Answering Questions about Ethanol | Top Marks QA #3


So today we’re going to talk about ‘Long Response Questions About Ethanol’. By this point of the HSC, you’ve probably realized that questions on ethanol are really common in all of your HSC exams. So the ability to get full marks on these questions, not slightly less than full marks, not six out of seven, It’s going to be really important even as soon as your half-yearlies and certainly through to the trials during your HSC. This video is going to talk about how to craft the pertinent answers to these questions.

The first key thing is specificity in your answer. The reality is that every student in the HSC has a rote-learned, memorized answer about ethanol. But every question is subtly different and you need to identify those differences and tailor your answer to the question. If the teacher realizes that you’ve just written down something word for word as you remembered it without actually thinking about the way they have asked this question, you will not get full marks. Things you need to think about are: Is the question talking about ethanol used as a fuel or as a source of petrochemicals or both? In each case, you need to be specific to what they’re asking about. Some questions ask about biomass and some jump straight to ethanol. If they’re asking about a biomass, you need to go through the steps to convert biomass to ethanol and describe them. Some questions are talking about comparing it to crude oil or in many questions that’s implied, but sometimes it isn’t. Very often though, it is useful to compare it to crude oil because if you’re discussing how useful ethanol is, it’s actually really important to know whether it is more useful than oil is because if not, then we wouldn’t be using it no matter how good it is. The final thing is, is the verb ‘assess’ or ‘evaluate’ or is it ‘discuss’ or ‘analyze’? Because assess and evaluate questions, (I hope you know already, but if you don’t) always need a judgment at the end. That’s one of your marks. If the verb is assess or evaluate, make a judgment and make it a strong one. You don’t say ethanol is okay or that it’s good and it’s bad. You either say it’s great or it’s not great. Now looking at how to structure your answer more specifically once you’re answering, I always break it into these three topics: the chemical viability, the economic viability and the environmental viability.

Chemical viability is a really great way to get out the equations that we need because the most simple thing in describing ethanol’s use as a fuel is saying that it can undergo combustion and writing up that equation and saying therefore that at least chemically, at least technically, it can be used as a fuel. Or when I’m talking all the way from biomass to petrochemicals, we can talk about the reactions in acid hydrolysis of cellulose to glucose, of fermentation to ethanol, of dehydration to ethene and then of polymerization to polyethylene. And writing up all those equations is a great way to snag some easy marks.

Now once we’ve described that it’s technically possible, we need to discuss whether it’s economically viable. In this case, we need to discuss the costs of things like distillation and the great energy cost there and about fermentation process and usually to compare this to what’s currently being used. Because if it’s more expensive than crude oil, then it’s probably not economically viable.

The final thing to talk about is environmental viability and there’s two main topics to talk about here. The first is renewability and we can definitely say that ethanol is a renewable resource because we can constantly produce new plants to create more ethanol. However, the tricky thing that often gets asked in questions is about carbon neutrality. Now while technically the amount of carbon produced by burning ethanol is exactly the same as the amount that was absorbed by a plant during its life where we got the ethanol from, we need to remember that the distillation process and some of the other processes as well involve a large energy input which of course is actually to come from burning either ethanol itself or fossil fuels and that’s actually going to undermine the statement that ethanol is carbon neutral. So all we can say it certainly will produce less carbon dioxide than burning petrol will. It’s not going to be perfectly carbon neutral and it’s important to recognize that and state that. Finally, we can discuss that ethanol actually burns more cleanly than petrol typically producing less carbon and carbon monoxide which are pollutants that are often come from incomplete combustion of petrol.

And as I said, finally, you have to make a judgment and your judgment will usually be to say, based on the criteria that are stated, ethanol is either useful or not useful (depending on your personal opinion) and depending on the way the question’s been asked. It actually doesn’t matter which way a judgment goes, but you do have to make one if the question asks for one.

I hope you found that useful. If you liked this topic or want to ask any further questions about it, leave your comment below or if there’s any other topics in HSC chemistry that you’d like to see me cover, then also ask that. Thank you.