First of all, split the soluble aqueous compounds into ions: The first steps of the procedure are the same as for writing ionic equations. In this example, magnesium is added to a solution of lead(II) nitrate:

Magnesium is a more reactive metal than lead, so will displace lead from its compounds. We can use another metal displacement reaction to illustrate how ionic half-equations are written. Write a half-equation for the oxidation and reduction reaction, balancing charges with electrons.Ionic half-equation simply refers to the fact that we simplify the half-equation by only showing the ions that undergo change. The two half-equations combined give the overall equation. Redox reactions are therefore two-part reactions, and it’s possible to split the overall chemical equation representing a redox reaction into two chemical equations, one of the reduction, one for the oxidation. These are known as half-equations. The metals will not be soluble.Īl (s) + Fe 3+ (aq) + 3NO 3 – (aq) → Al 3+ (aq) + 3NO 3 – (aq) + Fe (s)Ĭancel ions that appear on both sides of the equationĪl (s) + Fe 3+ (aq) → Al 3+ (aq) + Fe (s)Ī close relative of ionic equations, ionic half-equations are exclusively used for redox reactions. The name redox is derived from the fact that two reactions occur simultaneously, a reduction and an oxidation. Both of these salts will dissociate into ions. Identify the soluble compounds using the solubility rules or the aqueous state symbolįe(NO 3) 3 is a nitrate, so will be soluble, as will Al(NO 3) 3. Hopefully you can recognise this as a metal displacement reaction in which the more reactive metal aluminium displaces the less reactive metal iron from its compounds.Īl (s) + Fe(NO 3) 3(aq) → Al(NO 3) 3(aq) + Fe (s) When aluminium metal is added to iron(III) nitrate solution, the pale yellow colour disappears leaving a colourless solution containing metallic particles. Let’s see how this works for a reaction that is described in words (interpreting this and turning it into a balanced symbol equation is an important skill that can be tested in the exams): Cancel ions that appear on both sides of the equation.Split up the soluble compounds into ions.Identify the soluble compounds using the solubility rules or the aqueous state symbol.With the theory out of the way, learn these steps to write the ionic equation for any chemical reaction: The table below shows some examples of substances that can and can’t be ionised. Similarly, you wouldn’t ionise nitrate ions (NO 3 –), carbonate ions (CO 3 2-), phosphate ions (PO 4 3-) and ions like chlorate(I) (ClO –) and iodate(V) (IO 3 –). These polyatomic ions are stable in solution compared to the ions you’d form if they broke up further. Whilst sulfuric acid (H 2SO 4) ionises to form sulfate ions (SO 4 2-), you can’t ionise sulfate ions any further. Water does ionise in solution, forming H + and OH – ions, but as you’ll discover post-GCSE, its ionisation is negligible compared to other ionic substances. You should also not ionise water in ionic equations. When that happens, don’t ionise the precipitate that has the state symbol (s) – it is formed as a precipitate because it’s not soluble and so won’t break up to form ions. This is especially important for precipitation reactions, where solutions are mixed and an insoluble solid forms. Lastly, don’t ionise solids or gases present as reactants or products in chemical reactions, and don’t ionise liquids unless it’s a molten ionic compound. Insoluble, except sodium, potassium, barium and ammoniumĪnything that this table indicates is soluble can be ionised in a chemical reaction. Insoluble, except sodium, potassium and ammonium If the chemical equation doesn’t include state symbols, or if it’s an aqueous substance but you’re not 100% certain that it’s soluble, you can use the solubility rules:Ĭommon salts of sodium, potassium and ammonium (Note that Cl 2 in water does form ions, but it’s through reaction with water and not what we’re considering here).

The exception to this is when (aq) means a substance has dissolved without ionising.Īn example of this would be Cl 2(aq), or chlorine water, which you wouldn’t show ionising to form Cl + and Cl – for instance. If a substance has the state symbol aqueous in a chemical equation, it means that the substance is soluble and will usually be ionised. With very few exceptions, a substance in aqueous solution will be ionised, because the process of dissolving usually requires ionisation. This we show using the state symbol aq, meaning the substance has formed an aqueous solution. A substance being split up by the action of water means it has formed a solution in water – it has dissolved.

Complete ionic equation