Acids Notes

Edexcel GCSE Chemistry: Revision notes

Key facts

  • Acids release hydrogen ions (HX+\ce{H+}); alkalis release hydroxide ions (OHX−\ce{OH-}). pH 7 is neutral.
  • Acid + metal gives salt + hydrogen; + oxide or hydroxide gives salt + water; + carbonate gives salt + water + carbon dioxide.
  • Neutralisation is HX++OHX−→HX2O\ce{H+ + OH- -> H2O}.
  • Make a salt from an insoluble base with excess solid and filtration; from an alkali use titration.
  • Use the solubility rules to predict precipitates.
01234567891011121314AcidicNeutral 7Alkaline

Acids, alkalis and pH

Acids give hydrogen ions, alkalis give hydroxide ions, and pH 7 is neutral.

Acids in solution are sources of hydrogen ions (HX+\ce{H+}) and alkalis are sources of hydroxide ions (OHX−\ce{OH-}). The pH scale measures acidity: below 7 is acidic and above 7 is alkaline. (Higher tier) Each tenfold increase in HX+\ce{H+} concentration lowers the pH by 1.

01234567891011121314AcidicNeutral 7Alkaline

Litmus

In acid:
Red
In alkali:
Blue

Methyl orange

In acid:
Red
In alkali:
Yellow

Phenolphthalein

In acid:
Colourless
In alkali:
Pink

What colour is phenolphthalein in an alkaline solution?

Dilute, concentrated, weak and strong

Concentration is the amount dissolved; strength is how fully the acid ionises.

Dilute and concentrated describe the amount of acid dissolved in a given volume. Weak and strong describe the degree of dissociation: a strong acid fully dissociates into ions, a weak acid only partially, so most stays as molecules.

Dilute

Concentrated

Dilute and concentrated describe how much acid (second kind of particle) is dissolved in a given volume.

Concentrated or dilute

Describes:
Amount dissolved
Meaning:
Large or small amount in a given volume

Strong or weak

Describes:
Degree of ionisation
Meaning:
Fully or partially dissociated into ions

A student says a dilute acid must be a weak acid. Is she correct?

Reactions of acids

Acids react with metals, oxides, hydroxides and carbonates to make salts.

A base reacts with an acid to form a salt and water only, and alkalis are soluble bases. Neutralisation is the reaction between an acid and a base. For example, hydrochloric acid + sodium hydroxide gives sodium chloride + water.

51015202530354045502468101214xyneutral, pH 7pH
Illustrative pH as an alkali is added to an acid: the pH rises sharply through 7 at neutralisation.

Acid + metal

  • Salt + hydrogen

Acid + metal oxide

  • Salt + water

Acid + metal hydroxide

  • Salt + water

Acid + metal carbonate

  • Salt + water + carbon dioxide
  • NeutralisationHX++OHX−→HX2O\ce{H+ + OH- -> H2O}

Hydrogen

  • Lit splint gives a squeaky pop

Carbon dioxide

  • Limewater turns cloudy or milky

Which products form when an acid reacts with a metal carbonate?

Preparing salts

Use excess insoluble base and filtering, titration for soluble reactants, or precipitation for insoluble salts.

With an insoluble base, add excess so all the acid reacts, filter off the excess, then evaporate and crystallise. If both reactants are soluble you cannot filter off excess, so use titration with a burette, pipette and indicator to add exact volumes.

  1. 1

    Warm the acid

    in a water bath

  2. 2

    Add copper oxide

    in excess, so all the acid reacts

  3. 3

    Filter

    to remove the excess copper oxide

  4. 4

    Evaporate

    to concentrate the solution

  5. 5

    Crystallise

    then dry the crystals

Core practical: copper sulfate crystals from copper oxide

Insoluble base

  • Add excess solid
  • Filter, evaporate, crystallise

Soluble alkali

  • Titration
  • Exact volumes with an indicator

Insoluble salt

  • Mix two soluble solutions
  • Filter, wash and dry the precipitate

Why can't you use excess solid and filtering to make sodium sulfate from sodium hydroxide and sulfuric acid?

Solubility rules and precipitates

Use the solubility rules to decide which product is insoluble.

Using the rules you can predict whether mixing two solutions forms a precipitate, and name it. For example, lead nitrate and potassium iodide solutions form a precipitate of lead iodide.

  1. 1

    Mix lead nitrate and potassium iodide solutions

    both are soluble

  2. 2

    Swap partners

    lead iodide and potassium nitrate could form

  3. 3

    Check the rules

    lead iodide is insoluble; potassium nitrate stays dissolved

  4. 4

    Precipitate forms

    solid lead iodide appears in the mixture

Predicting a precipitate from the solubility rules.

Soluble

  • Sodium, potassium and ammonium salts
  • All nitrates
  • Most chlorides (not silver or lead)
  • Most sulfates (not lead, barium or calcium)

Insoluble

  • Common carbonates
  • Common hydroxides
  • (except sodium, potassium, ammonium)

Worked example

Silver nitrate solution is mixed with sodium chloride solution. Name the precipitate.

Silver nitrate solution is mixed with sodium chloride solution. Which precipitate forms?

Try an exam question

Describe how to prepare pure, dry crystals of copper sulfate from copper oxide and sulfuric acid.

[5 marks]

That's the notes covered.

Carry on to the next subtopic.