The Alkali Metals
7 marks · HardThis question is about the reactions of rubidium and caesium with water and the reactions of Group 1 metals with air.
Describe what you would observe when a small piece of caesium is added to water. Explain your answer in terms of the position of caesium in Group 1.
Sodium reacts with oxygen in the air to form a white solid of sodium oxide. Write a balanced chemical equation for this reaction, including state symbols.
Caesium oxide is also a white solid. Predict its formula and give a reason for your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show explosion or very violent reaction, production of gas/fizzing, white solid formed, link to caesium being below potassium and more reactive because outer electron is more easily lost due to greater atomic size/more shells
(b) 4Na(s) + O2(g) → 2Na2O(s) · must show balanced symbol equation, correct state symbols
(c) Cs2O; caesium, like other group 1 metals, forms an oxide with formula M2O because it forms +1 ions and oxygen forms -2 ions. · must show Cs2O, reason relating to group 1 always forming +1 ions and balancing oxide ion charge
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Marking points
B1 (a) · 1 mark · States that caesium reacts explosively or very violently when added to water.
B2 (a) · 1 mark · Describes observations: fizzing, rapid movement, gas production, or white solid formation.
B3 (a) · 1 mark · Explains that caesium is more reactive because its outer electron is further from the nucleus or more easily lost (due to increased atomic size, more electron shells, or decreased attraction).
B4 (b) · 1 mark · Gives a correctly balanced equation for sodium and oxygen.
B5 (b) · 1 mark · Includes the correct state symbols in the equation.
B6 (c) · 1 mark · Gives the formula Cs2O for caesium oxide.
B7 (c) · 1 mark · Explains the formula using the charges on Group 1 ion (+1) and oxide ion (O^2−) – two Cs+ for every O^2−.
Worked solution
a) When a small piece of caesium is added to water, it reacts extremely violently, much more so than potassium. The caesium skims rapidly across the surface, fizzing as hydrogen gas is produced, and the reaction may end with an explosion. A white solid of caesium hydroxide forms in the solution.
This violent reaction happens because caesium is further down Group 1 than potassium or sodium. Its outer electron is further from the nucleus, is less strongly attracted, and is more easily lost, making caesium the most reactive of these metals.
b) The balanced equation for the reaction of sodium with oxygen is:
c) As caesium is a Group 1 metal, its oxide will have the formula Cs2O. This is because all Group 1 metals form ions with a 1+ charge, and the oxide ion has a 2– charge. Two Cs+ ions are needed to balance one O2– ion, so the formula is Cs2O.