Reactive chemistry

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Chemical change

A physical change is a change in the physical properties of a substance or a change in state.

A chemical change leads to the formation of new substances with different compositions and different properties.

There are often signs that a chemical change has occurred, such as the evolution of a gas, production of an odour, formation of a precipitate, a change in temperature or a change in colour.

A chemical change can be represented by a chemical equation. A chemical equation contains information about the ratios and the physical state of each substance.

Mass is conserved in a chemical reaction because the number of atoms of each element present does not change. This is the law of conservation of mass.

Synthesis reactions

A synthesis reaction involves forming a compound from elements or from other, usually smaller, compounds.

Decomposition reactions

A decomposition reaction is a reaction in which one reactant is broken down into two or more products.

Heat energy is often used to bring about decomposition. This is referred to as thermal decomposition.

Electrical energy can be used to bring about decomposition. This is called electrolysis.

Light energy can be used to bring about decomposition. This is called photolysis.

Combustion reaction

In a combustion reaction, the reactant combines with oxygen to produce oxides. Combustion reactions release energy.

The products of the complete combustion of hydrocarbons and carbon-based fuels containing oxygen are carbon dioxide and water.

Incomplete combustion, resulting in the production of carbon monoxide and/or carbon and water, occurs when hydrocarbons and carbon-based fuels containing oxygen undergo combustion in a limited supply of oxygen.

Precipitation reaction

A precipitation reaction occurs when two solutions of compounds are mixed and a solid product is formed. The solid product is called a precipitate.

In a precipitation reaction, an insoluble compound is formed when two clear solutions are mixed together.

The SNAAP rule can be used to predict the solubility of an ionic compound.

Solubility tables can be used to predict which compound, if any, will precipitate in a reaction.

Ions that are not directly involved in the formation of the precipitate are called spectator ions.

Full and ionic equations can be written for precipitation reactions.

Ionic equations do not include spectator ions.

Reactions of acids and bases

Generalisations can be made about the likely products of reactions involving acids and bases:

acid + reactive metal → salt + hydrogen

acid + metal hydroxide → salt + water

acid + metal carbonate → salt + water + carbon dioxide

acid + metal hydrogen carbonate → salt + water + carbon dioxide

Each of these reactions can be represented by full and ionic equations

An ionic equation only shows those ions, atoms or molecules that take part in the reaction.

Spectator ions (ions that do not take part in the reaction) are not included in ionic equations.

Ionic equations are balanced with respect to both the number of atoms of each element and the change.

Removing toxins from food

Indigenous Australians have long utilised a range of chemical processes to remove toxins from native nuts and berries.

These techniques have included leaching fermentation, grinding, heating and grating.

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Predicting reactions of metals

Reactions of metals

Most metals combine with oxygen to form a metal oxide.

The reaction between a metal and oxygen involves a transfer of electrons from the metal atoms to the oxygen atoms

There is considerable variation in the reactivity of metals with oxygen. Group 1 and 2 metals react readily with oxygen. Most transition metals react slowly with oxygen at room temperature, but more quickly when heated. Silver and gold do not react with oxygen, even when healed.

Potassium, sodium and calcium displace hydrogen from cold water. Magnesium displaces hydrogen from water. A solution containing metal ions and hydroxide ions is also formed. Aluminium, zinc and iron displace hydrogen from steam. Tin, lead, copper and silver do not react with either water or steam.

Hydrogen ions are displaced from dilute acids by magnesium, aluminium, zinc, iron, tin and lead (in decreasing order of reactivity). Hydrogen gas and solutions containing the metal ions are formed by these reactions.

A more reactive metal will displace the ion of a less reactive metal from solution.

The activity series of metals

The activity series ranks metals in order of there relative reactivity.

Observations made of reactions of different metals are used to rank metals in order of most reactive to least reactive.

Metal atoms lose electrons in a reaction.

The atoms of more reactive metals lose electrons more readily than the atoms of less reactive metals.

The activity series can be used to predict the reactions of metals.

A metal will displace the ion of less reactive metals from solution.

During a displacement reaction, the metal atom loses electrons and the metal ion gains electrons.

The loss and gain of electrons can be represented by half-equations.

Metal displacement reactions can be represented by ionic equations derived by combining half-equations.

The activity series of metals is based on observations of the relative vigour of the reactions of various metals. Terms such as ‘violent’, ‘fast’, ‘slow’ and ‘barely reacts’ are used to describe the vigour of reaction. Reaction conditions are not standardised.

The table of standard reduction potentials ranks both metals and non-metals in order of reactivity. Their reactivity is assigned a numerical value, standard reduction potential, based on precise measurements made under standardised conditions.

Metal activity and the periodic table

The chemical reactivity of metals increases down a group and decreases across a period of the periodic table.

The atoms of metals lose electrons when they react.

A reactive metal loses its valence electrons more readily than a less reactive metal.

The relative reactivity of metals can be explained in terms of atomic radius, ionisation energy and electronegativity.

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Redox reactions and galvanic cells

Introducing redox reactions

Redox reactions involve the transfer of electrons from one species to another.

Oxidation and reduction always occur at the same time.

Oxidation numbers

Oxidation numbers are calculated according to a set of rules.

Free elements