Rubidium and caesium
3 marks · NormalA student is given the information below about the reactions of three Group 1 metals with water:
| Metal | Time taken to disappear in water (s) |
|---|---|
| Lithium | 45 |
| Sodium | 35 |
| Potassium | 18 |
Calculate the average decrease in time taken to disappear from lithium to potassium per step down the group.
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13.5 s per step (±0.5)
Common mistakes: Only subtracting lithium from potassium; Not taking average per step
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Marking points
M1 · 1 mark · Subtract time for potassium from time for lithium to find the total decrease
M2 · 1 mark · Divide by the number of steps between metals (2)
A1 · 1 mark · Final answer correct to nearest 0.5 s
Worked solution
Total decrease in time = 45 s (Lithium) – 18 s (Potassium) = 27 s
Number of steps: lithium to sodium, sodium to potassium = 2 steps
Average decrease per step = 27 s ÷ 2 = 13.5 s