Briefly covers C3 2.1-4

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Strong and Weak Acids/Alkalis

Acids form H+ ions when dissolved in water. This is when the H loses its electron so its simply a proton it is therefore a proton donor.

Bases form OH- ions when dissolved in water. Bases or alkalis are proton acceptors.

A strong acid or alkali is one that is 100% ionised in water. Hydrochloric, sulfuric and nitric acids are all examples of strong acids. Sodium hydroxide and potassium hydroxide are strong alkalis. Weak acids or alkalis are only partially ionised in water - like ethanol, citric acid and carbolic acid, or ammonia for alkalis.

We can tell if we have a strong or weak acid by its Ph, the lower the Ph the stronger the acid and the higher the Ph the stronger the alkali.

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Used to find volumes of acids and alkalis that react and neutralise completely.

Example Question:

25cm3 Of NaOH(aq) reacts with 20cm3, 0.5 mol/dm3 of HCl. Whats the concentration of NaOH?

  • First, write out the full balanced chemical formula - NaOH(aq) + HCl(aq) --> NaCl(aq) + H2O(l)
  • We know that one mole of NaOH reacts with one mole of HCl, so we need to find the number of moles of HCl, then we can find out the number of moles in HCl and then the volume. To do this we need to times the volume by the concentration of HCl and then divide by 1000 --> 20 x 0.5 / 1000 = 0.01 Therefore there are 0.01 moles of HCl in 20cm3.
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  • Because there there is one mole of both NaOH and HCl there is 0.01 moles of NaOH in 25cm3. To find out the concentration we have to divide the 0.01 by 25 then times by 1000 = 0.4.
  • Therefor the concentration of NaOH is 0.4 mol/dm3. ( Indicators - For a strong acid + weak alkali use methyl orange - For a weak acid + strong alkali use phenolphthanlein - you must know this!)
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