Rubidium and caesium
4 marks · NormalThe table below shows times taken for different alkali metals to disappear when added to water:
| Metal | Time (s) |
|---|---|
| Lithium | 50 |
| Sodium | 30 |
| Potassium | 11 |
State the relationship between the metal and the time taken to disappear in water.
Which metal is the most reactive? Explain how the data supports your answer.
Rubidium is below potassium in Group 1. Predict what you would expect to observe if rubidium was added to water.
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.
Show answer
(a) Down the group, the metals disappear faster in water (time decreases as you go down the group)
(b) Potassium is the most reactive; it disappears the fastest (in the shortest time). Lower time means it is reacting more quickly.
(c) Rubidium would react even more violently, possibly explosively, and disappear almost instantly.
Common mistakes: Not comparing time and reactivity; Ignoring the trend down the group; Saying sodium is the most reactive
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states the trend (down the group, time decreases/disappear faster)
B2 (b) · 1 mark · Correctly identifies potassium as most reactive
B3 (b) · 1 mark · Explanation: identifies that shortest time means most reactive
B4 (c) · 1 mark · Prediction: rubidium reacts more violently/explosively or faster
Worked solution
a) The lower the metal in Group 1, the faster it disappears in water (time gets shorter down the group).
b) Potassium is the most reactive because it disappears fastest (11 s is the shortest time).
c) Rubidium would react even more violently and could explode, disappearing almost instantly.