Question

When 1.045g of calcium oxide is added to 50.0mL of water at 25.0°C in a calorimeter, the temperature of water increases to 32.3°C. Assuming density of solution is 1.00g/mL and a specific heat capacity of 4.19J/g˚C?

What is the enthalpy change (kJ/mol) for the reaction?

CaO(s) + H2O(l)  Ca(OH)2(aq)

Answer 1

The heat of reaction is `DeltaH = -95.3 (kJ)/"mol"`

Explanation:

The energy absorbed by the water is

`E=mxxcxxDeltat = 50g xx 4.19J/(g°C) xx 7.3°C = 1529.35 J` or 1.53 kJ (rounded to three digits).

Since this was due to 1.045g of CaO, which has a molar mass of 56.1 g, the number of moles of CaO used was

`1.045g/56.1 g/"mol" = 0.01605 "mol"`

Therefore, the heat of reaction is

`1.53kJ -:0.01605 "mol" = 95.3 (kJ)/"mol"`

or. more correctly, -95.3 kJ/mol since the reaction is exothermic.