Balanced ionic equations - Higher

A balanced ionic equation shows the reacting ions in a chemical reaction. These equations can be used to represent what happens in precipitation reactions.

Half equations

A half equation is used to represent what happens when atoms or ions gain or lose electrons. In half equations:

  • electrons are shown as e-
  • the numbers of atoms of each element must be the same on both sides
  • the total charge on each side must be the same (usually zero)

These are half equations for some reactions where positive ions gain electrons:

Na+ + e- → Na

Pb2+ + 2e- → Pb

2H+ + 2e- → H2

Worked example

Balance the half equation for the formation of aluminium during electrolysis: Al3+ + e- → Al.

The balanced half equation is: Al3+ + 3e- → Al (because three negatively charged electrons are needed to balance the three positive charges on the aluminium ion).

Question

Write a balanced half equation for the formation of calcium from a calcium ion, Ca2+.

Ca2+ + 2e- → Ca

These are half equations for some reactions where negatively charged ions lose electrons:

2Cl- → Cl2 + 2e-

2O2- → O2 + 4e-

Question

Write a balanced half equation for the formation of bromine, Br2, from bromide ions, Br-.

2Br- → Br2 + 2e-

Ionic equations for precipitation reactions

In a typical precipitation reaction, two soluble reactants form an insoluble product and a soluble product.

For example, silver nitrate solution reacts with sodium chloride solution. The products are insoluble solid silver chloride and sodium nitrate solution:

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

The Na+ ions and NO3- ions remain separate in the sodium nitrate solution and do not form a precipitate. This means these can be ignored when writing the ionic equation. Only how the solid silver chloride forms needs to be shown:

Ag+(aq) + Cl-(aq) → AgCl(s)

In a balanced ionic equation:

  • the number of positive and negative charges on each side of the arrow are the same
  • the numbers of atoms on each side of the arrow are the same
Question

Explain why this ionic equation is balanced:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

There are the same numbers of atoms of each element on both sides of the equation. The total charge on both sides is also the same (zero).

Question

Balance this ionic equation, which represents the formation of a silver carbonate precipitate:

Ag+(aq) + CO32-(aq) → Ag2CO3(s)

2Ag+(aq) + CO32-(aq) → Ag2CO3(s)

Question

Balance this ionic equation, which represents the formation of an aluminium hydroxide precipitate:

Al3+(aq) + ...OH- (aq) → Al(OH)3(s)

Al3+(aq) + 3OH- (aq) → Al(OH)3(s)

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