Acids and Bases
Acid – a substance that acts as a donor of hydrogen ions
Base – a substance that acts as an acceptor of hydrogen ions
Alkali – soluble bases
| Acid | Base | ||
| Sour Taste | Bitter Taste | ||
| pH less than 7 | pH greater than 7 | ||
| In solution, contains hydronium ions (H3O+) | In solution, contains hydroxide ions (OH-) | ||
| Turns blue litmus red | Turns red litmus blue | ||
| Turns phenolphthalein colourless | Turns colourless phenolphthalein pink | ||
| Corrosive | Soapy feel | ||
| Reacts with metals to produce salt and hydrogen | Cannot react with metals | ||
| Examples of Acids | Examples of Bases | ||
| Hydrochloric Acid | HCl | Sodium Hydroxide | NaOH |
| Nitric Acid | HNO3 | Potassium Hydroxide | KOH |
| Sulphuric Acid | H2SO4 | Calcium Hydroxide | Ca(OH)2 |
| Ethanoic Acid | CH3COOH | Ammonia Solution | NH3 (aq) |
Hydronium Ion – same as a single proton because when a hydrogen atom loses an electron, only a proton remains. H+ is irresistibly attractive to water molecules and therefore it would form H3O+.
Dissociation – breaking apart
Strong Acids – in aqueous solutions, strong acids donate all their protons to water molecules.
Weak Acids – there is only a slight tendency to donate protons to water molecules, therefore an aqueous solution of a weak acid contains mainly undissociated molecules and a low concentration of H3O+.
| Strong Acids | Weak Acids | |
| Dissociation in Aqueous Solution | Completely dissociate | Partially dissociate |
| Equilibrium | None (forward only) | Equilibrium reaction |
| Electrolyte | Good | Poor |
| Electrical conductivity | Good | Poor |
| [H3O+] | Higher | Lower |
| pH value | Lower | Higher |
| Examples | HCl, HNO3, H2SO4 | +
CH3COOH, NH4 |
Amphiprotic – substances can act as both an acid and a base e.g. H2O, HCO3 , HSO4
Amphoteric – substances will undergo chemical reactions with both acids and bases
Neutralisation – an alkali or base can neutralise an acid by removing the H+ ions and converting them to water. Neutralisation always produces a salt.
Concentration – a measure of the amount of acid per dm3, refers to the proportion or ratio of acid to water in the solution
Concentrated Acids – high proportion of acid to water
Dilute Acids – low proportion of acid to water
Monoprotic – having one transferrable proton
Diprotic – having two transferrable protons
Titration – an indicator shows when the acid properties are just destroyed by the alkali. The salt can then be recovered by evaporating the water away allowing the salt to crystallise. This method is used when the base, acid and salt are all soluble.