Chapter 10 - Making Salts

What is a Salt?

• Salt is formed when the hydrogen in an acid is replaced by a metal (E.g. Sulphates come from Sulphuric acid, Chlorides come from Hydrochloric acid, Nitrates come from nitric acid).

Summary of Making Salts:

Is the salt soluble?

• If no -> Use a precipitation method. Mix two solutions, one containing the correct positive ion and one containing he correct negative ion.
• If yes -> Move to the next step...

Is it a sodium, potassium or ammonium salt?

• If no -> React an acid with an excess of a solid metal, metal oxide, hydroxide or carbonate.
• If yes -> Use a titration method. React an acid with a solution of sodiun hydroxide, potassium hydroxide, carbonate or ammonia solution.

Some salts are soluble, while others are insoluble. You need to memorise these:

• Nitrates -> NH4NO3 (Ammonium), KNO3 (Potassium), NaNO3 (Sodium), Ba[NO3]2 (Barium), Ca[NO3]2 (Calcium), Mg[NO3]2 (Magnesium), Al[NO3]3 (Aluminium), Zn[NO3]2 (Zinc), Fe[NO3]2 (Iron), Pb[NO3]2 (Lead), Cu[NO3]2 (Copper), AgNO3 (Silver)
• Chlorides -> NH4Cl, KCl, NaCl, BaCl2, MgCl2, AlCl3, ZnCl2, FeCl2, PbCl2, CuCl2, AgCl
• Sulphates -> [NH4]2SO4, K2SO4, Na2SO4, BaSO4, CaSO4, MgSO4, Al2[SO4]3, ZnSO4, FeSO4, PbSO4, CuSO4, Ag2SO4
• Carbonates -> [NH4]2CO3, K2CO3, Na2CO3, BaCO3, CaCO3, MgCO3, Al2[CO3]3, ZnCO3, FeCO3, PbCO3, CuCO3, Ag2CO3 
• Hydroxides -> NH4OH, KOH, NaOH, Ba[OH]2, Ca[OH]2, Mg[OH]2, Al[OH]3, Zn[OH]2, Fe[OH]2, Pb[OH]2, Cu[OH]2, Ag2OH

All the insoluble salts are highlighted in blue.

All the salts highlighted in pink are slightly soluble or almost insoluble.

Soluble salts (except Na, K and NH4) can be made by reacting:

• An acid and a moderately reactive metal             Note that the reaction needs to be heated unless
• An acid and a metal oxide or hydroxide               it involves carbonates or magnesium, in which
• An acid and a carbonate                                     case they do not need to be heated.

Making Magnesium Sulphate:

• Mg(s) + H2SO4(aq) -> MgSO4(aq) + H2(g)     [The MgSO4 solution would be crystallised]

Making Na, K and NH4 Salts using the Titration Method:

• You can do the titration reaction using NaOH, KOH or an NH4 solution, or with carbonates.
• All of these are alkaline so you can use indicator to see when the solution is neutral.
• Once you know how much acid is needed, you can react them without the indicator.

Making Sodium Sulphate Crystals:

• 2NaOH(aq) + H2SO4(aq) -> Na2SO4(aq) + 2H2O(l) 
• Methyl Orange was used as the indicator with 25cm2 of sodium hydroxide solution.

Precipitation Reactions:    (A reaction that produces a precipitate [a fine solid])

The precipitate forms because that is the insoluble salt, so does not form an aqueous solution.

What Happens in the Reaction?   (Using the example of Silver Chloride)

• AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)
• Silver nitrate solution contains both silver and nitrate ions that are attracted to each other (although not very strongly - not enough for then to stick together)
• Similarly, sodiun chloride contains sodium and chloride ions but their attractions are not strong enough to make them stick together.
• When you mix the silver nitrate solution and the sodium chloride solution, the silver and hloride ions have a strong attraction to each other, making them clump together - this is the precipitate of silver chloride.

The ionic half equation for this is:

• Ag+(aq) + Cl-(aq) -> AgCl(s)

It is the same principle for all insoluble salts.