Chemical reactions are also known as chemical changes. This refers to the changes 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.
There are several types of basic chemical reactions:
A + B → AB
AB → A + B
AB → CO2 + H2O
A + BC → B + AC
AB + CD → AD + CB
A-B=C → A=B-C
A + B → A+ + B-
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 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). 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).
The rate of a chemical reaction depends on:
Every chemical reaction is in theory reversible. In a forward reaction the reactants are converted to products. In a reverse reaction vice versa. Chemical equilibrium is the state in which the forward and reverse reaction rates are equal, thus preserving the amount of reactants and products.
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.
The concentrations of reactants and products determine the rate of forward and reverse reactions.
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.
ca:Reacció química de:Chemische Reaktion et:Keemiline reaktsioon es:Reacción química fr:Réaction chimique nl:Chemische reactie ja:化学反応 nds:Chemisch Reaktschonen pl:Reakcja chemiczna pt:Reacção química su:Réaksi kimiawi fi:Kemiallinen reaktio