Chemical reaction

A chemical reaction or is a reaction of two or more chemicals (reagents), yielding a chemical change and a product(s). A chemical change is defined as molecules attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within molecules. Chemical reactions usually involve the making or breaking of chemical bonds.

Types

There are six types of common chemical reactions. Most reactions will be classified as one of these, though there may be others that cannot be classified:

Synthesis

*Also called composition or direct combination.

*Two or more individual atoms, ions, or molecules coming together and forming a new substance.

*Only one product.

*Example: A + B → AB

Decomposition

*Also called analysis

*A chemical compound breaks apart into two or more individual atoms, ions, or molecules.

*Only one reactant.

*Example: AB → A + B

Combustion

*A specific type of decomposition, involving large amounts of light and heat. Combustion reactions typically involve carbon, hydrogen, and oxygen (the reactants) forming carbon dioxide (CO₂) and water (H₂O) (the products).

*Example: C_xH_x + O₂ → CO₂ + H₂O

Single displacement

*Also called single replacement.

*One compound and one element are displaced.

*One compound and one element on both sides.

*Example: AB + C → AC + B

Double displacement

*Also called double replacement, metathesis, or ion exchange

*Two compounds are displaced.

*Two compounds on both sides.

*Example: AB + CD → AC + BD

Neutralization

*Also called water-forming reactions.

*A specific type of double displacement reactions involving an acid and a base (reactants) neutralizing (canceling each other out) to form water.

In a rearrangement reaction, the atoms and bonds within a molecule rearrange to form an isomer of the original compound.
A-B=C → A=B-C
In an oxidation-reduction reaction (also known as a redox reaction), one reactant loses electrons (that is, it is oxidized), and the other reactant gains electrons (it is reduced). The oxidized reactant is the reducing agent and the reduced reactant is the oxidizing agent.
*Example: A + B → A⁺ + B^-
*Single displacement reactions are typically also redox reactions.

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.)

Energy and reactions

Net change in energy

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 enthalpy, a measure of energy which is equal to the heat transferred at constant pressure. ΔH is usually given in units of kJ (thousands of joules) or in kcal (kilocalories).

Exothermic reactions

If ΔH is negative for the reaction, then energy has been released. This type of reaction is referred to as exothermatic (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.

Endothermic reactions

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).
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Reactive intermediates

Reactions very seldom occur directly. Usually, reactants must collide to form an activated complex. This has a higher internal energy than the original reactants combined, having gained some from the kinetic energy of the collision. This energy allows for the rearrangement of bonds which constitutes the reaction. In some reactions, the reactants may pass through many reactive intermediates before becoming products.

Reaction rate

The rate of a chemical reaction depends on:

Reactant concentrations

Activation energy

Temperature

The presence or absence of a catalyst.

Reversibility

Every chemical reaction is, in theory, reversible. In a forward reaction the reactants are converted to products. In a reverse reaction products are converted into reactants.

Chemical equilibrium is the state in which the forward and reverse reaction rates are equal, thus preserving the amount of reactants and products. However, a reaction in equilibrium can be driven in the forward or reverse direction by changing reaction conditions such as temperature or pressure. Le Chatelier's principle can be used to predict whether products or reactants will be formed.

Although all reactions are reversible to some extent, some reactions can be classified as irreversible. An irreversible reaction is one that "goes to completion." This phrase means that nearly all of the reactants are used to form products. These reactions are very difficult to reverse even under extreme conditions.

Law of mass action

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

Catalyst

A catalyst increases the speed of a reaction by lowering the activation energy needed for the reaction to take place, and supplies enough energy for the reaction to happen. A catalyst is not destroyed or changed during a reaction, so it can be used again.