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Heat of Reaction Formula

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  • Last Updated : 27 Jan, 2022

The heat of reaction also known as Enthalpy of Reaction is the difference in the enthalpy value of a chemical reaction under constant pressure. It is the thermodynamic unit of measurement used to determine the total amount of energy produced or released per mole in a reaction. As a result, the heat of a chemical reaction may be defined as the heat released into the environment or absorbed while the reaction occurs at constant pressure and temperature. Joule (J) is the unit used to measure the total quantity of heat received or released. 

In chemical reactions, heat is mostly transferred between the reacting system as one medium and the surrounding as the other one. Before and after the chemical transformation, the amount of heat energy is the same. In other words, the heat acquired or lost in a reacting system is equivalent to the heat gained or lost in the surroundings.

What is Heat of Reaction?

In simple words, the Heat of Reaction is the heat required for effecting the given reaction which is negative for exothermic and positive for an endothermic reaction. Here for an endothermic reaction, ∆H is positive whereas ∆H is negative for those reactions which produce heat.

When the given reaction is carried out at constant volume, the heat required to affect the reaction is nothing but an increase in the Internal Energy (∆U) through the ∆H/∆U will be negative for Endothermic and positive for Exothermic reaction.

Formula for Heat of Reaction:

Q = m × c × ΔT


  • Q = Heat of Reaction,
  • m = mass of medium,
  • c = specific heat capacity of the reaction medium,
  • ∆T = difference in temperature of the medium.

Besides we also have another equation as,

Heat of Reaction = ΔH (products) – ΔH (reactants) 


  • ΔH = change in heat value

Sample Problems 

Problem 1: Calculate the heat change that occurs with ethanol combustion when a specified quantity of the substance is burned in air to increase the temperature from 28 to 42 degrees Celsius of 200 g of water, provided that water has a specific heat capacity of 4.2J/g.K.


It is given that, 

c = 4.2 Jg-1K-1,

m = 200 g,

ΔT = 42 – 28 ,

i.e. ΔT = 14 °C or 14 K

Here in the question, it is mentioned that a certain amount of ethanol is burned in order to raise the temperature of the water, implying that heat absorbed by water is evolved by the ethanol combustion process. The amount of heat lost in the combustion process is equal to the amount of heat gained by the water.

The amount of heat that has been changed can be determined by using formula,

Q = m × c × ΔT

Q = 200 × 4.2 × 14

Hence, Q = 11760 J

Problem 2: When sodium chloride is dissolved in 100 g of water at 25 °C, the resulting solution has a temperature of 21 °C after proper stirring. If the solution’s specific heat capacity is assumed to be 4.18 J / g °C, calculate the heat change during the dissolution process.


Here it is given that,

c = 4.18 J / g °C,

m = 100 g,

ΔT = 25 – 21,

i.e. ΔT = 4 K

The process results in a temperature drop, indicating that the salt dissolution tends to heat absorption from the system. Since, the heat lost by water is same as the heat absorbed by salt,

We have,

Q = m × c × ΔT

Q = 100 × 4.18 × 4

Hence, Q = 1672 J

Problem 3: When 240 grams of iron cools from 90 °Celsius to 25 °Celsius, how much heat is released? (Given: c = 0.452 J / g °C)


We have,

m = 240 g,

Specific heat capacity of Iron (c) = 0.452 J / g °C,

ΔT = Final temperature – Initial temperature = 25 – 90 = -65 °Celsius

We have the formula,

Q = m × c × ΔT

By putting given values in above equation we get,

Q = 240 × 0.452 × (-65)

hence, Q = -7051.2 J

i.e. Q = -7.05 KJ

Hence, 7.05 KJ heat is released when the process takes place.

Problem 4: With 650 KJ of energy, how much carbon can be heated from 20 degrees C to 100 °C? (Given: c = 4.184 J / g °C)


Here we are given with,

c = 4.184 J / g degrees C,

q = 650 KJ = 650000 J

ΔT = 100 – 20 = 80 Degrees Celsius

We are asked to find the Mass ( m ) so we have the formula,

Q = m × c × ΔT

the above equation will give us,

m = Q / ( c × ΔT )

by putting given values in above equation, we will get the actual mass of carbon required,

m =  650000 / ( 4.184 × 80 ) 

m = 1941.9 g 

i.e. m = 194 kg

Problem 5: What is the specific heat capacity of 60 grams of a substance that heats up from 30 °C to 40 °C when 968 J of energy were added?


It is given in the question that, 

m = 60 g

ΔT = 40 – 30 = 10 degrees Celsius

q = 968 Joules

We have to find the Specific heat capacity ( c ) so we have the formula,

Q = m × c × ΔT

the above equation will give us,

c = Q / ( m × ΔT )

by putting given values in the above equation we will get,

c = 968 / ( 50 × 10 )

c = 1.936 J / g °C

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