In
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
, reactivity is the impulse for which a
chemical substance undergoes a
chemical reaction, either by itself or with other materials, with an overall release of
energy.
''Reactivity'' refers to:
* the chemical reactions of a single substance,
* the chemical reactions of two or more substances that interact with each other,
* the systematic study of sets of reactions of these two kinds,
* methodology that applies to the study of reactivity of chemicals of all kinds,
* experimental methods that are used to observe these processes
* theories to predict and to account for these processes.
The chemical reactivity of a single substance (reactant) covers its behavior in which it:
* Decomposes
* Forms new substances by addition of atoms from another reactant or reactants
* Interacts with two or more other reactants to form two or more products
The chemical reactivity of a substance can refer to the variety of circumstances (conditions that include temperature, pressure, presence of catalysts) in which it reacts, in combination with the:
* Variety of substances with which it reacts
* Equilibrium point of the reaction (i.e., the extent to which all of it reacts)
* Rate of the reaction
The term ''reactivity'' is related to the concepts of ''
chemical stability'' and ''
chemical compatibility Chemical compatibility is a measure of how stable a substance is when mixed with another substance. If two substances can mix together and undergo a chemical reaction, they are considered compatible. So, incompatible is vice versa.
Chemical compati ...
''.
An alternative point of view
''Reactivity'' is a somewhat vague concept in chemistry. It appears to embody both thermodynamic factors and kinetic factors—i.e., whether or not a substance reacts, and how fast it reacts. Both factors are actually distinct, and both commonly depend on temperature. For example, it is commonly asserted that the reactivity of group one metals (Na, K, etc.) increases down the group in the periodic table, or that hydrogen's reactivity is evidenced by its reaction with oxygen. In fact, the rate of reaction of alkali metals (as evidenced by their reaction with water for example) is a function not only of position within the group but particle size. Hydrogen does not react with oxygen—even though the equilibrium constant is very large—unless a flame initiates the radical reaction, which leads to an explosion.
Restriction of the term to refer to reaction rates leads to a more consistent view. ''Reactivity'' then refers to the
rate
Rate or rates may refer to:
Finance
* Rates (tax), a type of taxation system in the United Kingdom used to fund local government
* Exchange rate, rate at which one currency will be exchanged for another
Mathematics and science
* Rate (mathema ...
at which a
chemical substance tends to undergo a
chemical reaction in time. In pure
compounds, reactivity is regulated by the physical properties of the sample. For instance, grinding a sample to a higher specific surface area increases its reactivity. In impure compounds, the reactivity is also affected by the inclusion of contaminants. In
crystalline compounds, the crystalline form can also affect reactivity. However, in all cases, reactivity is primarily due to the sub-atomic properties of the compound.
Although it is commonplace to make statements that substance 'X is reactive', all substances react with some reagents and not others. For example, in making the statement that 'sodium metal is reactive', we are alluding to the fact that sodium reacts with many common reagents (including pure oxygen, chlorine, hydrochloric acid, water) and/or that it reacts rapidly with such materials at either room temperature or using a Bunsen flame.
'Stability' should not be confused with reactivity. For example, an isolated molecule of an electronically excited state of the oxygen molecule spontaneously emits light after a statistically defined period. The half-life of such a species is another manifestation of its stability, but its reactivity can only be ascertained via its reactions with other species.
Causes of reactivity
The second meaning of 'reactivity', that of whether or not a substance reacts, can be rationalised at the atomic and molecular level using older and simpler valence bond theory and also atomic and molecular orbital theory. Thermodynamically, a
chemical reaction occurs because the products (taken as a group) are at a lower
free energy than the reactants; the lower energy state is referred to as the 'more stable state'.
Quantum chemistry
Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions ...
provides the most in-depth and exact understanding of the reason this occurs. Generally,
electrons exist in
orbital
Orbital may refer to:
Sciences Chemistry and physics
* Atomic orbital
* Molecular orbital
* Hybrid orbital Astronomy and space flight
* Orbit
** Earth orbit
Medicine and physiology
* Orbit (anatomy), also known as the ''orbital bone''
* Orbito ...
s that are the result of solving the
Schrödinger equation for specific situations.
All things (values of the ''n'' and ''m''
l quantum numbers) being equal, the order of stability of electrons in a system from least to greatest is unpaired with no other electrons in similar orbitals, unpaired with all degenerate orbitals half filled and the most stable is a filled set of orbitals. To achieve one of these orders of stability, an atom reacts with another atom to stabilize both. For example, a lone
hydrogen atom has a single electron in its 1s orbital. It becomes significantly more stable (as much as 100
kilocalories per mole, or 420
kilojoules
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied ...
per
mole) when reacting to form H
2.
It is for this same reason that
carbon almost always forms four
bonds. Its ground state
valence
Valence or valency may refer to:
Science
* Valence (chemistry), a measure of an element's combining power with other atoms
* Degree (graph theory), also called the valency of a vertex in graph theory
* Valency (linguistics), aspect of verbs rel ...
configuration is 2s
2 2p
2, half filled. However, the
activation energy to go from half filled to fully filled p orbitals is so small it is negligible, and as such carbon forms them almost instantaneously. Meanwhile, the process releases a significant amount of energy (
exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
). This four equal bond configuration is called sp
3 hybridization
Hybridization (or hybridisation) may refer to:
*Hybridization (biology), the process of combining different varieties of organisms to create a hybrid
*Orbital hybridization, in chemistry, the mixing of atomic orbitals into new hybrid orbitals
*Nu ...
.
The above three paragraphs rationalise, albeit very generally, the reactions of some common species, particularly atoms. One approach to generalise the above is the activation strain model
of chemical reactivity which provides a causal relationship between, the reactants' rigidity & their electronic structure, and the height of the reaction barrier.
The rate of any given reaction,
:
Reactants -> Products
is governed by the
rate law:
: