Neutralisation and salt formulae: Predict and write salts
Chemical changes • Reactions of acids
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Key concepts
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Definition of neutralisation
Neutralisation is the reaction between an acid and a base that reduces acidity because H+ ions from the acid combine with OH- ions from the base to form water. When H+ and OH- combine, water forms and the acidic property of H+ is removed. Neutralisation between acids and metal carbonates produces carbon dioxide as a direct result of carbonate decomposition by acid.
Predicting products from reactants
Acid + alkali (base containing OH-) produces a salt and water because H+ from the acid reacts with OH- from the alkali to give H2O, leaving the cation from the alkali and the anion from the acid to form the salt. Acid + metal produces a salt and hydrogen gas because H+ displaces H2 from the metal, so the metal cation combines with the acid anion and H2 evolves. Acid + carbonate produces a salt, water and carbon dioxide because the carbonate ion reacts with H+ to form CO2 and H2O while the remaining ions form the salt.
Common ions and their charges
Common positive ions include sodium Na+, potassium K+, calcium Ca2+, magnesium Mg2+ and aluminium Al3+. Common negative ions include chloride Cl-, sulfate SO4 2-, nitrate NO3 -, carbonate CO3 2- and oxide O2-. Ion charges are fixed for these common ions and determine how ions combine to form electrically neutral salts. The charge on each ion indicates how many of that ion are needed to balance the opposite charge.
Deducing salt formulae from ion charges
The formula of a salt arises from combining cations and anions in whole-number ratios that make the total positive and negative charge equal. For example, Ca2+ combines with Cl- in a 1:2 ratio to form CaCl2 because one Ca2+ neutralises two Cl-. Al3+ combines with SO4 2- in a 2:3 ratio to form Al2(SO4)3 because two Al3+ contribute +6 and three SO4 2- contribute −6.
Writing and naming salts
Salt formulae write the cation first and the anion second, using the smallest whole-number ratio of ions. Naming salts uses the name of the cation followed by the name of the anion, for example sodium chloride for NaCl or magnesium sulfate for MgSO4. When a polyvalent ion appears, parentheses surround a polyatomic ion when more than one is required, for example Ca(NO3)2.
Key notes
Important points to keep in mind