6 markers

Question 1

(6) Sodium chloride is an ionic compound.
It contains sodium ions, Na+, and chloride ions, Cl–
.
When molten sodium chloride is electrolysed, sodium metal and chlorine gas are
formed.
Describe how the sodium ions and chloride ions in solid sodium chloride are
converted into sodium and chlorine by electrolysis.

Answer 1

sodium chloride is melted and ions are free to move
at cathode
 {sodium / positive} ions move to cathode
 (sodium) metal forms at cathode
 sodium ions gain electrons ...
 to form atoms
 this is reduction
 Na+ + e-
  Na
at anode
 {chloride / negative} ions move to anode
 (chlorine) gas forms at anode
 chloride ions lose electrons ...
 to form atoms
 this is oxidation
 two chlorine atoms combine to form a chlorine molecule /
share electrons
 Cl-
  Cl + e-
 / 2Cl-
  Cl2 + 2e-

 2 Cl  Cl2

Question 2

(6) Describe a titration experiment to find the exact volume of hydrochloric acid
needed to neutralise 25.0 cm3
of sodium hydroxide solution and how you would
use this result to obtain pure, dry crystals of sodium chloride.

Answer 2

titration experiment
• rinse pipette with alkali and burette with acid
• measure alkali using a pipette
• into suitable container e.g. flask/beaker
• add a few drops of indicator / suitable named indicator (eg
methyl orange/phenolphthalein)
• flask on a white tile
• fill burette with acid
• read level/volume (of acid) in burette
• add acid from burette to the flask slowly / swirl the flask
• until {indicator just changes colour/correct colour change for
named indicator (eg methyl orange yellow to peach/orange,
phenolphthalein pink to colourless)/solution is neutral}
• read level/volume (of acid) in burette
• repeat experiment
• until concordant results

salt preparation
• mix the same volume of alkali with the volume of acid
determined from the first experiment but do not add
indicator (or add (activated) charcoal to remove indicator,
then filter)
• pour solution into an evaporating basin
• {heat solution/leave the water to evaporate} until pure salt
crystals are left

Question 3

(6) If nitrogen and hydrogen were reacted at 90 atm pressure and 300 °C, without a
catalyst, some ammonia would be formed eventually.
In the Haber process a pressure of 150 atm and a temperature of 450 °C are used,
in the presence of an iron catalyst.
Explain, with reasons, why the Haber process conditions are better for the
manufacture of ammonia.

Answer 3

Higher pressure:
 higher pressure gives increased yield
 equilibrium shifts to right hand side
 because decrease in number of molecules going from left to
right
 therefore decrease in volume
 favoured by increase in pressure
 reaches equilibrium faster
 because molecules closer together
 so get more frequent collisions
Higher temperature
 higher temperature reaches equilibrium faster
 because molecules move faster
 therefore more frequent collisions
 molecules have more energy
 therefore more collisions have required energy
 but yield will be lower
 because higher temperature favours endothermic reaction
 equilibrium shifts to left hand side
 which is decomposition of ammonia / ammonia reforms
elements
Use of catalyst
 catalyst causes reaction to reach equilibrium faster /
catalyst increases rates (of both forward and back reactions)
 lowers the activation energy (of both forward and back
reactions)
 reaction follows a new pathway
 does not affect yield
 equilibrium position not affected

Question 4

(6) A technician found some colourless crystals of a substance left, unlabelled, in a
beaker in a laboratory.
She knew the substance was one of potassium sulfate, potassium iodide, sodium
sulfate or sodium iodide.
Explain how, using chemical tests, the technician could find out if the substance
left in the beaker was potassium sulfate, potassium iodide, sodium sulfate or
sodium iodide.
You may include equations in your answer.

Answer 4

test for cation
• flame test
• if the flame is yellow/not lilac, sodium ions are present
• if the flame is lilac/not yellow, potassium ions are
present
test for iodide ions
• make a solution of the crystals in water
• add dilute nitric acid
• add silver nitrate solution
• if there is a yellow precipitate, iodide ions are present
• if there is no precipitate, sulfate ions are present
• Ag+ + I¯ → AgI
OR
• make a solution of the crystals in water
• add chlorine water
• then cyclohexane
• if the cyclohexane/top layer turns purple, iodide ions
were present
• if there is no colour change, sulfate ions are present
• Cl2 + 2I-
→ 2Cl-
+ I2
test for sulfate ions
• make a solution of the crystals in water
• add dilute {hydrochloric/nitric} acid
• add barium {chloride/nitrate} solution
• if there is a white precipitate, sulfate ions are present
• if there is no precipitate, iodide ions are present
• Ba2+ + SO4
2- → BaSO4

Question 5

(6) Sodium sulfate solution contains sodium ions, Na+, sulfate ions, SO4
2–,
hydrogen ions, H+, and hydroxide ions, OH–
.
Hydrogen is produced at one electrode and oxygen is produced at the other
electrode.
Explain how these products are formed from the ions in the electrolysis
process, indicating how you would identify the products.
You may give ionic equations in your answer.

Answer 5

electrolysis process
 ions move when current passed
 negative ions move to anode
 overall decomposition of water
 2H2O → 2H2+O2
Anode/ positive electrode
 sulphate ions move to anode
 hydroxide ions move to anode
 hydroxide ions lose electrons/oxidation
 hydroxide ions form oxygen
 half equation: 4OH- → O2 + 2H2O + 4e-
 half volume gas at this electrode
 oxygen test: glowing splint in gas relights
cathode/ negative electrode
 sodium ions move to cathode
 hydrogen ions move to cathode
 hydrogen ions gain electrons / reduction
 hydrogen ions form hydrogen
 half equation: 2H+ + 2e- → H2
 double volume gas at this electrode
 hydrogen test: lit splint in gas burns/ pops
 hence double volume of hydrogen gas

Question 6

(6) When mixed with a small volume of soap solution, hard water does not form a
lather.
You are provided with three unlabelled samples of water.
t one is soft water
t one is permanent hard water
t one is temporary hard water
You are provided with soap solution and the usual laboratory apparatus.
Describe tests that you should carry out on each sample to identify the type of
water in each sample.

Answer 6

soft
 add soap (solution)
 shake/ mix
 lather (immediately)
 no scum/ no precipitate

permanent hard
 add soap (solution)
 shake
 no lather / less than with 
soft water
 scum/ precipitate
 boiled sample
 same results / boiling does not change
 becomes soft after ion exchange but not after boiling

temporary hard
 add soap (solution)
 shake
 no lather / less than with soft water
 scum/ precipitate
 boiled sample
 after boiling precipitate / (lime)scale formed
 lather (immediately)