Why Chemical Reactions Occur

Fire is an energy-releasing chemical reaction.

A chemical reaction occurs when one or more substances changes to produce one or more different substances. The key: For reaction to occur, there must be incentive. One incentive is energy, but some other incentives allow escape of a product from the reaction.

Solubility Incentive

If a solution of sodium chloride is combined with a solution of silver nitrate, the reaction occurs,

NaCl + AgNO3 --> NaNO3 + AgCl.

When a silver ion does come in contact with a chloride ion, silver chloride, which is not soluble, precipitates out. This reduces the number of silver and chloride ions available, driving the reaction forward.

Volatility Incentive

If a reaction occurs in which a gas is produced and is allowed to escape the reaction vessel, volatility is one incentive driving this reaction. For instance,

Mg + 2 HCl --> MgCl2 + H2.

The hydrogen gas (H2) quickly escapes into the atmosphere.

Permeability Incentive

If a combination of chemicals is reacted in a semi-permeable membrane, if one of the products can penetrate that membrane, the reaction is driven forward.

Energy Incentive

If reactants can give products plus energy (usually heat), then the products are stabilized by that amount of energy. This common incentive to chemical reaction is termed the "enthalpy of reaction."

Reversible Reactions

Many reactions are reversible and the direction the reaction takes may depend upon conditions, including some of those listed above. Some reactions can be forced to occur by the addition of energy.

References

• University of Waterloo: "Enthalpies of Reactions"

Resources

• Chest of Books: "Chemical Affinity"