New Reactions

Law of conservation of mass

In a chemical reaction, the total mass of reactants equals the total mass of products; atoms are rearranged, not created or destroyed.

Closed system / open system

A closed system prevents matter entering or leaving; an open system allows gases to escape or enter.

Reactant / product

Reactants are the starting substances (left of the arrow); products are the substances formed (right of the arrow).

Balanced chemical equation

An equation where the number of atoms of each element is equal on both sides, with correct state symbols (s), (l), (g), (aq).

Ionic compound

A compound formed from a metal and non-metal, held together by electrostatic attraction between oppositely charged ions.

Covalent (molecular) compound

A compound formed between non-metals, where atoms share electrons.

Polyatomic ion

A charged group of covalently bonded atoms.

Synthesis reaction

Two or more reactants combine to form a single product (A + B → AB).

Decomposition reaction

A single reactant breaks down into two or more products (AB → A + B).

Displacement reaction

A more reactive element displaces a less reactive element from a compound (single or double displacement).

Neutralisation reaction

An acid reacts with a base to produce a salt and water (acid + base → salt + water).

Precipitation reaction

Two aqueous solutions react to form an insoluble solid (precipitate), predicted using solubility rules.

Precipitate / soluble / insoluble

A precipitate is the insoluble solid formed in a precipitation reaction; soluble substances dissolve in water (aq), insoluble substances do not (s).

Spectator ion

An ion present in solution that does not take part in the reaction and appears unchanged on both sides of the equation.

pH

A measure of the hydrogen-ion concentration of a solution; pH < 7 acidic, pH = 7 neutral, pH>7 basic. Measured with indicators or a pH meter.

Exothermic reaction

A reaction that releases energy to the surroundings, usually as heat; temperature of the surroundings increases.

Endothermic reaction

A reaction that absorbs energy from the surroundings; temperature of the surroundings decreases.

Reaction rate

How quickly reactants are converted into products; affected by concentration, surface area, temperature, and catalysts.

Collision theory

Reactions occur when particles collide with sufficient energy and correct orientation; more frequent and/or more energetic collisions increase the rate.

Catalyst

A substance that increases the rate of a reaction without being consumed; lowers the energy required for a successful collision.