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Heat of Reaction formula

The heat of reaction which is also known as Enthalpy of Reaction is the change in the enthalpy of a particular chemical reaction that occurs at a constant pressure. It is the thermodynamic unit of measurement applied in calculating the amount of energy per mole either released or produced in a reaction.  

Heat of a chemical reaction can therefore be defined as the heat evolved in the surroundings or absorbed when the reaction takes place at constant pressure and temperature. The amount of heat absorbed or evolved is measured in Joule (J). Mostly heat transfer takes place between the reacting system as one medium and surrounding as the other in chemical reactions.

Please note that the amount of heat energy before and after the chemical change remains same. In other words, the heat lost or gained in a reacting system is equal to heat lost or gained in the surrounding.

Therefore, the heat of reaction formula is given by

Q = mcΔT


m is the mass of the medium,

c is the specific heat capacity of the medium,

ΔT is the difference in temperature of the medium.

Example 1

Calculate the heat change which accompanies the combustion of ethanol when a certain mass of substance is burnt in air to raise the temperature of 200g of water initially at 28oC to 42oC, given that the specific heat capacity of water is 4.2Jg-1K-1.


Given parameters are

m = 200g

c = 4.2 Jg-1K-1

ΔT = 42 – 28

      = 14oC or 14 K

According to the question, a certain mass of ethanol is burnt to raise the temperature of 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 be given by


Q = 200 × 4.2 × 14

Therefore, Q = 11760 J

Example 2:

If Sodium chloride is dissolved in 100g of water at 25oC, the solution obtained after proper stirring have a temperature of 21oC. Determine the heat change during the process of dissolution if specific heat capacity of the solution is assumed to be 4.18 Jg-1K-1.


Given parameters are,

m= 100g

c= 4.18 J g-1K-1

ΔT = 25 – 21

= 4 K

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 Heat lost by water

We have the formula,

Q = mCΔT

= 100 × 4.18 × 4

Therefore, Q = -1672J