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Inchemistry,reactivity is the impulse for which achemical substance undergoes achemical reaction, either by itself or with other materials, with an overall release ofenergy.
Reactivity refers to:
The chemical reactivity of a single substance (reactant) covers its behavior in which it:
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:
The termreactivity is related to the concepts ofchemical stability andchemical compatibility.
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 ofalkali 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 also of 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 therate at which achemical substance tends to undergo achemical reaction in time. In purecompounds, 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. Incrystalline 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," each substance reacts with its own set of reagents. For example, the statement that "sodium metal is reactive" suggests that sodium reacts with many common reagents (including pure oxygen, chlorine,hydrochloric acid, and water), either at room temperature or when using aBunsen burner.
The concept of 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.[citation needed] 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.
The second meaning ofreactivity (i.e., whether or not a substance reacts) can be rationalized at the atomic and molecular level using older and simpler valence bond theory and also atomic and molecular orbital theory. Thermodynamically, achemical reaction occurs because the products (taken as a group) are at a lowerfree energy than the reactants; the lower energy state is referred to as the "more stable state."Quantum chemistry provides the most in-depth and exact understanding of the reason this occurs. Generally,electrons exist inorbitals that are the result of solving theSchrödinger equation for specific situations.
All things (values of then andmlquantum numbers) being equal, the order of stability of electrons in a system from least to greatest is;
To achieve one of these orders of stability, an atom reacts with another atom to stabilize both. For example, a lonehydrogen atom has a single electron in its 1s orbital. It becomes significantly more stable (as much as 100kilocalories per mole, or 420kilojoules permole) when reacting to form H2.
It is for this same reason thatcarbon almost always forms fourbonds. Its ground-statevalence configuration is 2s2 2p2, half-filled. However, theactivation energy to go from half-filled to fully-filled p orbitals is negligible, and as such, carbon forms them almost instantaneously. Meanwhile, the process releases a significant amount of energy (exothermic). This four equal bond configuration is called sp3hybridization.
The above three paragraphs rationalize, albeit very generally, the reactions of some common species, particularly atoms. One approach to generalize the above is theactivation strain model[1][2][3] of chemical reactivity which provides a causal relationship between, the reactants' rigidity and their electronic structure, and the height of the reaction barrier.
The rate of any given reaction:
is governed by therate law:
where therate 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), andk is the reaction constant, which is constant for one given set of circumstances (generally temperature and pressure) and independent of concentration. The reactivity of a compound is directly proportional to both the value ofk and the rate. For instance, if
then
wheren is the reaction order ofA,m is the reaction order ofB,n + m is the reaction order of the full reaction, andk is the reaction constant.
