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 its 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. Lets 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 cant be ionised. Similarly, you wouldnt 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 youd form if they broke up further. Whilst sulfuric acid (H 2SO 4) ionises to form sulfate ions (SO 4 2-), you cant ionise sulfate ions any further. Water does ionise in solution, forming H + and OH - ions, but as youll discover post-GCSE, its ionisation is negligible compared to other ionic substances. You should also not ionise water in ionic equations. When that happens, dont ionise the precipitate that has the state symbol (s) - it is formed as a precipitate because its not soluble and so wont break up to form ions. This is especially important for precipitation reactions, where solutions are mixed and an insoluble solid forms. Lastly, dont ionise solids or gases present as reactants or products in chemical reactions, and dont ionise liquids unless its 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 doesnt include state symbols, or if its an aqueous substance but youre not 100% certain that its soluble, you can use the solubility rules:Ĭommon salts of sodium, potassium and ammonium (Note that Cl 2 in water does form ions, but its through reaction with water and not what were 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 wouldnt 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. We can write the balanced equation for this:Ī substance being split up by the action of water means it has formed a solution in water - it has dissolved. In the neutralisation reaction between dilute hydrochloric acid and sodium hydroxide solution, a salt and water are formed. This is very useful, particularly when dealing with redox reactions which are complicated enough without spectator ions getting in the way. The purpose of ionic equations is they simplify chemical equations by eliminating the species that arent undergoing any change. The other ions that dont participate are called spectator ions. In other words, the ions that react together in solution and form new substances. Ionic equations are chemical equations that show only ions that participate in a chemical reaction. Which substances ionise in ionic equations?.
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