement reaction, e.g

CuSO4 (aq) + Zn (s) —> ZnSO4 (aq) + Cu (s)

(b) Consider the sign of the enthalpy change - does it suggest the reaction is exothermic or endothermic? How is that relevant?

(c) (i) Start from concentration (in mol dm^-3) = moles / volume (in dm^3)

(c)(ii) Start from enthalpy = -(energy released in kJ) / moles

(c)(iii) What can you divide both sides of the equation by so that you leave only ΔT on the right?

(c)(iv) Assume that the solution has a density of 1 g cm^-3 and so the 50 cm^3 of copper sulphate solution weighs 50 g. How might your answers to (c)(ii) and (c)(iii) help you?

(d)(i) and (ii) - repeat the process you used in parts (c)(i) - (iv), but account for the new masses of each solution (again, assume the density is 1 g cm^-3) and new concentrations.

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