In chemistry, an exothermic reaction is one that releases heat. It is
the opposite of an endothermic reaction. Expressed in a chemical
equation:
reactants -->product + energy
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 solution in which the reaction is taking place gains heat, the
reaction is exothermic.
In an exothermic reaction the total energy absorbed in bond breaking is less than the total energy released in bond making.
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.
For an exothermic reaction, this gives a negative value for ΔH as a
larger value is subtracted from a smaller value. For example, when
hydrogen burns:
In Chemistry an endothermic reaction is one in which the products have
more energy than the reactants, and thus a net input of energy, usually
in the form of heat, is required. Endothermic reactions are often
described as reactions that "feel cold", and contrast with exothermic
reactions, in which heat is released.
Although the process of bond breaking amongst reactants in a chemical
process requires an initial input of energy (the activation energy), in
the case of an endothermic reaction, the energy released when bonds are
formed to create reactants is less than that required to break the bonds
in the products; bonding electrons in the products are therefore at a
higher energy than the reactants. Heat energy from the material
surrounding the reactants is usually what breaks their bonds, so as heat
energy is transferred from the surroundings to the reactants, the
surroundings get colder. This is often summarized in a chemical equation
as follows:
Reactants + Energy → Products
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