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Chemical reactions (also called chemical changes) are transformations in the structure of molecules. Such reactions can result in molecules attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangements of atoms within molecules. Chemical reactions usually involve the making or breaking of chemical bonds.

Types

There are several types of chemical reactions:

• Synthesis consists of 2 or more individual molecules coming together and forming a new substance.
• Decomposition is the opposite of synthesis, and in this type reaction individual atoms are no longer bonded.
• In single displacement reactions, one of the atoms' formula is exchanged with another one's.
• In double displacement reactions, both reactants' bonded atoms are exchanged.

A chemical reaction does not change the nucleus of the atom in any way, only the interaction of the electron clouds of the involved atoms. (Changes in the composition of the nuclei of atoms are called nuclear reactions, and are not considered chemical reactions, although chemical reactions may follow a nuclear transformation.)

A chemical reaction almost always involves a change in energy, conveniently measured in terms of heat. The energy difference between the "before" and "after" states of a chemical reaction can be calculated theoretically using tables of data (or a computer). For example, consider the reaction CH₄ + 2 O₂ → CO₂ + 2 H₂O (combustion of methane in oxygen). By calculating the amounts of energy required to break all the bonds on the left ("before") and right ("after") sides of the equation, we can calculate the energy difference between the reactants and the products. This is referred to as ΔH, where Δ (Delta) means difference, and H stands for heat, a measure of energy. ΔH is usually given in units of kJ, or thousands of joules. If ΔH is negative for the reaction, then energy has been released. This type of reaction is referred to as exothermic (literally, outside heat, or throwing off heat). An exothermic reaction is more favourable and thus more likely to occur. Our example reaction is exothermic, which we already know from everyday experience, since burning gas in air gives off heat.

A reaction may have a positive ΔH. This means that, to proceed, the reaction requires an input of energy from outside. This type of reaction is called endothermic (literally, inside heat, or absorbing heat).

Reaction rate

The rate of a chemical reaction depends on:

• Reactant concentrations
• Activation energy
• Temperature
• The presence or absence of a catalyst or an inhibitor
• The reactivity of the reactants

Reversability

Every chemical reaction is in theory reversible. In a foreward reaction the reactants are converted to products. In a reverse reaction vice versa. Chemical equilibrium is the state in which the foreward and reverse reaction rates are equal.

Although all reactions are reversible to some extent, some reactions can be classified as irreversible. An irreversible reaction occurs when at equilibrium almost all of the reactant molecules have been converted to product.

Law of mass action

The concentrations of reactants and products determine the rate of foreward and reverse reactions.

Catalyst

A catalyst is not created or destroyed in a chemical reaction, but helps to increase the reaction rate.

See also

• Chemical synthesis
• Chemical equation
• Stoichiometry