The Alkali Metals
4 marks · HardA student investigates Group 1 (alkali) metals and their compounds.
Rubidium and caesium are placed below potassium in Group 1 of the Periodic Table. Predict and explain how the reaction of caesium with water is expected to be different from lithium, sodium, and potassium.
The student has solid samples of lithium, sodium, and potassium. She cuts a fresh surface and leaves each in air. State what she would observe for each metal after a few minutes. Explain the reason for any differences in what she sees.
Group 1 metal ions form colourless compounds unless the other ion is coloured. The student prepares potassium sulfate crystals and notices that the crystals are white and dissolve easily in water. Give two reasons, based on the properties of Group 1 metal compounds, to explain these observations.
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(a) — · must show Caesium reacts even more violently than potassium with water, Reaction is faster, may be explosive, Explained by outer electron being further from nucleus/more easily lost
(b) — · must show Lithium surface tarnishes slowly (remains shiny longer), Sodium surface tarnishes rapidly, Potassium tarnishes extremely rapidly, Difference due to increasing reactivity down the group
(c) — · must show Group 1 metal ions are colourless so their compounds are white/colourless, Group 1 compounds are mostly soluble in water
Common mistakes: Stating caesium is less reactive than potassium; Omitting the speed or violence of reactions; Confusing compound colour origins
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Marking points
M1 (a) · 2 marks · Predicts caesium reacts more violently than K (explosive, very fast, or stronger than K/N/Li/Na).
M2 (a) · 2 marks · Mentions reason: outer electron further from nucleus or more easily lost for caesium.
Worked solution
a) Caesium is expected to react much more vigorously and explosively with water compared with lithium, sodium, and potassium. The reaction will be faster and more violent even than potassium, producing hydrogen gas extremely rapidly and possibly causing an explosion.
This is because the outer electron in caesium atoms is further from the nucleus and more weakly held, so it is lost even more easily. This makes caesium the most reactive of these Group 1 metals.
b) Lithium, when cut and exposed to air, will remain shiny for a longer period but eventually turns dull as it reacts slowly with oxygen to form a layer of lithium oxide. Sodium will lose its shine and tarnish much more rapidly, forming sodium oxide. Potassium will tarnish extremely quickly, almost immediately losing its shine, due to its greater reactivity with oxygen. The differences happen because reactivity increases down the group from lithium to potassium.
c) Potassium sulfate crystals are white because Group 1 metal ions are colourless and do not give colour to their salts, unless the other ion is coloured (which sulfate is not). They dissolve easily in water, as most Group 1 metal compounds are readily soluble. These are both typical properties of Group 1 metal compounds.