Everything about Reactivity totally explained
Reactivity refers to the
rate 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.
Causes of reactivity
In general, any time a
chemical reaction occurs it's due to the chemical being able to enter a more stable state.
Quantum chemistry provides the most in depth and exact understanding of the reason this occurs.
Electrons exist in
orbitals that are the result of solving the
Schrödinger equation for specific situations.
All things (values of the
n and
ml 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. In order to achieve one of these orders of stability, an atom will react with another atom, thereby stabilizing both atoms. 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 per mole) when reacting to form H
2.
It is for this same reason that
carbon will almost always form four
bonds. Its ground state
valence 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's negligible, and as such carbon will form them almost instantaneously, meanwhile the process releases a significant amount of energy (
exothermic). This four equal bond configuration is called sp
3 hybridization.
Chemical kinetics and reactivity
The rate of any given reaction,
Reactants → Products
is governed by the
rate law:
where the
rate is the change in the molar concentration in one second in the rate-determining step of the reaction (the slowest step), [A] is the product of the molar concentration of all the reactants raised to the correct order, known as the reaction order, and
k is the reaction constant, which is constant for one given set of circumstances (generally temperature and pressure) and independent of concentration. The greater the reactivity of a compound the higher the value of
k and the higher the rate. For instance, if,
A+B → C+D
Then:
where n is the reaction order of A, m is the reaction order of B, n+m is the reaction order of the full reaction, and k is the reaction constant.
Further Information
Get more info on 'Reactivity'.
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