Question

1. The heats of formation of CO₂(g) and H₂O(l) are -394 and -285.8 kJ/mole, respectively. Using the data for the following combustion reaction, calculate the heat of formation of C₃H₄(g).

C₃H₄(g) + 4 O₂(g) -> 3 CO₂(g) + 2 H₂O(l)                   DH^o = -1,939.1 kJ

2. A 10.0 gram sample of CaO is dissolved in water in a calorimeter whose specific heat capacity is 4.37 kJ/K. The temperature decreases 2.70 K. Calculate the DH^o for the following reaction.

CaO(s) + H₂O(l) -> Ca(OH)₂(aq)

Answer 1

Heat of combustion = sum of heats of formation of products- sum of heats of formation of reactants= 3* heat of formation of CO2+2* heat of formation of water- { 1* heat of formation of C3H4+4* heat of formation of O2}

Where 3,2, 1 and 4 are coefficients of CO2, H2O, C3H4 and O2 respectively

Since heat of formation of an element in its standard state =0

3*(-394)+2*(-285.8)- heat of formation of C3H4= -1939.1 Kj

-1753.6+1939.1= heat of formation of C3H4

Heat of formation of C3H4= 185.5 Kj/mole

2.

There is a drop in temperature, the heat is absorbed

Heat absorbed = specific heat of calorimeter* drop in temperature= 4.37* 2.7 KJ=11.8 KJ

This much heat is absorbed by 10 gm Cao. Moles of CaO= mass of CaO/molar mass= 10/56 = 0.18

deltaH= 11.8 Kj/0.18 mol = 65.55 Kj/mole ( since the reaction is endothermic, deltaH is +ve)