State Ranking Acidic Environment Long Response Sample
2009 HSC sample question response
Post written by Patrick Shi (12th in the state Chemistry). See all articles first and personally get in touch with our state rankers here
Question 23 (6 marks)
The graph shows the variation in concentration of reactant and products as a function for the following system.
Identify and explain each of the changes in conditions that have shaped the curves during the time the system was observed
Sample Answers
HSC Answer (Poor Answer)
Concentrations vary at 4 to 8 minutes, 10 to 12 minutes and 14–16 minutes. At 4 to 8 minutes the system is moving to the right. There is no indication that Cl2 or CO have been initially removed or that COCl2 was initially added, hence the temperature has been changed. The reaction is endothermic to the right, so this means heat has been added (temperature has been raised). The sharp drop in [CO] at 10 minutes means that CO has been removed. The system then moves to the right, seen as an increase in [CO] and [Cl2] and a decrease in [COCl2]. At 14 min all concentrations abruptly decrease, all by a 2/3 factor hence the volume of the system has increased. As the right side is proportionally more affected than the left hand side, the system moves to the right to compensate, hence [CO], [Cl2] increases and [COCl2] decreases.
Answers could include:
- Where the graph shows horizontal lines for concentrations, equilibrium has been achieved.
- Discussion on the rate of change
- Discussion on equilibrium states
- Change at 14min may have occurred because of mechanical or chemical removal of all species, but this is very unlikely
My Answer (Good Answer)
4 Minutes
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At 4 minutes, [Identify] Cl and CO concentrations increase whilst COCL2 decreases.
[This is a prime example of a temperature stimulus that affects the equilibrium, because there are no sharp increases or decreases in the graph].
[Explain] Therefore the equilibrium must have shifted right. A temperature change must have affected this system namely an increase in temperature. Via Le Chatelier’s, to counteract heating, the system must shift right to the endothermic side
10 Minutes
At 10 minutes, [Identify] Cl2, CO increase, whilist COCL2 decreases. There is also a sharp decrease in CO.
[This is a prime example of an addition or removal of a substance, since only one substance has as sudden change]
[Explain] The equilibrium must have shifted right. A sudden drop in CO means that CO was removed from the system. Hence by le Chatelier’s, the system will shift right to replenish that lost CO.
14 Minutes
At 14 minutes, [Identify] Cl2, CO, COCl2 all have a sharp decrease in concentration. Over time, Cl2 and CO increase, whilst COCl2 increases
[This is a prime example of a volume increase or decrease since all substances of the equation are affected by a sharp increase or decrease]
[Explain] The volume of the system must have increased by 33% (Take a look at CO, it starts at 0.06 and ends at 0.04. Also an increase in volume leads to a decrease in concentration)
A increase in volume means an decrease in pressure. A decrease in pressure of the system means that the equilibrium will shift right to the side of more moles, favouring the forward reaction.
Comments
- 6 mark question, wanting us to identify and explain. As such, we had to address both directive terms in the question
- Before I start the question, I tell myself that it will be 2 marks allocated to each time intervals (4mins, 10mins and 14mins)
- Take note how I put titles (4 minutes) in order to structure my answer more clearly unlike the HSC version
Post written by Patrick Shi (12th in the state Chemistry)
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