Balanced equations: atoms and total charge must match on both sides
Definition
Electron
A negatively charged subatomic particle that can be transferred between species during a redox reaction.
- Half equations show the change for one reactant or product and include any electrons transferred.
- Ionic equations show only the particles that react or are formed.
- Both types must have the same number of atoms of each element on both sides.
- The total electrical charge must also be the same on both sides.
- Balance using whole-number coefficients in front of formulae.
- Never change a chemical formula or the charge on an ion to make an equation balance.
Key Idea
- Atoms are conserved, so the number of each type of atom must match.
- Charge is conserved, so the charges on each side must add to the same total.
- Check atoms and charge separately before deciding an equation is balanced.
Half equations: electrons balance the change in charge
Definition
Ion
An atom or group of atoms with an overall electrical charge because electrons have been lost or gained.
- Write the correct formulae for the particle before and after the change.
- Balance the atoms using coefficients before balancing the charge.
- Add the charges on each side to find the total charge.
- Add electrons to the more positive side until the total charge is equal on both sides.
- Place electrons on the right when the particle becomes more positively charged (it loses electrons, so this is oxidation).
- Place electrons on the left when the particle becomes less positive or more negative (it gains electrons, so this is reduction).
- Include state symbols when they are given or can be worked out.
Example
- Magnesium: one magnesium atom forms one Mg2+\text{Mg}^{2+}Mg2+ ion, so two electrons go on the right.
- Mg(s)→Mg2+(aq)+2e−\text{Mg(s)} \rightarrow \text{Mg}^{2+}\text{(aq)} + 2\text{e}^-Mg(s)→Mg2+(aq)+2e−
- This has one atom on each side and a total charge of zero on each side.
- Chlorine: one Cl2\text{Cl}_2Cl2 molecule forms two Cl−\text{Cl}^-Cl− ions, so two electrons go on the left.
- Cl2(g)+2e−→2Cl−(aq)\text{Cl}_2\text{(g)} + 2\text{e}^- \rightarrow 2\text{Cl}^-\text{(aq)}Cl2(g)+2e−→2Cl−(aq)
- This has two atoms on each side and a total charge of −2-2−2 on each side.
Ionic equations: remove particles that stay unchanged
Definition
Aqueous
Dissolved in water and shown using the state symbol (aq)\text{(aq)}(aq).
- Begin with a balanced full symbol equation that includes state symbols.
- Separate aqueous ionic compounds into their individual ions.
- Keep solids, liquids and gases together as complete formulae.
- Cancel any spectator ions that appear unchanged on both sides.
- Write the remaining particles as the balanced ionic equation.
- Check the atoms and the total charge after cancelling ions.
- For example, magnesium displaces copper from aqueous copper sulfate:
- Mg(s)+CuSO4(aq)→MgSO4(aq)+Cu(s)\text{Mg(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{Cu(s)}Mg(s)+CuSO4(aq)→MgSO4(aq)+Cu(s)
- The aqueous ionic compounds can be separated into their ions:
- Mg(s)+Cu2+(aq)+SO42−(aq)→Mg2+(aq)+SO42−(aq)+Cu(s)\text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{Mg}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} + \text{Cu(s)}Mg(s)+Cu2+(aq)+SO42−(aq)→Mg2+(aq)+SO42−(aq)+Cu(s)
- The sulfate ions appear unchanged on both sides, so they are spectator ions and are cancelled:
- Mg(s)+Cu2+(aq)→Mg2+(aq)+Cu(s)\text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Mg}^{2+}\text{(aq)} + \text{Cu(s)}Mg(s)+Cu2+(aq)→Mg2+(aq)+Cu(s)
- An ionic equation can also be built by adding two half equations together.
- If the half equations have different numbers of electrons, multiply one or both until the electron numbers match.
- Cancel the electrons when the half equations are added, because they are transferred between the particles.
Common Mistake
- Do not separate a solid, liquid or gas into ions.
- Do not cancel an ion unless its formula, charge and state are identical on both sides.
- Do not leave spectator ions in the final ionic equation.
- Do not leave electrons in an overall ionic equation made by combining half equations.
Final checks: change coefficients, never formulae or ion charges
Definition
State symbol
A symbol in a chemical equation that shows whether a substance is solid, liquid, gas or dissolved in water.
- A balanced half equation must contain electrons and must balance both atoms and total charge.
- A balanced ionic equation must contain only the particles that react or are formed.
- Use the lowest whole-number coefficients in the finished equation.
- Keep the correct formula for every atom, molecule and ion.
- Keep state symbols, because they show which substances can be separated into aqueous ions.
- Work out charge by multiplying each ion's charge by its coefficient.
- Recheck after every change, because balancing charge can affect the number of particles shown.
Self review
- What two quantities must be equal on both sides of a half equation?
- On which side are electrons placed when magnesium forms Mg2+\text{Mg}^{2+}Mg2+?
- Which substances are separated into ions when writing an ionic equation?
- What happens to spectator ions in the final ionic equation?
- What happens to electrons when two half equations are combined?
