Exothermic reaction

A thermite reaction using Iron(III) Oxide

An exothermic reaction is a chemical reaction that releases energy in the form of heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:

reactants → products + energy

Overview

An exothermic reaction is a chemical reaction that is accompanied by the release of heat. In other words, the energy needed for the reaction to occur is less than the total energy released. As a result of this, the extra energy is released, usually in the form of heat.

When using a calorimeter, the change in heat of the calorimeter is equal to the opposite of the change in heat of the system. This means that when the medium in which the reaction is taking place gains heat, the reaction is exothermic.

The absolute amount of energy in a chemical system is extremely difficult to measure or calculate. The enthalpy change, ΔH, of a chemical reaction is much easier to measure and calculate. A bomb calorimeter is very suitable for measuring the energy change, ΔH, of a combustion reaction. Measured and calculated ΔH values are related to bond energies by:

ΔH = energy used in bond breaking reactions − energy released in bond making products

A sketch of an exothermic reaction

by definition the enthalpy change has a negative value:

ΔH < 0

For an exothermic reaction, this gives a negative value for ΔH, since a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction). For example, when hydrogen burns:

2H₂ + O₂ → 2H₂O

ΔH = −483.6 kJ/mol of O₂

Examples of exothermic reactions

• Combustion reactions of fuels
• Neutralization reactions such as direct reaction of acid and base
• Adding concentrated acid to water
• Adding water to anhydrous copper(II) sulfate
• The thermite reaction
• Reactions taking place in a self-heating can based on lime and aluminum
• The setting of cement and concrete
• Many corrosion reactions such as oxidation of metals
• Most polymerisation reactions
• The Haber-Bosch process of ammonia production

Key points

• The concept and its opposite number endothermic relate to the enthalpy change in any process, not just chemical reactions.
• In endergonic reactions and exergonic reactions it is the sign of the Gibbs free energy that determines the equilibrium point, and not enthalpy. The related concepts endergonic and exergonic apply to all physical processes.
• the conceptually related endotherm and exotherm are concepts in animal physiology.
• In quantum numbers, when any excited energy level goes down to its original level for example: when n=4 fall to n=2, energy is released so, it is exothermic.
• Where an exothermic reaction causes heating of the reaction vessel which is not controlled, the rate of reaction can increase, in turn causing heat to be evolved even more quickly . This positive feedback situation is known as thermal runaway. An explosion can also result from the problem.

Measurement

Heat production or absorption in either a physical process or chemical reaction is measured using calorimetry. One common laboratory instrument is the Differential scanning calorimeter or DSC, where a small sample is slowly heated in a controlled way and the heat flow into or from the sample chamber is monitored. The technique can be used to follow chemical reactions as well as physical processes, such as melting and boiling (both of which are endothermic). Crystallization and condensation, the reverse processes, are both exothermic.

See also

• Chemical thermodynamics
• Differential scanning calorimetry
• Endergonic
• Exergonic
• Endergonic reaction
• Exergonic reaction
• Exothermic
• Endothermic reaction
• Endotherm
• Exotherm

External links

• GCSE Science: Reactions