Question 22N.2.hl.TZ0.5
| Date | November 2022 | Marks available | [Maximum mark: 10] | Reference code | 22N.2.hl.TZ0.5 |
| Level | hl | Paper | 2 | Time zone | TZ0 |
| Command term | Calculate, Deduce, Discuss, Sketch, State, Suggest, Write | Question number | 5 | Adapted from | N/A |
Lignite, a type of coal, contains about 0.40 % sulfur by mass.
Calculate the amount, in mol, of sulfur dioxide produced when 500.0 g of lignite undergoes combustion.
S (s) + O2 (g) → SO2 (g)
[2]
«» 2.0 «g» ✔
«» = 0.062 «mol of SO2» ✔
Award [2] for correct final answer.
Accept 0.063 «mol».
This question caused more problems than might have been anticipated with an average mark of 1.4/2. Many students had difficulty in calculating 0.4% of 500 g, with many getting 200 g (40% - again showing the need to read the question carefully). Even when they managed this, some used the molar mass of sulfur dioxide rather than that of sulfur.
Write an equation that shows how sulfur dioxide can produce acid rain.
[1]
SO2 (g) + H2O (l) → H2SO3 (aq)
OR
SO2 (g) + ½O2 (g) → SO3 (g) AND SO3 (g) + H2O (l) → H2SO4 (aq)
OR
SO2 (g) + ½O2 (g) + H2O (l) → H2SO4 (aq) ✔
Accept ionized forms of acids.
This proved quite challenging with about half the candidates producing many imaginative, but incorrect and often unbalanced equations, for SO2 producing acid rain.
Deduce the Lewis (electron dot) structure for sulfur dioxide.
[1]
OR
✔
Do not penalise missing formal charges.
Just over half of the students knew one of the contributing resonance Lewis structures for sulfur dioxide.
Sodium thiosulfate reacts with hydrochloric acid as shown:
Na2S2O3 (aq) + 2HCl (aq) → S (s) + SO2 (aq) + 2NaCl (aq) + H2O (l)
The precipitate of sulfur makes the mixture cloudy, so a mark underneath the reaction mixture becomes invisible with time.
Suggest two variables, other than concentration, that should be controlled when comparing relative rates at different temperatures.
[2]
Any two of:
depth/volume «of solution» ✔
colour/darkness/thickness/size/background of mark ✔
intensity of lighting in the lab ✔
Accept same size flask.
Accept position of observation/person observing.
Accept same equipment/apparatus.
Do not accept catalyst/particle size/pressure/time.
As is often the case with questions referencing practical skills, this was not well answered with the average being just over half marks with the mark achieved being largely unrelated to performance on the rest of the paper. It is unclear why factors that would have no effect and which cannot be controlled anyway (pressure and size of particle precipitated), are non-variables (such as not adding a catalyst), or are one of the actual variables measured (time/temperature) were more attractive to students than more obvious factors, such as the flask being used, the volumes of reagents, the mark being observed and the person observing it. Another shortcoming was that, even though two variables were specified, students frequently gave more and invoking the "list principle" which often resulted in the additional incorrect answers cancelling out marks already gained for correct responses.
Discuss two different ways to reduce the environmental impact of energy production from coal.
[2]
Any two of:
remove sulfur from coal ✔
add lime during combustion ✔
not allow sulfur oxides to be released into the environment ✔
reduce proportion/percentage of energy/power produced by «the combustion of» coal ✔
Accept any valid method to wash coal and remove sulfur content for M1.
Accept any valid combustion/post-combustion method to remove sulfur oxides.
Accept any suggestion that would reduce the amount of coal that is burnt or would reduce the damage caused.
Do not accept answers that only reduce production of SO2/CO2 from other fuels.
Accept “improve efficiency of energy production from coal”.
Accept “use coal of lower sulfur content” OR “cleaner coal”.
The average for this very open ended question was just over half marks. Many correct answers were possible, however some that occurred quite frequently (catalytic converters) are not relevant to the process and others (using public transport) only affect pollution from other sources, not the burning of coal.
SF4Cl2 can form two isomers, one which is polar and another non-polar. Deduce the 3-dimensional representations of both isomers of SF4Cl2.
[2]
Non-polar isomer:
✔
Polar isomer:
✔
Accept other methods of clearly representing 3D structure.
Many students understood the significance of cis/trans orientation, but frequently the diagrams produced did not convey a sufficient 3D impression to merit both marks, resulting in an average score of about 60%.