Calculating heat absorbed or released during reaction
Polystyrene cup calorimeter
Reaction occurs in water
Reaction occurs in water
For the purpose of describing energy changes in a chemical reaction, the reaction is considered to be the system.
During a chemical reaction
The quantity of heat transferred during a reaction occurring in aqueous solution can be calculated, and the direction of heat transfer determined if the reaction is done in a polystyrene cup calorimeter and the temperature change of the solution is measured. The calculation uses
During a chemical reaction
Energy released as heat (q) may be transferred to the surroundings.
Energy required may be transferred as heat from the surroundings.
In either case heat transfer is complete and:
Energy required may be transferred as heat from the surroundings.
In either case heat transfer is complete and:
q(reaction) = q(sys) = -q(surr)
sys = system, surr = surroundings
sys = system, surr = surroundings
The quantity of heat transferred during a reaction occurring in aqueous solution can be calculated, and the direction of heat transfer determined if the reaction is done in a polystyrene cup calorimeter and the temperature change of the solution is measured. The calculation uses
- the temperature change in K (or °C):
Step 1: - ΔT = T(final) - T(initial)
- the specific heat capacity of the solution (c in J K–1 g–1)
This is usually assumed to be the same as water.
- If the temperature change of the calorimeter itself is ignored,
Step 2:
q(surr) = q(soln)
q(surr) = q(soln)
q(soln) = m(soln) × c*(soln) × ΔT(soln)
q(surroundings) may be either positive or negative because ΔT may be positive or negative
*Note that c(solution) is the specific heat capacity of the solution - NOT the concentration of the solution.