Higher and foundation tier

Metal oxides

Metals react with oxygen to produce metal oxides.

metal(s) + oxygen(g) metal oxide(s)
No doubt at some time in your science lessons you will have held a piece of magnesium ribbon in a Bunsen flame and cautiously observed the bright flash from the burning magnesium. However not all metal oxygen reactions are as violent this.

magnesium burning in air

Burning copper metal

copper metal burning If a square of copper metal is held with a pair of tongs in a hot Bunsen flame for about 30 seconds the shiny bronze coloured copper turns black. No flames or bright flashes are produced. The copper metal is reacting with the oxygen in the air, it is being oxidised. An equation for this reaction is:

copper(s) + oxygen(g) → copper oxide(s)
In both the examples above the metals join with oxygen from the air and turn into a metal oxide. The metal is oxidised. These metal reactions with oxygen are called oxidation reaction. Oxidation simply means adding oxygen to a substance.

The reactivity series

By observing how violent and fast the reactions of metals are when they are burned, react with water or react with acids it is possible to rank the metals in order of reactivity. The reactivity series is simply a list with the most reactive metal at the top and the least reactive metal at the bottom.

Reaction of metals with water- oxidation and reduction

Metals react with water to produce a metal hydroxide solution and hydrogen gas. We can show this as:

metal(s) + water(l) metal hydroxide(aq) + hydrogen(g)
As an example consider the reactions of the alkali metal lithium with water: :
lithium(s) + water(l) lithium hydroxide(aq) + hydrogen(g)
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)

group1 and 2 metals The alkali metals and the metals radium, barium, strontium and calcium from group 2 of the periodic table (The alkaline earth metals), all react violently with water. However even a fairly reactive metal like magnesium reacts very slowly with water and it would take days to collect enough hydrogen for a pop test! Remember the trends in the reactivity of metals, the metals become more reactive as you descend a group in the periodic table. The reason for this is because the metals lose their outer electrons more readily as you descend the group and so their reactivity increases.

When the metals react with water they are losing their electrons and forming positively charged ions e.g.

Metal - e → metal+
In the case of lithium reacting with water, we have:
Li(s) - e → Li+(aq)
The lithium hydroxide formed is an ionic compound with positive lithium ions and negatively charged hydroxide ions, OH-(aq). Metals when they react lose electrons to form positive ions. We defined oxidation earlier as the addition of oxygen to a substance, however we can extend this definition to include the loss of one or more electrons. When metals react with oxygen to form metal oxides the metals lose electrons. The oxygen gains electrons from the metal to form a negatively charged ion. The gain of electrons is often called reduction e.g.
copper(s) + oxygen(g) → copper oxide(s) -word equation
2Cu(s) + 02(g) → 2CuO(s) - symbolic equation
2Cu(s) + 02(g) → 2Cu2+O2-(s) - ionic equation
In the ionic equation, we can clearly see that the copper atoms lose 2 electrons to form copper ions with a 2+ charge while the oxygen atoms gain 2e and form oxide ions with a 2- charge. The copper is oxidised and the oxygen is reduced. Reactions where one substance is reduced and another is oxidised are called redox reactions.

Key Points

Practice questions

Check your understanding - Questions on metal reactivity