Question 22N.2.sl.TZ0.3
| Date | November 2022 | Marks available | [Maximum mark: 8] | Reference code | 22N.2.sl.TZ0.3 |
| Level | sl | Paper | 2 | Time zone | TZ0 |
| Command term | Label, Outline, Predict, Show, State, Write | Question number | 3 | Adapted from | N/A |
Consider the following reaction:
Cu2+ (aq) + Fe (s) → Fe2+ (aq) + Cu (s)
State the ground-state electron configuration for Fe2+.
[1]
1s22s22p63s23p63d6
OR
[Ar]3d6 ✔
Surprisingly poorly answered. Only 20% of the candidates recognized that the 4s electrons are lost before the 3d electrons and calculated the number of electrons in Fe2+ correctly. A small proportion of candidates did not list the subshells in the correct order.
The mass spectrum for copper is shown:
Source: WebElements, n.d. Copper: isotope data [online] Available at:
https://www.webelements.com/copper/isotopes.html [Accessed 6 October 2021].
Show how a relative atomic mass of copper of 63.62 can be obtained from this mass spectrum.
[1]
OR
AND «%» AND «%» ✔
A very well answered question. 70% of the candidates were able to show how the relative atomic mass of copper could be calculated using data from the mass spectrum given. Some candidates did not obtain the same value of the relative atomic mass because they did not read the percentage abundance accurately (69% and 31%). 11% of the candidates did not attempt the question.
The diagram shows an unlabelled voltaic cell for the reaction:
Cu2+ (aq) + Fe (s) → Fe2+ (aq) + Cu (s)
Label the diagram with the species from the equation and the direction of electron flow.
[2]
all 4 species correctly labelled ✔
arrow showing electron flow from anode to cathode in the external circuit ✔
Accept any soluble salt of copper(II) for Cu2+ and any soluble salt of iron(II) for Fe2+.
Do not apply ECF for M2.
Half of the candidates labelled the voltaic cell components and identified the direction of electron flow correctly. The most common mistake was reversing the anode and the cathode components. Other candidates mixed up the components in unusual ways perhaps indicating that they did not have the opportunity to conduct this lab. This question had the highest discrimination index on the paper.
Write the half-equation for the reaction occurring at the anode (negative electrode).
[1]
Fe (s) → Fe2+ (aq) + 2e− ✔
Accept equilibrium arrow.
Do not award ECF for Cu (s) → Cu2+ (aq) + 2e−.
This was a well answered question. 60% of the candidates deduced the half-equation for the reaction occurring at the anode. The question also discriminated well between high-scoring and low-scoring candidates.
The diagram includes a salt bridge that is filled with a saturated solution of KNO3. Outline the function of the salt bridge.
[1]
«keep» each half-cell/electrolyte «electrically» neutral ✔
Accept balance charges/ions.
Accept allow ion flow «between cells».
The majority of the candidates outlined the function of the salt bridge. One of the common answers that did not score the mark was that the salt bridge allowed electrons to flow (without mentioning ions).
Predict the movement of all ionic species through the salt bridge.
[2]
NO3– to anode/Fe/left ✔
K+ «and Fe2+» to cathode/Cu/right ✔
Accept other specific anions in addition to nitrate for M1.
Award [1 max] for “anions/negative ions to anode AND cations/positive ions to cathode”.
This was a challenging question for the candidates. Many candidates did not identify K+ and NO3– as the ions in the salt bridge and only discussed the flow of the Fe2+ and Cu2+ ions. The average mark was 0.5 out of a total of 2 marks.
