9.2.3 HSC Question Feedback

Generally very good.

I was impressed with your mathematical answers. The only concern here was converting between heat per gram and heat per mole (molar heat of combustion).

We have derived the equations for converting these in our notes but you don’t need to derive them I your answers, you can just rely on this equation:

Heat per mole = heat per gram x Molar mass

D_cH = J/g x M  (and obviously this means J/g = D_cH / M)

So the equations you need for heat of combustion are (and you should know most of these without needing to ‘memorise’ them):

q = mCDT

J/g = q/m_fuel

D_cH = q/n_fuel

n_fuel = m_fuel/M_fuel

D_cH = J/g x M_fuel

In the written responses the answers were generally thorough and demonstrated a dee understanding. One major point is that in 9.2.3 we are focussing on the ‘real’ world – ie humans ARE using glucose (from sugarcane) to make ethanol. This is different from 9.2.2 where the focus was on the POTENTIAL to convert cellulose into fuels (ethanol) and materials (ie via ethene) – ie not something that humans are actually doing. So when it comes to discussing ethanol you don’t need to start the process at cellulose (biomass) unless specifically asked.

Tips for specific questions

2002

Q1) Anything above 36^oC starts to harm the yeast (its enzymes don’t function as efficiently)

2003

Q6) This is converting between J/g and D_cH. However, you need to think: Is it easier to convert all four J/g data values to D_cH to find the right one OR just convert the single DcH value to J/g to find the right one?...

2003

Q16) In this questions and ALL fermentation experiment questions – the focus is on i) Did you specify the correct chemicals (glucose, water, yeast) ii) was it an open container & iii) did you specify that you measured the mass before AND after fermenting (ie to measure mass loss). You do NOT need to go into detail about our airlock system UNLESS it asks about how you prevented oxygen entering and proved CO₂ was produced. All you need to say (at the very most) is that “an airlock system was used to allow CO₂ to escape but no air to enter”.

2004

Q25) Whenever you are given a graph (with numerical axes) or a table with numerical data YOU MUST REFER SPECIFICALLY TO THE DATA (ie numbers) in your answer. Eg “ethanol’s heat of combustion is 29.7 kJ/mol compared to octane’s 47.9 kJ/mol…”

2005

Q20) You should be able to do from Glucose to pure ethanol BUT do not start from cellulose (see notes above) – in addition the Q specified for the process in the flow chart and there is no cellulose in the flow chart.

2006

Q2) You need to know that concentrated sulfuric acid is a dehydrating agent (thus used for dehydration of ethanol into ethene). Dilute sulfuric acid is used for hydrating ethene into ethanol (this was the whole focus of our Experiment #3)

Q18) Just be careful to read the question and use your mole ratios correctly – many students were mixing up and finding masses of ethanol rather then glucose.

2007

Q24) a) If you are asked to plot a graph with molar mass or molecular weight as the axid you have to calculate and plot it correctly.

Q24) b) When determining values from a graph you should mark the value on your graph with lines to the axis.

2008

Q22) for this Q I would show the chemical reasons why ethanol is theoretically CO₂ neutral. But then discuss the reasons why it does produce some net CO₂, then I would MAKE A JUDGEMENT on the article. If you do not put a judgement on an assess or evaluate question you will lose 1-2 marks.

Q24) An excellent combination of moles J/g and D_cH. I strongly recommend that you do this question again if you made a mistake. Knowing how to answer this question will greatly increase your understanding of these sort of calculations.

2009

Q17) If you read the question carefully you will see that you have to explain the similarities AND differences between water and ethanol solubility. You need to draw a diagram of both with the polarity labelled. You need to make a statement about like dissolves like. You need to state why both can dissolve polar and hydrogen-bonded substances. But then you need to say why ethanol can dissolve non-polar substances BUT WATER CANNOT – without this last statement you have not explained the differences.

Q20) a) When heat is lost to the surroundings the questions can be tricky. If calculating the heat of combustion this means that the heat you measured will be half what is produced so you need to double the calculated heat of combustion. But if calculating the amount of fuel that must be burned you need to double the theoretical amount of  fuel as half the heat is being lost. This is a question to do again for study.

Q20) b) A real sneaky question – most of the time the two ERRORS are heat loss and incomplete combustion BUT this question wanted two methods to reduce heat loss (only) – e insulate the container and move the flame as close as possible to the container.

2010

Q11) To answer this question you had to think – what chemical decolorises Br-water (alkene) & what chemical is dehydrated by sulfuric acid to make an alkene (alkanol)…

Q23) b) A nasty question. Similar to 2003 Q6 – I would convert the D_cH value to a J/g to figure out which fuel. You should get 36 kJ/g which does not match any of the results. BUT this is the results of a STUDENTS experiment. Thus all the values are going to be UNDERESTIMATES. Thus the student would have always got a number LOWER than the theoretical value – ie lower then 36. So the correct sample must be…

2011

Q17) This is a DEMONSAUD Question - HINT - you have ben given q and DcH for propan-1-ol...what can you find the number of moles of...?

2012

Q30bii) Why might we add ethanol to a solution when trying to dissolve something in water?