i) State the temperature used in this process. [1 mark]
ii) State and explain the effect on the rate of reaction of using a higher temperature than you suggested in (i). [4 marks]
As the forward reaction is exothermic high temperatures favour the backwards reaction as it is endothermic & equilibriums seek to reverse change. therfore raising the temperature would decrease the yeild of SO3(g) Equilibrium will move to the left.
iii) State, with a reason, the effect on the yield of sulphur trioxide of using a higher temperature than you suggested in (i). [2 marks]
Higher Pressure would increase the yeild as their are less moles on the products side so the equilibrium will move to the right.
I suspect that the exam board would give her 1/7 marks, though there's a possibility she may get the 2 marks for part (iii) for her answer to (ii). However, I don't want to bank on that, as even with those 2 marks, the student has thrown away marks by careless misreading. There must be some way I can teach her to take the time to read the questions carefully before starting?
But this gets onto another issue, which is that all of my students always have trouble reading for content. Dyslexia makes that worse, as does having English as an Additional Language, but even the native-born British students with no apparent disabilities seem to have reading difficulties. I think they panic about running out of time and rush their way through the paper, when they would do so much better reading it carefully before starting. Most of them also have trouble with the idea of active checking and the way the brain caches what it thinks is there, so will happily read out loud what they thought they wrote on the exam paper rather than what they've actually written.
I was taught at school to always read the entire exam paper before I started to answer anything. This was reinforced by an exam paper we were given once, at the age of 10, which included instructions such as stand up and shout "hello", and make a hole through the centre of the exam paper. The very last instruction said "Now that you have read the entire exam paper, simply write your name at the top of it and sit quietly. Try not to laugh at your classmates." I was the only student in that class who managed to follow the instructions, and it's something I have never forgotten. Reading the entire exam paper before you start is a good idea, as all exams have the potential to embarrass you horribly, though it's not normally that blatant!
My belief, which is shared by the various exam boards examiners' reports, is that you should:
a) Read the entire exam paper before you begin answering anything. Don't get distracted by an easy question that you can immediately scribble down the answer to. Read the whole thing.
b) As you go through the exam paper, read the entire question again before answering it.
The reasons for doing this are fourfold. Firstly, the subconscious mind is capable of an awful lot of parallel processing. If you read the entire paper before starting, then while your conscious mind works on question 1, your subconscious is busy working on questions 2-7. This is useful, because it saves you time in the long run and leads to more coherent answers. Also, you may find that you have no idea how to answer some of the questions when you read them the first time, yet by the time you come to answer them, your brain will have remembered how to do them. This wouldn't happen if you hadn't already read the paper.
You might feel that you're saving time by reading the question only once, but in fact you work more quickly if you've read the paper and already know what the next question will be about.
Secondly, in order to get marks, it's essential to answer the question as written on the paper - not another similar question you've seen before. Reading each question through twice before you begin, plus a third time as you work, helps to ensure that you actually read the words and take them in. If you read three times that the question is about rate of reaction not yield of product, you have a higher probability of answering the right question.
The fact that the exam board have put some words in bold text means "Pay particular attention to this word to make sure that you're answering the question they've set, and not another similar question that you've seen before".
Thirdly, if you don't read the entire question before you start, you may well waste time writing an answer for part (i) of a question that is actually the answer for part (ii) or (iii). You will then either have to repeat yourself, or draw arrows all over the answer pages - both of which look messy, and do not favour the giving of marks. Most exam mark schemes allow a certain amount of discretionary, or "benefit of the doubt" marks, which can be awarded if the student has almost said the right thing. These tend to only be awarded to students who show themselves to be well-organised and capable. It's difficult to impress the examiner with crossings-out and arrows everywhere!
Fourthly, often a later part of a question contains a clue that will help you with an earlier part. This is particularly common in organic chemistry questions where one substance is converted to another via a complicated route, or in calculations. For example:
(b) The standard enthalpy of formation of 1,2-dibromoethane is -37.8 kJ mol^-1. Suggest the main reason for the difference between this value and your calculated value in (a) (ii).
This tells you that you're expecting an answer with an approximately similar magnitude to -37.8 kJ mol^-1. -16 kJ mol^-1 might be a reasonable answer considering issues such as heat losses to the surroundings. But +30 kJ mol^-1 implies that you've made a mistake in your calculation as it's the wrong sign, and -1160 mol^-1 is so far away that clearly something is very wrong. A wise student would immediately notice the discrepancy and carefully recheck their calculation.
I think I'm going to copy & paste this entire text into OpenOffice & print it out to return along with my marked exam papers :/ But any further ideas for getting this idea into the kiddies' heads would be ever so appreciated.