Heat Of Reaction Formula and Sample Example

Heat Of Reaction Formula :

The heat of Reaction formula quality of change heat Q is equal to the m (kg) in kilogram and multiply the specific heat capacity of the medium c (j/K.g) in joule per kilogram and multiply the difference temperature of the medium ΔT(k or oc ) in Kelvin or degree Celsius. Hence the Heat of Reaction formula can be written as

Q= m (kg)*c (j/K.g) *ΔT(k or oc )


M → mass of the medium in kg

C→ specific heat capacity of the medium in J/Kg.K

ΔT → difference in temperature of the medium in K or 0C

Sample Example :

Example 1:

Calculate the heat change which accompanies the combustion of ethanol when a certain mass of a substance is burnt in air to raise the temperature of 400g  of water initially at 300  c  to 40 0C given that the specific heat capacity of water is 4.2 Jg-1 K-1?

Solution :

Given parameters are


C=4.2 Jg-1 K-1


ΔT=100 c or 10K

According to the question, a certain mass of ethanol is burnt to raise the temperature of the water, which means heat absorbed by water is evolved from the combustion reaction of ethanol.

Heat lost in the combustion reaction is equal to heat gain by water

Quantity of heat changed can given by


Q=400 ×4.2 ×10


Example 2:

Calculate the heat change during the process of dissolution if a specific heat capacity of the solution is assumed to be is 4.18 Jg-1 K-1? In a sodium chloride is dissolved in 200g of water at 450 C the solution obtained after proper stirring have  a temperature of 380C.

Solution :

Given parameters are


C=4.18 Jg-1 K-1



The process involves drop in temperature which indicates that dissolution of salt absorbed heat from the system

Since, heat absorbed by the salt is equal to the heat lost by water

We have the formula


Q=200 ×4.18 ×7


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