Enthalpy change calculations
1) When gaseous hydrogen and oxygen are heated in a test tube, droplets of liquid water form on the sides
of the test tube.
Calculate f H (H2O(ℓ)), given the following data:
f H  (H2O(g)) = – 242 kJ mol–1
vap H  (H2O(ℓ)) = + 44 kJ mol–1
2) i)
Ammonia can be oxidised to produce nitrogen, N2, and steam as shown in the equation below:
4NH3(g)
+
3O2(g)

2N2(g)
+
6H2O(g)
D r H = –1267 kJ mol–1
Calculate the energy produced when 50.0 g of ammonia reacts as shown in the equation above.
3) fH (HBr, g) is –36.2 kJ mol–1. Calculate the heat produced by the formation of 50.0 g of HBr(g)
from its elements in their standard states.
4) The experimental value for fH(H2O, ℓ) is –286 kJ mol–1 and the enthalpy of formation of water in
the gas state, fH (H2O, g) is –241 kJ mol–1
Using the information, calculate the vapH(H2O), and also the heat required to vaporise 100 g of water.
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