Chemical Reactions

Inquiry question: What are the products of a chemical reaction?

When a chemical change occurs, one or more of the following observations can be made:

• A new substance is formed

• A temperature change

• Visible light or sound emission 

• A change in colour 

• A new odor is produced

A chemical change is often not reversible. For example, the combustion of petrol to form carbon dioxide and water cannot be reversed. In contrast, physical changes such as dissolving salt into water are easily reversible.

Predicting Reactions of Metals

Inquiry question: How is the reactivity of various metals predicted?

Water is slow to react with metals other than the very active metals, such as sodium and potassium. Magnesium will react slowly with hot water; zinc and iron will react with steam at high temperatures; and less reactive metals, such as lead and copper, show no reaction with water.

Dilute hydrochloric acid and dilute sulfuric acid react with some metals and hydrogen gas is released. This type of reaction is a displacement reaction where hydrogen is displaced from solution as hydrogen gas and the metal forms metal ions in solution. For the reactive metals, bubbles of colourless hydrogen gas are observed on the metal's surface. The reactive metals will dissolve in the excess acid to form solutions of the metal salt. (Note that nitric acid should not be used because NO gas is released.)

Very active metals, such as sodium, burn readily in air or pure oxygen to produce peroxides (e.g. Na2O2) of the active metal. Reactive metals, such as magnesium and aluminium, burn brightly (especially when powdered) to form metallic oxides. Less active metals may only become coated in an oxide film on heating whereas noble metals, such as gold, do not react.

The following generalisations can be made about metal reactivity and the periodic table: 

• The most reactive metals are found in Group 1 (K, Na) and group 2 (Ba, Ca, Mg).

• Group 13 (Al) metals are less reactive than Groups 1 and 2, although some transition metals (e.g. Fe, Zn) have similar reactivity to Group 13.

• The soft metals at the bottom of Group 14 are less reactive than Group 13 metals.

• The least reactive metals are found in the lower periods of the transition metals (Ag, Au) or in the soft, heavy metal region (Pb).

Rates of Reactions

Inquiry question: What affects the rate of a chemical reaction?

Various factors affect the rate of a chemical change:

• temperature

• concentration

• gas pressure

• particle size and surface area

• catalysts

In order for chemicals to react their atoms or molecules must collide with sufficient kinetic energy in order that bonds can be broken.

Some useful formulae are: