Balanced ionic equations - Higher
A balanced ionic equationA chemical equation that shows how positively charged ions join with negatively charged ions to make a compound. shows the reacting ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. in a chemical reaction. These equations can be used to represent what happens in precipitation reactionA reaction in which an insoluble solid is formed when certain solutions are mixed..
Half equations
A half equationAn equation, involving ions and electrons, that describes the process happening at an electrode. is used to represent what happens when atomThe smallest part of an element that can exist. or ions gain or lose electronSubatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.. In half equations:
- electrons are shown as e-
- the numbers of atoms of each elementA substance made of one type of atom only. must be the same on both sides
- the total chargeProperty of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative 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 solubleAble to dissolve in solvent. For example, sugar is soluble in water because it dissolves to form sugar solution. reactantA substance that reacts together with another substance to form products during a chemical reaction. form an insolubleUnable to dissolve in a particular solvent. For example, sand is insoluble in water. productA substance formed in a chemical reaction. 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+ ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. and NO3- ions remain separate in the sodium nitrate solution and do not form a precipitateA suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction.. 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 atomThe smallest part of an element that can exist. 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)