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chemical reaction, process by which one or more substances may be transformed into one or more new substances. Energy is released or is absorbed, but no loss in total molecular weight occurs. When, for example, water is decomposed, its molecules, each of which consists of one atom of oxygen and two of hydrogen, are broken down; the hydrogen atoms then combine in pairs to form hydrogen molecules and the oxygen atoms to form oxygen molecules. In a chemical reaction, substances lose their characteristic properties. Water, for example, a liquid which neither burns nor supports combustion, is decomposed to yield flammable hydrogen and combustion-supporting oxygen. In some reactions heat is given off (exothermic reactions), and in others heat is absorbed (endothermic reactions). Furthermore, the new substances formed differ from the original substances in the energy they contain. Chemical reactions are classified according to the kind of change that takes place. When a compound, which consists of two or more elements or groups of elements, is broken down into its constituents, the reaction is called simple decomposition. When two compounds react with one another to form two new compounds, the reaction is called double decomposition. In so-called replacement reactions the place of one of the elements in a compound is taken by another element reacting with the compound. When elements combine to form a compound, the reaction is termed chemical combination. Oxidation and reduction
reactions are extremely important. Reversible reactions are those in which the chemical change taking place may be paralleled by another change back to the original substances. The rates at which chemical reactions proceed depend upon various factors, e.g., upon temperature, pressure, and the concentration of the substances involved and, sometimes, upon the use of a chemical called a catalyst
. In some chemical reactions, such as that of photographic film, light is an important factor. The changes taking place in a chemical reaction are represented by a chemical equation
. An element's activity, i.e., its tendency to enter into compounds, varies from one element to another.
A process in which atoms
of the same or different elements
rearrange themselves to form a new substance. While they do so, they either absorb heat
or give it off.
Note: click on a word meaning below to see its connections and related words.
chemical reaction has one meaning:
: (chemistry) a process in which one or more substances are changed into others
A chemical reaction
or is a reaction of two or more chemicals
), 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
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:
- 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
- Also called analysis
- A chemical compound breaks apart into two or more individual atoms, ions, or molecules.
- Only one reactant.
- Example: AB → A + B
- A specific type of decomposition, involving large amounts of light and heat. Comustion reactions typically involve carbon, hydrogen, and oxygen (the reactants) forming carbon dioxide (CO2) and water (H2O) (the products).
- Example: CxHx + 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
- 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
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
). 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
are converted to products
. In a reverse reaction
products are converted into reactants.
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.
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.
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