Ionic equations: show only the particles that change
Definition
Ionic equation
An equation that shows only the ions and substances that change during a reaction, with the spectator ions left out.
Definition
Spectator ion
An ion that is unchanged by the reaction and appears on both sides of the full ionic equation, so it is cancelled out.
- Ionic equations are written for reactions in solution, where the ions of a dissolved ionic compound are free to move apart.
- A balanced ionic equation conserves every element and has the same total charge on each side.
- A coefficient in front of a formula multiplies the particles, and it multiplies the charge they carry as well.
- Balance an ionic equation using coefficients, never by changing a subscript or a charge.
- Cancelling the spectator ions leaves a short equation that holds for every reaction of that type, whichever soluble compounds were mixed.
Key Idea
- An ionic equation strips out the ions that only watch, leaving the chemistry that actually happens.
- Atoms and total charge must both balance, so a charge check is part of balancing.
Writing an ionic equation: split the aqueous substances, then cancel
Definition
Precipitate
An insoluble solid that forms when two solutions are mixed.
- Start from a balanced symbol equation with a state symbol on every substance.
- Split each aqueous ionic substance into its separate ions, keeping each ion's charge.
- Keep solids, liquids, gases and precipitates as complete formulae, because their particles are not free to move apart.
- Cancel any ion that appears unchanged on both sides of the arrow.
- Write out the particles that are left, then check the atoms and the total charge.
Example
- Mixing aqueous sodium chloride with aqueous silver nitrate forms a silver chloride precipitate.
- Balanced symbol equation:
- NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(aq)\text{NaCl(aq)} + \text{AgNO}_3\text{(aq)} \rightarrow \text{AgCl(s)} + \text{NaNO}_3\text{(aq)}NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(aq)
- Split the aqueous substances into their ions:
- Na+(aq)+Cl−(aq)+Ag+(aq)+NO3−(aq)→AgCl(s)+Na+(aq)+NO3−(aq)\text{Na}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)} + \text{Ag}^{+}\text{(aq)} + \text{NO}_3^{-}\text{(aq)} \rightarrow \text{AgCl(s)} + \text{Na}^{+}\text{(aq)} + \text{NO}_3^{-}\text{(aq)}Na+(aq)+Cl−(aq)+Ag+(aq)+NO3−(aq)→AgCl(s)+Na+(aq)+NO3−(aq)
- Na+\text{Na}^{+}Na+ and NO3−\text{NO}_3^{-}NO3− are unchanged, so they are the spectator ions.
- Cancelling them leaves:
- Ag+(aq)+Cl−(aq)→AgCl(s)\text{Ag}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)} \rightarrow \text{AgCl(s)}Ag+(aq)+Cl−(aq)→AgCl(s)
- Each side has one silver atom, one chlorine atom and a total charge of zero.
Common ionic equations: match the ions to the product they form
Definition
Neutralisation
The reaction in which hydrogen ions from an acid join with hydroxide ions from an alkali to form water.
- An acid produces H+\text{H}^{+}H+ ions in aqueous solution, and an alkali produces OH−\text{OH}^{-}OH− ions.
- In neutralisation, hydrogen ions and hydroxide ions join to make water: H+(aq)+OH−(aq)→H2O(l)\text{H}^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)} \rightarrow \text{H}_2\text{O(l)}H+(aq)+OH−(aq)→H2O(l)
- With a carbonate, hydrogen ions release carbon dioxide and water: CO32−(aq)+2H+(aq)→CO2(g)+H2O(l)\text{CO}_3^{2-}\text{(aq)} + 2\text{H}^{+}\text{(aq)} \rightarrow \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}CO32−(aq)+2H+(aq)→CO2(g)+H2O(l)
- Two hydrogen ions are needed there because they balance the carbonate ion's 2−2-2− charge and supply the two hydrogen atoms in the water.
- With a metal oxide, hydrogen ions release the metal ion into solution and form water: CuO(s)+2H+(aq)→Cu2+(aq)+H2O(l)\text{CuO(s)} + 2\text{H}^{+}\text{(aq)} \rightarrow \text{Cu}^{2+}\text{(aq)} + \text{H}_2\text{O(l)}CuO(s)+2H+(aq)→Cu2+(aq)+H2O(l)
- The copper oxide equation has one copper atom, one oxygen atom and two hydrogen atoms on each side, with a total charge of +2+2+2 on each side.
Common Mistake
- Do not write H2O(aq)\text{H}_2\text{O(aq)}H2O(aq) in a neutralisation equation, because the water formed is a liquid.
- Do not leave Cl−(aq)\text{Cl}^{-}\text{(aq)}Cl−(aq) on the right of the silver chloride equation, because the chloride ion has become part of the solid.
Checking an ionic equation: atoms and total charge both balance
- Count each element on the left and the right once the spectator ions have been cancelled.
- Add up the charges on each side, including any coefficients written in front of the ions.
- Check that every state symbol still matches its substance, especially for solids, gases and water.
- Use the smallest whole-number coefficients that balance both the atoms and the total charge.
- An equation whose atoms balance but whose charges do not is still wrong, so check both every time.
Self review
- What does a balanced ionic equation show?
- What is a spectator ion, and why is it removed?
- Which two quantities must be equal on both sides of an ionic equation?
- Write the ionic equation for neutralisation.
- Why does AgCl\text{AgCl}AgCl stay as a complete formula instead of being split into ions?
