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WJEC GCSE Additional Science Teacher’s Notes 9 Alkali metals and halogens _The alkali metals (pages 83–91)__________________ PRACTICAL Observing patterns of reactivity – the alkali metals (pages 87–89) Safety: take care with these demonstrations and ensure that you consult the relevant CLEAPSS guidance. 1. Which alkali metal reacts most vigorously with: a air Potassium b water Potassium c chlorine Potassium 2. Using your observations, arrange the three alkali metals in order of their reactivity (from least to most reactive). Lithium then sodium then potassium 3. Using a Periodic Table, arrange all the Group 1 metals in order of reactivity. Lithium, sodium, potassium, rubidium, caesium, francium 4. How does reactivity vary as you move down Group 1 of the Periodic Table The elements get more reactive as you move down the Group. 5. Look again at the electronic structure of the alkali metals in Table 9.1. Is there a pattern between the electron structure and the reactivity of the alkali metals Can you explain this pattern Alkali metals all have a single electron in their outer shell. The further down the group, the weaker this single electron is held to the atom (there are more electron shells beneath it). The weaker the electron is held, the more reactive the metal. 6. Predict the observations that you mighht make from the reactions of rubidium with oxygen (in air), water and chlorine. Rubidium will react much more violently with all three reactants, exploding on contact (see Discussion Point). 7. Why are the reactions of potassium with chlorine, and any of the other alkali metals with any of the other chemicals, not allowed to be carried out in schools The reactions are far too violent to be carried out using normal school facilities. 8. Use the word and balanced symbol equations for lithium to produce similar word and balanced symbol equations for the reactions of sodium and potassium with oxygen, water and chlorine. 24
WJEC GCSE Additional Science Teacher’s Notes sodium + oxygen → sodium oxide 4Na(s) + O 2 (g) → 2Na 2 O(s) sodium + water → sodium hydroxide + hydrogen 2Na(s) + 2H 2 O(l) → 2NaOH(aq) + H 2 (g) sodium + chlorine → sodium chloride 2Na(s) + Cl 2 (g) → 2NaCl(s) potassium + oxygen → potassium oxide 4K(s) + O 2 (g) → 2K 2 O(s) potassium + water → potassium hydroxide + hydrogen 2K(s) + 2H 2 O(l) → 2KOH(aq) + H 2 (g) potassium + chlorine → potassium chloride 2K(s) + Cl 2 (g) → 2KCl(s) 9. Bromine is a halogen like chlorine but it is less reactive than chlorine gas. Predict the reactions of bromine with lithium, sodium and potassium. Lithium burns readily in bromine, as do sodium and potassium, the reactions becoming more violent from sodium to potassium. 10. Write word and balanced symbol equations for the reactions of bromine with lithium, sodium and potassium. lithium + bromine → lithium bromide 2Li(s) + Br 2 (g) → 2LiBr(s) sodium + bromine → sodium bromide 2Na(s) + Br 2 (g) → 2NaBr(s) potassium + bromine → potassium bromide 2K(s) + Br 2 (g) → 2KBr(s) Discussion point Rubidium and caesium react far too violently with oxygen, water and chlorine to be performed even by demonstration in schools. You can, however, see video clips online of these reactions occuring under very special controlled conditions. The best example of this is Brainiac: Science Abuse! www.youtube.com/watchv=eCkolYB_8co Show the YouTube clip in conjunction with Question 6 above. PRACTICAL Flame tests of alkali metal salts (pages 90–91) Safety – take care with these practicals and ensure that you consult the relevant CLEAPSS guidance. 1. Were there any differences in the colours of the different alkali metal salts using the three different methods The flame colours will be very similar with each method. 25
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