Question 19M.3.hl.TZ1.2
| Date | May 2019 | Marks available | [Maximum mark: 8] | Reference code | 19M.3.hl.TZ1.2 |
| Level | hl | Paper | 3 | Time zone | TZ1 |
| Command term | Estimate, Outline, Predict, State, Suggest | Question number | 2 | Adapted from | N/A |
Powdered zinc was reacted with 25.00 cm3 of 1.000 mol dm−3 copper(II) sulfate solution in an insulated beaker. Temperature was plotted against time.
Estimate the time at which the powdered zinc was placed in the beaker.
[1]
100 «s» [✔]
Note: Accept 90 to 100 s.
Nearly everyone correctly estimated 100s as the time when powdered zinc was added to the beaker.
State what point Y on the graph represents.
[1]
highest recorded temperature
OR
when rate of heat production equals rate of heat loss [✔]
Most scored the mark in for stating that point Y either indicated the maximum temperature or the end of the reaction.
The maximum temperature used to calculate the enthalpy of reaction was chosen at a point on the extrapolated (dotted) line.
State the maximum temperature which should be used and outline one assumption made in choosing this temperature on the extrapolated line.
Maximum temperature:
Assumption:
[2]
Maximum temperature:
73 «°C» [✔]
Assumption:
«temperature reached if» reaction instantaneous
OR
«temperature reached if reaction occurred» without heat loss [✔]
Note: Accept “rate of heat loss is constant” OR “rate of temperature decrease is constant”.
Stating the maximum temperature that should be used in calculations was less well answered, with answers between 63 and 65, or 78 commonly given instead of the correct answer of 73°C. Most candidates managed to score the second mark for stating “no heat loss”.
To determine the enthalpy of reaction the experiment was carried out five times. The same volume and concentration of copper(II) sulfate was used but the mass of zinc was different each time. Suggest, with a reason, if zinc or copper(II) sulfate should be in excess for each trial.
[1]
Any one of:
copper(II) sulfate AND mass/amount of zinc is independent variable/being changed
OR
copper(II) sulfate AND with zinc in excess there is no independent variable «as amount of copper(II) sulfate is fixed» [✔]
copper(II) sulfate AND having excess zinc will not yield different results in each trial [✔]
zinc AND results can be used to see if amount of zinc affects temperature rise «so this can be allowed for» [✔]
zinc AND reduces variables/keeps the amount reacting constant [✔]
Many candidates struggled to explain their choice of which reagent should be in excess. This question proved quite difficult with many candidates seeming to confuse independent and dependent variables.
The formula q = mcΔT was used to calculate the energy released. The values used in the calculation were m = 25.00 g, c = 4.18 J g−1 K−1.
State an assumption made when using these values for m and c.
[2]
Note: Accept “copper(II) sulfate/zinc sulfate” for “solution”.
Explaining the assumptions made when using values for m and c was challenging in. Many referred to the accuracy of the data when using m = 25.00g or said that no mass was lost during the reaction. Most knew that the value of c used was for water and suggested that the water was pure, but did not say that the specific heat of solution was assumed to be the same as that of water.
Predict, giving a reason, how the final enthalpy of reaction calculated from this experiment would compare with the theoretical value.
[1]
lower/less exothermic/less negative AND heat loss/some heat not accounted for
OR
lower/less exothermic/less negative AND mass of reaction mixture greater than 25.00 g
OR
greater/more exothermic/more negative AND specific heat of solution less than water [✔]
Most scored a mark for predicting how the calculated enthalpy value would compare with the theoretical value.