Question 2
- An electrochemical cell is constructed with copper and silver electrodes.
- State the type of reaction occurring at each electrode.
- Write a balanced equation for the overall cell reaction.
- Draw and label a diagram for the cell.
-
- Explain briefly the term cracking.
- State two conditions under which cracking takes place.
- What is the importance of cracking in the petroleum industry?
- (i) Write the electron configuration for:
- 29Cu;
- 29Cu+.
- (i) Define the term oxidation number.
[9 marks]
(ii) In what way is the electron configuration of copper different form the general trend across the first transition element?
[5 marks]
(ii) Propan-1-ol can be oxidized to propanoic acid.
- State the reagent necessary for this reaction to occur.
- In an experiment, 5.67 g of propan-1-ol was oxidized to the acid with a 64% yield. Calculate the mass of propanoic acid produced.
[Molar mass: C2H5CH2OH = 60; C2H5COOH = 74]
[6 marks]
Observation
A fair attempt was made on the question and the performance of the candidate was below average.
In part (a), few candidates were able to state the type of reaction occurring at each electrode when an electrochemical cell is constructed with copper and silver electrodes. However, majority of them could not draw and label the cell.
In part (b), majority of the candidates explained the term cracking and stated the conditions under which cracking takes place.
In part (c), majority of the candidates could not write the electron configuration for 29Cu and 29Cu+.
In part (d), majority of the candidates defined the term oxidation number correctly.
The expected answers include:
-
(i) Cracking is the breakdown (either thermally or catalytically) of long chain / large hydrocarbons molecules / petroleum fractionsinto short chain / smaller one
(ii)
- presence of (a suitable) catalyst
- high temperature
- high pressure
(iii) - yields more useful and high quality petroleum products -
(i)
I. 29Cu - 1s22s2 2p2 3s2 3p6 3d10 4s1
II. 29Cu+ - 1s2 2s2 2p6 3s2 3p6 3d10(ii) It has 3d104s1 instead of 3d9 4s2
