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Exothermic Reaction

Exothermic Reactions- Exothermic and Endothermic Reactions, Exothermic Reactions, Identification, Examples, Practice Problems, FAQs

It's a very cold day and she is clad in a jacket. It’s night time and seen warming herself in front of a fireplace.

Burning of fuelsNone of the above

It is pretty cold and snowing out there at this beautiful hill station and this piercing cold is going down all the way to her bones.

But this fire here is surely keeping her warm. The heat feels heavenly in this weather.

But how do we look at this process from a thermodynamic point of view?

Let’s look at what is happening to this wood first. What the fire is doing is consuming this wood and giving out heat. This means that this combustion process is giving out heat and so is called exothermic in thermodynamics.

Exo stands for external and’ thermo’ for heat. That makes the word exothermic refer to a process where there is an exit of heat or one that gives out heat to the surroundings.

But if you think of her as the system, she is absorbing the heat. This process is called endothermic in thermodynamics.

Endo stands for internal and therefore the word endothermic collectively refers to the entry of heat or a process that absorbs heat.

So ya, chemical processes or reactions are broadly classified into these two types: Exothermic & Endothermic.

Now, I hope this raises a logical question in your mind:

How and why is heat released or absorbed in a chemical reaction? Let’s discuss it in detail!

And let me reveal to you that we have a separate branch of chemistry concerned with the actual quantities of heat released or absorbed during chemical reactions -‘Thermochemistry’.

Table of Contents

  • Exothermic and Endothermic Reactions
  • Exothermic Reactions
  • Identification of Exothermic Reactions
  • Examples of Exothermic Reactions
  • Practice Problems
  • Frequently Asked Questions – FAQs

Exothermic and Endothermic Reactions

Burning of any material involves chemical transformation and also releases heat energy during the reaction. Many, chemical reactions also involve the release of energy. Neutralisation of acid and a base also releases heat energy. The exothermic reaction is defined as a reaction where the system (where the reaction is occurring) releases the energy into the surrounding in the form of heat.

There are also chemical reactions which can take place only on receiving heat. Calcium carbonate, for example, decomposes to form carbon dioxide and oxygen only when heated or only by absorbing ea energy from an external source.

Such, chemical changes, requiring heat energy to undergo are called endothermic processes in thermodynamics.

So, chemical processes or reactions are broadly classified into these two types: Exothermic & Endothermic reactions.

So, the endothermic reactions absorb energy from the surrounding in the form of heat. On the other hand, an exothermic reaction releases heat energy into the surrounding of the system.

Photosynthesis is an example of an endothermic process. In the photosynthesis process, plants absorb energy from the sun and convert carbon dioxide and water into glucose and oxygen. The reaction between sodium and chlorine, which produces a bright yellow light and heat energy is an example of an exothermic reaction.


Exothermic Reactions

Let’s consider a reaction:

A + B → C + D; ΔH = -ve

ΔH=HP-HR=-ve for an exothermic reaction


HP = Summation of enthalpies of product

HR = Summation of enthalpies of reactants

In the exothermic reactions, the total enthalpy of the products is less than the total enthalpy of the reactants. i.e., HP<HR

The difference in the enthalpy heat energy is released, as a result of the reaction happening where reactants are converted into products. It occurs as a result of the dissociation of the bonds between the molecules. The heat energy is then released through the formation of new bonds.

The energy released is expressed as negative ΔH (-ΔH), with the reaction as given below.

CH4 + 2O2 → CO2 + 2 H2O, - ΔH= X joules

The negative sign indicates that the heat energy is released and it is an exothermic reaction.

Identification of Exothermic Reactions

Procedures to differentiate between exothermic and endothermic reactions are as follows:

  • Monitoring the change in temperatureThe temperature of a system (where the reaction is occurring) rises in an exothermic reaction when the heat energy is given out to the surrounding. Whereas the temperature of the reaction decreases when energy is absorbed. You can monitor changes in temperature by placing a thermometer in the reaction mixture.
  • Enthalpy change of reaction (rH )The enthalpy change when the reactants are converted into products in a particular chemical reaction at any temperature and pressure is known as the enthalpy change of a reaction, which is represented as and is calculated as follows:

For example, consider the reaction given,

N2(g) + 3H2(g) → 2NH3(g)

  • rH = 2Hf(NH3, g) – [Hf(N2, g) + 3Hf(H2, g)]
  • Where Hf = Enthalpy of formation

The reactions that absorb heat energy are known as endothermic reactions. In such cases, the enthalpy of the reactants is less than the enthalpy of the products so that rH is positive.

