AQA Chemistry 5.4.2.2–5.4.2.4 · Combined and Triple · Required practical
Making salts and neutralisation
Acid plus an insoluble base makes a salt. Add the solid until no more reacts, filter, crystallise: the required practical, with the reason for every step.
Triple Science · Higher tier
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Salty water.
You have dissolved salt in a glass of water and now you want the salt back.
If you had to guess, would you pour it through a filter, or let the water evaporate?
Which salt?
The acid names the salt.
Acids are neutralised by metal oxides and metal hydroxides (bases; soluble hydroxides are alkalis) to produce salts and water, and by metal carbonates to produce salts, water and carbon dioxide. Hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates. The metal comes from the base or carbonate.
acid + metal oxide salt + water
acid + metal carbonate salt + water + carbon dioxide
CuO + H2SO4 CuSO4 + H2O
Zinc carbonate is added to hydrochloric acid. Name the salt.
Pick one.
Required practical · Combined RP8 · Chemistry RP1
A pure, dry sample of copper sulfate.
Preparation of a pure, dry sample of a soluble salt from an insoluble oxide or carbonate, using a Bunsen burner to heat dilute acid and a water bath or electric heater to evaporate the solution. Step through; before each step, say why.
Measure dilute sulfuric acid into a beaker and warm it gently with a Bunsen burner. Warm acid reacts faster with the solid. Before the next step: why keep adding copper oxide until some is left over?
With a carbonate instead of an oxide: add it until the fizzing stops and some solid is left over. That is how you know the carbonate is in excess.
Neutralisation · the ionic equation
Acids produce hydrogen ions, H+, in aqueous solution; alkalis produce hydroxide ions, OH−. In neutralisation between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water. The other ions (Na+ and Cl−, for sodium hydroxide and hydrochloric acid) are unchanged and stay in solution as the salt.
H+(aq) + OH−(aq) H2O(l)
Triple · when both reactants dissolve
Titration.
Sodium chloride from sodium hydroxide and hydrochloric acid cannot use the excess-solid method: there is no solid to filter off. Instead the volumes of acid and alkali that react with each other are measured by titration using a suitable indicator, then the reaction is repeated with those volumes and no indicator, and the solution crystallised. The method and the calculation are in Titrations.
Think again · the last step
“Boil the filtrate until it is dry and you have your crystals.”
Your call
ChooseHow should the copper sulfate solution be turned into crystals?
Sort
Which acid made it?
Place each salt.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- The method for a pure, dry sample of a soluble salt.
- Explain
- Why excess solid; why filter; why not boil dry.
- Name
- The salt from a given acid and base.
Key fact
Warm the acid, add the insoluble solid until no more reacts, filter off the excess, heat on a water bath until crystals start to form, leave to cool, pat dry. Excess solid means no acid is left in the salt.
Key note · AQA 5.4.2.2–5.4.2.4 · Combined RP8 · Chemistry RP1
Making salts and neutralisation
- Acids are neutralised by metal oxides and metal hydroxides (bases) to produce salts and water, and by metal carbonates to produce salts, water and carbon dioxide.
- Hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates. The metal comes from the base.
- Required practical (Combined RP8 = Chemistry RP1): warm the acid; add the insoluble oxide or carbonate until no more reacts (excess, so all the acid is used); filter off the excess solid; heat the filtrate on a water bath until crystals start to form; leave to cool; pat dry.
- With a carbonate: add until the fizzing stops and solid is left over.
- Neutralisation: H⁺(aq) + OH⁻(aq) → H2O(l).
- Triple: a salt from two soluble reactants is made by titration, then repeating without indicator.
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure why the solid has to be in excess?
Required practical numbering: Combined Science RP8 is the same practical as Chemistry RP1.