Hess's Law Question

Question

1.255 g Mg turnings are added to 101.450 grams of hydrochloric acid solution. The temperature of the solution rises from 20.2 °C to 75.9 °C. What is the heat flow for the reaction, q_rxn, in kJ?

EXPRESS YOUR ANSWER AS A NEGATIVE NUMBER.

Answer 1

The reaction is Mg (s) + 2 HCl (aq) ====> MgCl2 (aq) + H2 (g)

The total mass of the mixture is 1.255 + 101.450 = 102.705 g or 0.102705 kg

As we don't know the specific heat of the mixture, and the mixture is aqueous, we can assume that the specific heat is the same as water which is 4.182 kj / kg.

The heat released by the reaction can be found by the equation ΔH = cm Δt = -4.182 kj/kg°C x 0.102705 kg x (75.9 - 20.2)°C = - 23.9 kj.