Answer to Question #23939 in Physical Chemistry for Simon

Question

In all the following questions, assume that the densities and specific heat capacities of the solutions are the same as pure water
i.e. ρ = 1.0 gcm-3 and c = 4.18 Jg-1K-1

1. Zinc will displace copper from copper (II) sulphate solution according to the following equation:
CuSO4(aq) + Zn(s)  Cu(s) + ZnSO4(aq)
If an excess of zinc powder is added to 50 cm3 of 1.0 moldm-3 copper(II) sulphate, the temperature increases by 6.3 oC. Calculate the enthalpy change for the reaction.

Answer 1

q (enthalpy change) = (mass of solution) x (specific heat capacity) x (temperature change)

q = 50g (from the 50cm cubed) x 4.18 x 6.3 = 1316.7 J

also, number of moles = concentration x volume divided by 1000

n = 1.0 x 50 / 1000 = 0.05 moles

so 0.05 moles = 1316.7 J

1 mole = 1316.7 divided by 0.05 = 26334 J and if we divide that by 1000 = 26.334 kJ

or q = - 26.334kJ/mol