4.12 calculate molar enthalpy change from heat energy change

Enthalpy Calculations

For reactions carried out at constant pressure the heat energy change is called the enthalpy change.

To standardise this we find the molar enthalpy change which is referred to asH. 

Heat Given Out = Mass x Specific Heat Capacity x Temperature Rise

To convert the heat given out into molar enthalpy change you have to find out how energy 1 mol of the substance produces.

e.g.

Reaction between Magnesium and Hydrochloric Acid

Temp Rise = 10 degrees

Moles of acid = 0.005

Heat given out = 50 x 4.2 (specific heat capacity of water) x 10J = 2100J = 2.10kJ

0.005 mol of acid produces 2.10kJ

1 mol of acid produces = 2.10/0.005 = 420kJ

H = -420kj/mol