Required Practicals (Paper 2)

Question 1

Describe a method to investigate the rate of reaction by colour change or turbidity?

Answer 1

1. Using a measuring cylinder, add 40cm³ of dilute sodium thiosulfate solution to a conical flask.
2. Place the conical flask on a piece of paper with a black cross drawn on it.
3. Using a different measuring cylinder, add 10cm³ of hydrochloric acid to the flask and gently stir
4. Look down through the reaction mixture. When the cross can no longer be seen (mixture opaque), it signal the end of the reaction, so record the time on the stop clock.
5. Calculate rate of reaction (1000/s)
6. Repeat steps 1 to 5 with different volumes of sodium thiosulfate solution (using different volumes of HCl and sodium thiosulphate)

Keep temperature the same

Question 2

Describe a method to investigate the rate of reaction by production of gas?

Answer 2

1. Support a gas syringe with a stand, boss and clamp.
2. Using a measuring cylinder, add 50cm³ of HCl to a conical flask.
3. Add the same amount calcium carbonate to the flask. Immediately connect the gas syringe and start a stop clock.
4. Every 10 seconds, record the volume of gas produced.
5. Calculate rate of reaction
6. Repeat steps 1 to 5 with different concentrations of hydrochloric acid

Question 3

A coloulrless solution contains potassium ions, but the identity of the negatively charged ion is a mystery. Describe a series of tests which would enable you to deduce if the negatively charged ion was iodide, carbonate, sulfate, chloride or bromide

Answer 3

• take small samples from the solution for the following tests
• add some dilute acid to one of the samples
• if it fizzes and the gas turns limewater cloudy then the carbonate ion is present
• add some dilute barium chloride solution to one of the samples
• if a white precipitate forms then sulfate ions are present
• add some dilute silver nitrate to one of the samples
• if a white precipitate forms then the chloride ion is present
• if a cream precipitate forms then the bromide ion is present
• if a yellow precipitate forms then the iodide ion is present

knowledge of RP7 relies on content learnt from 8.3, so make sure you cover this

Question 4

Devise a series of experiments to distinguish between the following compounds.

• sodium chloride
• sodium carbonate
• potassium chloride
• potassium iodide
• potassium sulfate

Answer 4

• first, do a flame test
• two compounds will give a yellow colour which identifies them as the sodium compounds
• three compounds will give a lilac colour, which identifies them as the potassium compounds
• then, test with dilute nitric acid, followed by silver nitrate…
• the sodium carbonate will fizz, and the gas will turn limewater cloudy
• the sodium chloride and potassium chloride will produce a white precipitate
• the potassium iodide will produce a yellow precipitate
• the potassium sulfate will not produce a precipitate
• (the potassium sulfate will produce a precipitate with barium chloride)

knowledge of RP7 relies on content learnt from 8.3, so make sure you cover this

Question 5

Aluminium, calcium and magnesium ions all form white precipitates when NaOH solution is added to them, how would you distinguish between them?

Answer 5

Only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution, so the one that does will contain aluminium ions

Then use flame test of calcium and magnesium ion containing substances

Calcium burns with an orange-red flame, so the one that does will contain calcium ions

Magnesium does not burn with a uniquely coloured flame

Question 6

Describe how to carry out a flame test

Answer 6

• Dip loop of unreactive metal (e.g. platinum or nichrome) in dilute acid
• Then hold it in blue flame until there is no colour change to decontaminate the loop
• because the test will only work if there is one metal ion present
• Dip loop in sample and hold it at end of blue flame
• Use the colour observed to identify the metal ion present