Live practice

Exothermic reactions release heat to the surroundings. For example, burning natural gas releases heat:

Endothermic reactions absorb energy from the surroundings. For example, calcium carbonate absorbs heat and decomposes:

During respiration, glucose is oxidised by oxygen in the cells. Carbon dioxide and water are formed, and energy is released:

Because energy is released to support the activities of the body, respiration is an exothermic reaction.

In a combination reaction, two or more reactants combine to form a single product. For example:

In a decomposition reaction, a single reactant splits into two or more simpler products. For example:

The patterns are opposite: combination is many-to-one, whereas decomposition is one-to-many.

Heat (thermal decomposition):

Light (photochemical decomposition):

Electricity (electrolytic decomposition):

In a displacement reaction, a more reactive element replaces a less reactive element from its compound:

Here iron displaces copper.

In a double displacement reaction, ions of two compounds exchange partners:

Here \(\ce{Ba^{2+}}\) and \(\ce{Na+}\) exchange anions, and insoluble barium sulphate precipitates.

Copper is more reactive than silver, so it displaces silver from silver nitrate solution. One copper atom forms \(\ce{Cu^{2+}}\), requiring two nitrate ions; therefore two silver ions are displaced:

A precipitation reaction is a reaction between solutions in which an insoluble solid, called a precipitate, is formed.

For example, barium chloride and sodium sulphate solutions form a white precipitate of barium sulphate:

Another example is the formation of yellow lead(II) iodide:

(a) Oxidation is the gain of oxygen by a substance.

Copper gains oxygen and is oxidised.

Sodium gains oxygen and is oxidised.

(b) Reduction is the loss of oxygen by a substance.

Copper(II) oxide loses oxygen and is reduced to copper.

Zinc oxide loses oxygen and is reduced to zinc.

The shiny brown element is copper. On heating in air, copper combines with oxygen and forms black copper(II) oxide:

Therefore, \(X\) is copper and the black compound is copper(II) oxide.

Rusting requires both oxygen and moisture. A coat of paint forms a protective barrier between the iron surface and the surrounding air and water. Since oxygen and moisture cannot reach the iron, oxidation is prevented and the article is protected from rusting.