Enthalpy and temperature change

Click to return to index

Suppose you don't let any of the heat (- 891 kJ mol-1) escape from the flame (this is adiabatic conditions). You know that the mixture will be hot. To work out how hot, you need to know what you are heating up and what the heat capacities of the materials are.

In air, for every one oxygen there are four nitrogen. I could therefore write the reaction as

CH4(g) + 2 O2(g) + 4 N2(g) ® CO2(g) + 2 H2O(g) + 4 N2(g)

The heat produced by the reaction is heating the reaction products and the nitrogen. Note that it's going to be hot so the water produced will be a gas.

Standard molar heat capacities

 

Cp / J K-1 mol-1

 

CO2(g)

37

Click for answer

N2(g)

29

 

H2O(g)

34

Click for help

Calculate the reaction temperature if the process is adiabatic.