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Endothermic Reactions

Endothermic Reactions: Introduction, Energy Diagram, Applications, Identification, Practice Problems & Frequently Asked Questions(FAQs)

Have your neck or shoulder ever felt strained? 

Do you suffer from back pain? 

What would you require in order to get rapidly relieved from that agonizing pain? Any ideas? 

You need an instant cold pack, however!


An instant cold pack, as its name implies, is essentially a pair of pouches connected by a thin membrane that we typically use to relieve back and ankle discomfort.

One of them is made of water, while the other is made of ammonium nitrate. You might be wondering how an immediate cold pack fits into the category of an endothermic reaction at this point. Let's see how this works, though.

Water turns ice-cold as a result of a chemical reaction that occurs when the tiny barrier that was preventing water and ammonium nitrate from mingling is ruptured, making it useful for treating injuries like ankle sprains and painful backs.

In other words, the interaction between ammonium nitrate and water absorbs heat from the water, making the entire process an endothermic reaction.

Table of Contents:

  • Introduction of Exothermic and Endothermic reactions
  • Energy Diagram for Endothermic reactions
  • Applications of Endothermic reactions
  • Ways to identify Exothermic and Endothermic reactions
  • Practice Problems
  • Frequently Asked Questions(FAQs)

Introduction of Exothermic and Endothermic reactions:

Energy transfer occurs in all physical or chemical changes. Energy is neither created nor destroyed during any physical or chemical activity, according to the law of conservation of energy. In other words, the universe's total energy is conserved. Definitions of the system and the surroundings are necessary in order to better comprehend the energy changes that occur throughout a reaction. The precise area of matter being researched during an experiment or observation is referred to as the system. The whole of the universe outside the system is referred to as the surroundings.

For a chemist, the system consists of the specific chemicals that are reacting and that are formed, and the surrounding refers to the area around those chemicals in the room. Energy is transferred between the system and its surroundings during the majority of processes. If the system loses a specific quantity of energy, it implies that the environment will gain the same amount of energy. The reverse is also true, that if the system gains a given amount of energy, then the same amount of energy is provided by the surroundings.

When heat is taken in by the system from the surroundings, a chemical reaction or physical change is said to be endothermic. An endothermic process causes the system to absorb heat from its surroundings, which causes the surroundings' temperature to drop. The letter ‘q’ stands for the amount of heat a process requires. An endothermic process has a positive sign of ‘q’ because the system is gaining heat. If heat is released from the system into the environment, the reaction or change is exothermic. The temperature of the surroundings rises as a result of the system's heat transfer to the environment. Due to the system's heat loss, an exothermic process has a negative sign for ‘q’.


Energy Diagram for Endothermic reactions:

A reaction that is endothermic will absorb heat from its surroundings. This type of reaction involves a change in positive energy, which results in a rise in the enthalpy of the system. A net energy absorption process occurs in an endothermic reaction because more energy is required to break the bond than is released during production.

Let’s consider a reaction: 

A  + B  →  C +  D;  ΔH= +ve 


for an endothermic reaction. 

Enthalpy increases during the transformation of reactants into products in endothermic processes, i.e.

Because they generate more energy products than reactants, endothermic reactions are frequently less spontaneous. An endothermic reaction takes heat from the surrounding environment, lowering the temperature in the system in which the reaction is occurring because the products of this reaction have much more energy than the reactant. As a result, the enthalpy change is positive. Enthalpy, the difference in thermal energy during the transmission of reactions to the product, also rises as the reaction proceeds.

Applications of Endothermic reactions:

1. Photosynthesis: In contrast to mammals (like humans), plants produce their own food. Photosynthesis, the method through which plants produce their food, is seen as the outcome of an endothermic reaction.

Energy from the sun, or sunshine, is absorbed by the leaves during the procedure of photosynthesis, which eventually produces food when combined with water and carbon dioxide. Consequently, a plant develops as a result.

2. Sublimation: The mechanism of sublimation is the next example of an endothermic reaction. The sublimation of dry ice would be a classic example of sublimation. Just to be clear, dry ice is nothing more than solid form carbon dioxide marketed in a packaged form.

Sublimation is a sort of phase transition in which a substance bypasses the liquid stage and goes straight from the solid to the gaseous state. In the case of dry ice, the process of sublimation is an endothermic one in which solid carbon dioxide (dry ice) draws heat from the environment.

3. Drying of Clothes: Evaporation is an example of an endothermic reaction in practice. The process by which wet clothing is dried is a common illustration of evaporation. When a molecule directly converts from a liquid state to a gaseous state, it is known as evaporation.

When wet garments are being dried, the droplets of water inside the garment ( system) absorb heat from the air. As a result, the system's (wet clothes) enthalpy, or internal energy rises.

