CSEC Chemistry: Reactivity of Metals

You probably remember some of the main chemical properties of metals:

  • they react with water to form a base and liberate hydrogen gas

  • they react with oxygen to form basic oxides

  • they react with dilute acids (except nitric acid) to form a salt and liberate hydrogen gas

  • they show properties of reducing agents

  • they form ionic compounds

Many metals exhibit these chemical properties, but there are differences in the reactivity of the metals that causes some metals to react differently, or to not give these typical reactions.

The reactivity of a metal is given in terms of its relative chemical reactivity. This is an indication of the ease with which the atoms of the metal atoms ionise. The greater the ease of ionization of the metal, the more rapidly it will react with chemical reagents.

Essentially, the greater the ease of ionization of the metal, the more reactive it is.

It is fairly simple to compare the reactivity of metals within the same period or group. Metallic character (metallic ease of ionization) decreases from group I to III but increases as a group is descended.

CSEC will generally focus on the following metals: zinc, iron, sodium, calcium, aluminium, magnesium, copper and lead.

The reactivity of metals can be much more difficult to ascertain when they come from various positions across various groups or periods. Chemists can determine the reactivity of metals by conducting experiments which involve exposing metals to a variety of reagents, including water, oxygen, dilute (non-oxidizing)* acids and metal salts in aqueous solution.

*non-oxidizing acids refers to acids that give anions that do not act as oxidizing agents. For example, nitric acid is an oxidizing acid since it produces nitrate ions which act as oxidizing agents.

Reactivity of Metals with Oxygen

Metals will react with oxygen to form metallic oxides. The difference between the metals is how quickly or spontaneously this reaction occurs.

  1. The more reactive metals, like sodium and calcium, react spontaneously with oxygen in the air.

  2. Other metals like magnesium and aluminium react more slowly and become covered in an oxide layer.

  3. Zinc and iron react even more slowly in dry air (air without moisture, since metals like iron will rust easily in moist air).

  4. Copper takes several years to tarnish in air.

Essentially, the more spontaneous the reaction, the more reactive the metal.

It is these reactions with oxygen which contribute to the corrosion of metals exposed to the environment. Corrosion, as you might remember, is the reaction of metals with several substances in the environment to form most commonly oxides, but also carbonates, sulphates, sulphides, and hydroxides. This corrosion takes away from the structural integrity of the metal.

Reactivity of the Metals with Water

  1. More reactive metals like potassium, sodium and calcium will react with cold water to form metal hydroxides

  2. Iron and Zinc react with steam to form oxides

  3. Copper and silver will give no reaction with either water or steam

Reactivity of the Metals with Dilute Non-Oxidizing Acids

The non-oxidizing acids we refer to most commonly are hydrochloric acid or sulphuric acid. It should be noted that nitric acid has oxidizing properties even when dilute.

  1. Metals like magnesium, zinc and iron will react safely and with decreasing vigour with dilute non-oxidizing acids

  2. Lead shows little reactivity, due in part to the fact that the salt formed is insoluble

  3. Copper and similar metals give no reaction

Displacement Reactions and Reactivity

The reactivity of metals can also be determined by reacting a specific metal with the solutions of salts of other metals.

For example, when a strip of magnesium is dropped into a solution of copper (II) sulphate, the blue colour of the solution fades and eventually becomes colourless, as the magnesium is used up.

This is because the more reactive magnesium displaces the less reactive copper from the solution containing copper (II) ions. The equation can be expressed like this:

Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)

Conversely, if copper is added to a solution of magnesium sulphate, no reaction will occur, since copper is less reactive than magnesium.

In every reaction involving a metal, the metal atom releases electrons, and become oxidised to cations. This means that they act as reducing agents.

M→Mⁿ⁺ + ne⁻

When metals react with oxygen, oxygen atoms are reduced to oxide ions:

O₂(g) + 4e⁻→2O²⁻

Hydrogen cations (protons) are reduced to hydrogen molecules (hydrogen gas) when metals react with water or dilute acids. However, this only occurs for metals more reactive (i.e. that are more powerful reducing agents) than hydrogen.

2H⁺(aq) + 2e⁻ → H₂(g)

When a more reactive metal is added to a solution containing ions of a less reactive metal, the metal ions are reduced to metal atoms:

M(s) + X²⁺(aq) → X(s) + M²⁺(aq)

The Reactivity Series of Metals

Experiments similar to those listed above have been used by chemists to rank metals in terms of their reactivity and reducing power. The metal with the greatest reducing power is found at the top of the series while the least powerful is placed at the bottom.

For our purposes, we only need a shortened form of the reactivity series containing the most common metals.

Hydrogen is also included in the reactivity series as a reference point, since it will help us predict reactions of metals with acids and water (which contain hydrogen).

As you can see, the reactivity series lists in descending order of reducing power, reactivity and ease of ionization.

This helps us to predict several things:

What will a metal do in a displacement reaction?

Metals higher in the series will displace metals lower in the series from aqueous solutions of their salts.

Metals above hydrogen in the series will displace hydrogen from aqueous solutions of acids and from water and steam.

Will compounds of the metal be decomposed by heat?

The more reactive metals (at the top of the reactivity series) form the most stable compounds, like sodium hydroxide and sodium carbonate. These compounds do not decompose when heated. The less reactive metals form less stable compounds which will decompose when heated.