Salts and Electrolysis

ACIDS AND ALKALIS

Pure water is neutral and has a pH of 7.  Acids are substances that produce hydrogen ions, H+(aq), when they are added to water.  When we dissolve a substance in water, we make an aqueous solution.  The state symbol, aq, shows that the ions are in aqueous solution.  Hydrogen ions make solutions acidic because they have pH values of less than 7.

Bases react with acids and neutralise them.  Alkalis are bases that dissolve in water to make alkaline solutions.  Alkalis produce hydroxide ions, OH- (aq), in the solution.  Alkaline solutions have pH values of more than 7.

The pH scale has values from 0 to 14, 0 being the most acidic and 14 being the most alkaline.  Indicators have different colours in acidic and alkaline solutions.  Universal indicator and full-range indicators have different colours at different pH values.

MAKING SALTS FROM METALS OR BASES

Acids will react with metals that are above hydrogen in the reactivity series.  However, the reactions of acids with very reactive metals, eg sodium and potassium, are too violent to be done safely.  When acids react with metals they produce a salt and hydrogen gas.

Acid + metal ----> a salt + hydrogen

eg sulfuric acid + zinc ----> zinc sulfate + hydrogen

H2SO4(aq) + Zn(s) ----> ZnSO4(aq) + H2(g)

Metal oxides and metal hydroxides are bases.  When an acid reacts with a base a neutralisation reaction takes place and a salt and water are produced.

Acid + base ----> a salt + water

eg Hydrochloric acid + magnesium oxide ----> magnesium chloride + water

2HCl(aq) + MgO(s) -----> MgCl2(aq) + H2O(l)

These reactions can be done to make salts.  A metal, or a base that is insoluble in water, is added a little at a time to the acid until all of the acid has reacted.  The mixture is then filtered to remove the excess solid reactant, leaving a solution of the salt.  The solid salt is made when water is evaporated from the solution so that it crystallises.

Chlorides are made from hydrochloric acid, nitrates from nitric acid and sulfates from sulfuric acid.

MAKING SALTS FROM SOLUTIONS

We can make soluble salts by reacting an acid and an alkali.

eg acid + alkali ----> a salt + water

We can represent the neutralisation reaction between an acid and an alkali using this equation:

H+(aq) + OH-(aq) -----> H2O(l)

There is no visible change when acids react with alkalis so we need to use an indicator or pH meter to show when the reaction is complete.  The solid salt can be obtained from the solution by crystallisation.

Ammonia solution is an alkali that does not contain a metal.  It reacts with acids to produce ammonium salts, such as ammonium nitrate, NH4NO3.  Ammonium salts are used as fertilisers.  We can make insoluble salts by mixing solutions of soluble salts that contain the ions needed…