4. Cooking food: Food preparation involves an endothermic reaction. Let's say you're preparing Noodles. The noodles use or perhaps I should say absorb, the energy of the pan while it is cooking. As a result, it is an endothermic reaction.

Ways to identify Exothermic and Endothermic reactions:

Monitoring the change in temperature:

The reaction mixture’s temperature rises in an exothermic reaction as energy is liberated. In an endothermic reaction, energy is absorbed, and the temperature drops. Placing a thermometer in the reaction mixture allows you to keep track of temperature fluctuations.

Calculation of enthalpy change in reaction :

The enthalpy change , often known as the heat of reaction, is a measurement that illustrates the energy of the reactants with the energy of the products and can be used to categorize the net energy output or input of chemical reactions.

A measure of internal energy is enthalpy. In order to find the enthalpy change , you must first determine the difference between the enthalpies of the reactants and products. 

This can be done mathematically by writing:

ΔH = energy used to break the bonds of reactants + the energy released in making product bonds

The chemical reaction is exothermic if is negative, as more energy is produced during the formation of the products that are consumed to disintegrate the reactants. The chemical reaction is endothermic if is positive, meaning that less energy is released when the products are created than was required to break down the reactants.

Practice Problems:

Q1. The reaction shown below is an illustration of     


(A) Endothermic reaction
(B) Exothermic reaction
(C) Both Endothermic and Exothermic
(D) Neither Endothermic nor Exothermic

Answer: (B)

The reaction produces energy (), hence energy is released. If any reaction releases energy it is known as an exothermic reaction.

Q2. Among the following, which is considered an endothermic reaction?

(A) Combustion reaction
(B) Thermal decomposition
(C) Both A and B
(D) None of the above

Answer: (B)

When a decomposition reaction is triggered by thermal energy, it is referred to as a thermal decomposition reaction. In other words, heat must be applied to the reactants in a thermal breakdown reaction to provide energy. Due to the energy needed to dissolve chemical bonds and separate the constituent parts, such reactions are typically endothermic.

Q3. Which of the following would be best for performing a reaction where the temperature needs to be checked?

(A) A polystyrene cup
(B) A burette
(C) A glass beaker
(D) A measuring cylinder

Answer: (A)

This is due to the polystyrene cup's role in protecting the reaction mixture from fluctuations in temperature or heat. Therefore, a reaction occurring in a polystyrene cup that is covered will not be impacted by variations in ambient temperature.

Q4. The enthalpy of reactants is and the enthalpy of products is . What is the heat of reaction?





Answer: (D)

Formula for calculating the heat of reaction is:



Heat of reaction

Frequently Asked Questions(FAQs):

Q1. What causes the ammonium chloride and barium hydroxide reaction container to become cold? What sort of reaction is this?
Answer: Because the reactants absorb the energy needed for their reaction to take place from their surroundings, the vessel employed in the reaction between ammonium chloride and barium hydroxide gets chilly. Because the reactants are added to the reaction vessel, they absorb the energy from the vessel, making it seem cold. Due to the fact that this reaction absorbs heat, it is endothermic.

Q2. Is the process of respiration endothermic or exothermic?
Answer: It is exothermic. As we all know, when we breathe, we take in oxygen and exhale carbon dioxide. When glucose is burned with oxygen, it also produces water and carbon dioxide, which is the same carbon dioxide that we exhale when we breathe. We obtain energy from the food during this entire process, which generates a lot of heat. A reaction is described as being exothermic if heat is produced as a byproduct. As a result, breathing out or breathing in produces heat, which is then dispersed throughout the body as energy. This is why breathing in or out feels warm.

Q3. What causes endothermic reactions to be typically non-spontaneous?
Answer: Endothermic reactions are those that require substantial energy to occur. The external energy is frequently present as heat. Endothermic processes tend to draw heat from their surroundings, which causes their environments to chill. Endothermic reactions are typically non-spontaneous because the products they produce have higher energies than the reactants. Enthalpy changes for endothermic reactions are usually positive.

Q4. Why does energy release occur in some chemical processes while energy absorption occurs in others?
Answer: In general, an exothermic reaction occurs when a molecule of reactant in a certain reaction has greater energy than the molecules of the product, releasing the energy. In contrast, an endothermic reaction is one in which energy is absorbed from the environment because the molecules of the product have greater energy than the reactants.

The energy that needs to be given to the reactants in order for them to break through the energy barrier and react is referred to as activation energy. The potential energy of the product in exothermic processes is often lesser than that of the reactant, but in endothermic reactions, the potential energy of the product is greater than that of the reactants.

Related Topics:

Isothermal Process First Law of Thermodynamics
Thermodynamic Processes Degrees of Freedom
Heat Capacity Cp- Cv relation

Zeroth Law of Thermodynamics


Exothermic reactions


Difference between Exothermic and Endothermic reactions

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