Heat of Reaction
Therefore, the heat of reaction formula is given by
Q = mcΔT
Where,
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.
Solved Examples
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 200g of water initially at 28oC to 42oC, given that the specific heat capacity of water is 4.2Jg-1K-1.
Solution:
Given parameters are
m = 200g
c = 4.2 Jg-1K-1
ΔT = 42 – 28
ΔT = 14oC or 14 K
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 be given by
Q = mcΔT
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 a specific heat capacity of the solution is assumed to be 4.18 Jg-1K-1.
Solution:
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 the heat lost by water,
We have the formula,
Q = mCΔT
Q = 100 × 4.18 × 4
Therefore, Q = -1672J