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Exothermic and Endothermic Reactions

02nd Jan 2022 @ 10 min read

Physical Chemistry

Exothermic and endothermic reactions are basic concepts taught in chemistry. In this article, we will start with definitions of them and explore some examples and try to understand them in the view of thermodynamics.

Definition

The term exothermic consists of two parts: “Exo,” which means outside, and “thermic,” which means heat. So, an exothermic reaction is any reaction that releases heat outside.

In formal words, we define an exothermic reaction as a reaction that has the heat of reaction (or enthalpy of reaction) negative. This statement will be explained in the latter part of the article.

In contrast, an endothermic reaction is a reaction that has the heat of reaction positive. In endothermic reactions, the heat is absorbed from the surroundings into the system.

The processes that describe these reactions are called exothermic and endothermic processes.

Examples:

Here are some typical examples of exothermic and endothermic reactions.

Combustion of fuel

If you have a vehicle that runs on gasoline or any other petrochemicals, you have experienced that your vehicle’s engine gets heated after a drive. And there is a cooling system installed to maintain its temperature.

Now, the question is why the vehicle gets heated. The answer is in chemistry. The engine fuel is a mixture of various hydrocarbons and petrochemicals. We can represent them empirically as CxHy (C is carbon and H stands for hydrogen, while x and y are arbitrary integers). On combustion, a large amount of energy is released from them. Most of this energy is utilized to drive the vehicle via an internal combustion engine. But some of it gets dissipated in the form of heat to surroundings, which causes heating of the vehicle.

The chemical reaction for combustion is as follow:

CxHy + (x+0.25y) O2 --> x CO2 + 0.5y H2O, (ΔH = − ve)

Where does the heat energy come from? In an exothermic reaction, heat is released from the reacting chemicals themselves. In chemicals, the energy is stored in the form of chemical bonds. Under certain conditions, these bonds could break, and the energy is liberated, usually in the form of thermal energy (heat). The breaking of bonds leads to the formation of new bonds, i.e., other chemicals. In the above example, the hydrocarbons break to form smaller species like carbon dioxide and water vapor.

The energy of products is always less than the energy of reactants in an exothermic reaction. That’s why we say an exothermic reaction is always accompanied by a negative heat of reaction (ΔH = − ve).

Neutralization Reaction

Neutralization reactions are very popular reactions demonstrated in chemical laboratories. In neutralization, the acid reacts with the base to form salt and water.

Acid + Base --> Salt + Water, (ΔH = − ve)

A classical example is a reaction between sodium hydroxide (NaOH) and hydrogen chloride (HCl).

HCl + NaOH --> NaCl + H2O, (ΔH = − ve)

Neutralization is an exothermic change. The enthalpy change is negative. If you have ever carried out titration between less weak acid and base, you might have experienced warming of the beaker.

Explosion

The explosion is always an exothermic phenomenon. Explosives are made of substances that contain a very high amount of chemical energy per unit volume. When triggered, this immense energy is unleashed into the environment in the form of heat and light. This covers firecrackers to lab reactions, to nuclear bombs.

Sodium in water is a popular example of exothermic reactions.

Explosion
Sodium in water (Image credit: Paulin/Flickr)

Photosynthesis

Photosynthesis is a chemical process in which plants convert carbon dioxide into glucose by absorbing sunlight. Photosynthesis is considered an endothermic process since energy in the form of light is absorbed. The enthalpy of products is more than the enthalpy of reactants, making the heat of reaction positive.

6CO2 + 6 H2O + light --> C6H12O6 + 6O2, (ΔH = + ve)

Cracking

Cracking is a vital part of chemical operations in the petroleum industry. In cracking, large chain alkanes are broken into smaller alkenes. The cracking reactions result in the breaking of carbon-carbon and carbon-hydrogen bonds, and the energy is absorbed during the process. Thus, it is an endothermic reaction.

Salt to water

Adding salt to water is endothermic.When we add salt into water, it dissociates into ions. This involves breaking ionic bonds. Ionic bonds are governed by the electrostatic force between anion and cation. To overcome this force, external energy must be supplied or absorbed. The ionic components break and absorb energy from the surrounding causing cooling of the solution.

Some examples are NaCl in water, KCl in water, NH4NO3 in water, etc.

Thermodynamic Explanation

Till now, we have understood some basic overview of the topic. Thermodynamics will give us more insight into them.

As explained earlier, chemicals store energy in them in the form of chemical bonds. The bonds that are weak and unstable are likely to go under a chemical change. The breaking of weaker and unstable bonds results in the formation of new, stable, and strong bonds. The energy difference between them is released into the environment.

In an endothermic process, everything is the reverse of the above—the product has more energy than the reactant.

As chemists, we are interested in measuring the energy difference between reactants and products, which is termed as the heat of reaction or the enthalpy of reaction.

Heat of reaction (ΔH) = Enthalpy of product − Enthalpy of reactant

If ΔH is negative, the reaction is exothermic, and it is endothermic if positive.

We can better understand this by the energy profile diagram.

Exothermic Reaction

Energy profile in an exothermic reaction As we can see in the above figure, the reactants are at higher energy than the products.

The enthalpy difference between them is the heat of reaction or heat released. Initially, the energy of the reaction mixture is increased. This increment is called activation energy, which can be thought of as an initial push or trigger. After activation, the reaction mixture reduces into products, and the energy is released in this phase of the reaction.

Endothermic Reaction

Energy profile in an endothermic reaction.

The energy profile in an endothermic reaction is the opposite of exothermic, and the reactants are at lower energy than products. The difference between them is the energy supply or heat absorbed.

Spontaneity

The spontaneity of the reaction is measured by the change in enthalpy and entropy. If a chemical reaction has a negative heat of reaction, it signifies the reaction is likely to be spontaneous. On the other side, reactions with positive enthalpy change are less spontaneous.

Note: The spontaneity of a chemical change is not only governed by the change in the enthalpy but also the change in entropy. There are chemical reactions that have a negative heat of reaction but are not spontaneous.

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Physical Chemistry Chemical Reactions Exothermic Endothermic

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