Question

For the reaction A + 2B → C, 0.050 mol A and 0.10 mol B are mixed in aqueous solution in a coffee cup calorimeter containing a total of 50 mL of water. The temperature of the water increases by 5.5K. What is ΔH_rxn?

a. +1200 kJ/mol

b. -1.2 kJ/mol

c. -23 kJ/mol

d. none of the above

Answer 1

Given reaction is:

A+2B → C

Initial concentration of A = 0.05 mol

Initial concentration of B = 0.1 mol

Total volume of water = 50 mL

∆T of water = 5.5 K

The heat flow of the reaction is calculated by:

q = mc∆T

Here m = mass of water = density x volume = (1g/mL)x50mL = 50 g

c = specific heat of water = 4.184 J/^oCg = 4.184x10³ J/Kkg

q = (50x10^-3kg)x(4.184x10³ J/Kkg )x5.5 K = 1150.6 J = 1.1506 kJ

That is the product of the reaction evolved 1150.6 J of heat, which was lost to the water.

Therefore: ∆H_rxn = -q_water = -1.1506 kJ

Expressing the value in kJ/mol as:

∆H_rxn = -q_water/n = -(1.1506 kJ)/0.05 mol = -23 kJ/mol.

So option c) is correct.