Calculating heat absorbed or released during reaction


Bomb calorimeter
Reaction occurs in a vessel surrounded by water
Polystyrene cup calorimeter
Reaction occurs in water



 
For the purpose of describing energy changes in a chemical reaction,the reaction is the system.
The internal energy of a chemical reaction system is the kinetic and potential energy of the substances of which it is composed.

During a chemical reaction 
Energy released as heat (q) may be transferred to the surroundings.
Energy required may be transferred as heat from surroundings.

 
In either case:
q(reaction) = q(system)
q(system) = -q(surr)

The quantity of heat transferred and the direction of heat transfer as a result of a reaction can be calculated if the reaction is done in a calorimeter, and the temperature change of the surroundings is measured. 

The calculation uses 
 
  • the temperature change in K (or °C):
    ΔT = T(final) - T(initial)
  • the heat capacities (C in J K–1) of the calorimeter and  surroundings of the reaction
    C for a specific mass of substance is calculated from its mass and the specific heat capacity (c in J K–1 g–1)
     

For calorimeters where the surroundings is water AND the temperature change of the calorimeter itself can be ignored
q(surroundings) = q(water)
q(surr) =m(water)× c(water) × ΔT(water) 
q(surroundings) may be either positive or negative because ΔT may be positive or negative 

The temperature change of polystyrene cup calorimeters can be ignored!! 
Dr Sheila Woodgate
© The University of Auckland 2002