The Alkali Metals
5 marks · EasyRubidium (Rb) and caesium (Cs) are alkali metals placed below potassium in Group 1 of the Periodic Table.
Rubidium reacts with water to produce rubidium hydroxide and hydrogen gas. Write a balanced equation, including state symbols, for this reaction.
In an experiment, a student places a small piece of caesium in a trough containing 100 cm³ of water. The student collects the hydrogen gas given off and measures its volume as 45 cm³ at room temperature and pressure (rtp).
Calculate the number of moles of hydrogen gas produced. (Molar volume of gas at rtp = 24 000 cm³/mol)
Caesium is stored under oil. State what would be observed if a freshly cut piece of caesium was quickly exposed to air, and explain why this observation occurs.
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) 2Rb(s) + 2H2O(l) → 2RbOH(aq) + H2(g) · must show correct balancing, correct state symbols
(b) 0.00188 mol hydrogen gas mol · must show correct calculation using volume and molar volume
(c) The caesium will tarnish almost instantly; a dull coating forms quickly. This is because caesium reacts very rapidly with oxygen in air, forming caesium oxide. · must show immediate tarnishing/formation of dull coating, explanation referring to reaction with oxygen forming an oxide
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correct balanced chemical equation, including all state symbols.
M1 (b) · 1 mark · Uses formula: moles = volume / molar volume, substitutes values.
A2 (b) · 1 mark · Correct final answer to 2 or 3 significant figures (0.00188 mol or 1.88 × 10⁻³ mol). (follow-through)
B1 (c) · 1 mark · States rapid tarnishing or immediate formation of a dull/oxide layer.
B2 (c) · 1 mark · Explains this is due to reaction with oxygen in air, forming caesium oxide.
Worked solution
a) The balanced equation for rubidium reacting with water is:
b) Moles of hydrogen gas = volume / molar volume = 45 cm³ / 24 000 cm³/mol = 0.001875 mol = 0.00188 mol (to 3 significant figures).
c) If caesium is exposed to air, the shiny surface will quickly become dull as a layer of caesium oxide forms almost instantly. This is because caesium reacts extremely rapidly with oxygen in air, causing immediate tarnishing due to the formation of an oxide coating.