Groups 1 & 2 (Edexcel International AS Chemistry): Exam Questions

This question is about Group 1 metals.

When potassium is placed into a beaker of cold water, potassium hydroxide and hydrogen are formed.

i) Write the equation for this reaction. Include state symbols.

(2)

ii) This is a redox reaction.

State which element is oxidised and which is reduced.

Justify your answer by giving the initial and final oxidation numbers of any element that changes oxidation state.

(2)

iii) The reaction of potassium with water is very vigorous and a flame is seen.

State the colour of the flame.

(1)

The label has come off a bottle known to contain M, a Group 1 metal which is stored in oil. A student carried out an experiment to determine the identity of M.

Procedure

Step 1

A small piece of M was wiped with tissue paper to remove the oil.
The piece of M was weighed and placed in a beaker of distilled water.

Step 2

After the reaction had finished, the contents of the beaker and washings were transferred to a 250.0 cm³ volumetric flask. The solution was made up to the mark with distilled water and mixed thoroughly.

Step 3

A pipette was used to transfer 25.0 cm³ portions of this solution to conical flasks. Each portion was then titrated with hydrochloric acid of concentration 0.400 mol dm^–3.

Results

The reaction taking place is shown.

MOH (aq) + HCl (aq) → MCl (aq) + H₂O (l)

i) The indicator used was phenolphthalein. State the colour change at the end-point.

(2)

from ........................................... to ......................................

ii) Calculate the relative atomic mass of M and use it to identify the Group 1 metal, M.

(4)

Another student repeated the experiment, using a different sample of metal M, but did not wipe off the oil before weighing it.

State how this would change the calculated value of the relative atomic mass of M. Justify your answer.