The reactions that release heat, i.e., the reactions that are accompanied by the evolution of heat are known as exothermic reactions. In such cases, the enthalpy of the products is less than the enthalpy of the reactants, so that rH is negative.

Examples of Exothermic Reactions

- Real-life Examples:

1. Burning of a Candle: The burning of the candle releases energy into the atmosphere, something that can be felt when you place your finger close to the flame. The flame is hot, implying heat is released into the environment and is an example of an exothermic reaction.

2. Formation of ice cubes

As previously explained, any change in the state of matter is associated with the absorption or release of heat. Similarly, for water to be converted to ice cubes in the fridge, heat is released.

3. Laundry detergent and water

When you place a little bit of the detergent in the palm of your wet hand, you can feel slight warmth. This is because when detergent reacts with the molecules of water, it releases energy in the form of heat.

Chemical Reactions that are Exothermic:

1. The dilution of sulfuric acid and water.

2. The reaction of calcium oxide and water.

CaO(s) + H2O(l) Ca(OH)2 (aq.) , H= -ve

Change in enthalpy (H) for this reaction is negative as a lot of heat liberate with hissing sound during the reaction.

3. Formation of sodium chloride from sodium and chlorine

.4. A few other examples are neutralisation reactions, burning of substances, burning of fuels, deposition of dry ice, respiration, dissolution of sulphuric acid into water etc.

Practice Problems

Q1. The reactions which can not occur spontaneously are:

  1. Exothermic
  2. Endothermic
  3. Isothermic
  4. None of the above.

Answer: (B)

Solution: Endothermic reactions absorb heat from the surroundings, so this reaction will occur only when the heat will be provided. So they do not occur spontaneously.

Q2. Which of the following is not an example of an exothermic reaction?

a. Reaction of carbon with oxygen forming carbon dioxide
b. Melting of ice
c. Burning of fuels
d. None of the above

Answer: (B)

Solution: The melting of ice is the process, where ice melts into the water at its melting point i.e., 100 ℃. This melting process happens when the ice absorbs the heat from the surrounding. When the heat is absorbed, the force that holds the molecules together weakens up and solid ice is converted into liquid water, the process of absorbing heat from the surroundings is known as the endothermic process. In all other options, heat energy is being released making them exothermic processes.

Q3. Which of the following reaction is an endothermic reaction?

a. C+O2→ CO2
b. N2+O2 → 2NO
c. 3H2+N2 → 2NH3
d.  PCl3+Cl2 → PCl5

Answer: (B)

Solution: An endothermic reaction is the type of process where an increase in the enthalpy of the system occurs. Enthalpy change of those reactions is positive. The formation of NO is an example of an endothermic reaction as energy will be required to break the triple bonds in N2.

Q4. The enthalpy of reactants is 15 JKg-1 and the enthalpy of products is 40 JKg-1, what is the heat of reaction?

A) -10 JKg-1
B) 10 JKg-1
C) -25 JKg-1
D) 25 JKg-1

Answer: (D)
Formula for calculating the heat of reaction is:


Given, enthalpy of products (HP) = 40JKg-1

Enthalpy of reactants  (HR) = 15JKg-1

Heat of reaction (ΔH)= 40 - 15 = 25 JKg-1

Frequently Asked Questions – FAQs

Q1. In a chemical reaction, why is heat released or absorbed?
Chemical bonds are either broken or formed in any chemical reaction. Heat is released when chemical bonds are formed, and heat is absorbed when chemical bonds are broken. Since molecules want to remain together, forming chemical bonds between them requires less energy than breaking them, resulting in heat being absorbed from the surroundings.

Q2. Define the enthalpy of a reaction.
The enthalpy of a reaction is defined as the heat energy change that occurs when reactants become products. ΔH is positive if heat is absorbed during the reaction; if heat is released, ΔH is negative.

Q3.During the endothermic reaction, what type of energy conversion will take place?
This is the conversion of kinetic energy to chemical energy. Heat is absorbed and converted into chemical energy during an endothermic reaction.

Q4. Why is respiration regarded as an exothermic reaction?
Since energy is released during the process of respiration, it is an exothermic process. The carbon dioxide in the food is broken down to form glucose during the process. This glucose combines with oxygen in our cells, releasing a large amount of energy.

Q5.What is the main difference between an exothermic and an endothermic process?
The main difference between exothermic and endothermic reactions is that an endothermic reaction absorbs energy in the form of heat from the surroundings, whereas an exothermic reaction releases energy to the surroundings.

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