The Experiment
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| Experiment set-up. Note the use of the Al-can to conduct heat to the water, the glass rod to support the can (to reduce heat loss to the supporting apparatus, the thermometer suspended in the middle of the water to accurately measure the water temperature & that the can has been adjusted to ensure that is at the top of the flame. |
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| Note the carbon (soot) deposit on the base of the can (a section near the top has been scraped off for comparison to the bare metal). Note that the cap has been put over the flame to extinguish it. Note the incorrect naming 'butanol' on the burner, it was the specific butanol isomer: 'butan-1-ol'. |
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| For comparing 'sootiness' |
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| Hmm - the bottom-left butan-1-ol is interesting. The soot is mainly at the side of the beaker and clean in the middle. This suggests that the flame was closer to the can than in the other experiments. |
Results
Team
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Chemical
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Al-can mass (g)
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Beaker + water mass (g)
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Initial Spirit Burner mass (g)
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Final Spirit Burner mass (g)
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Initial Water Temperature (oC)
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Final Water Temperature (oC)
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JKMAH
|
methanol
|
11.77
|
111.97
|
304.57
|
303.93
|
21
|
34
|
LJ
|
12.71
|
112.05
|
247.06
|
246.4
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21
|
32
|
|
CV
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ethanol
|
11.88
|
111.36
|
233.82
|
231.58
|
21.5
|
89.5
|
OW
|
11.93
|
111.1
|
309.1
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308.26
|
21
|
40
|
|
ICPC
|
propan-1-ol
|
12.06
|
109.4
|
307.65
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307.27
|
22
|
35
|
MHAP
|
12.08
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111.38
|
276.31
|
275.69
|
22
|
35
|
|
AS
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butan-1-ol
|
11.77
|
109.45
|
263.86
|
263.43
|
24
|
40
|
NG
|
11.95
|
110.3
|
259.38
|
259.03
|
21
|
31
|
Heat of Combustion Experiment
Analysis
How to use Excel for this experiment VIDEOBlank Excel table for data entry HERE
2013/14 Data check
- first methanol water mass: 100.2g
- second methanol temperature change: 11oC
- first ethanol fuel mass: 2.24g
- second ethanol heat released: 8.08kJ
- first propan-1-ol HPG: 14.3 kJ/g
- second propan-1-ol molar heat: 536.86 kJ/mol
Worksheet Feedback
Surfing Chemistry Questions
* SHOW WORKING!
* Always put correct units with your answers. Molar heat of combustion must be in kJ/mol (or J/mol)
* in q = mCDT, the m is the mass of water. q is the amount of heat given to the water so the m, C and DT are all about water!
* Be careful when defining molar heat of combustion "The energy released when 1 mole of fuel undergoes complete combustion" is the bare minimum required.
* Whenever talking about error in a heat of combustion experiment you should mention these errors before anything else: a) Heat loss to the environment, b) incomplete combustion.
Worksheet 9: Ethanol as a Fuel
The level of effort and care taken with this worksheet was very disappointing. When you do rushed sloppy work and cut corners with your explanations and calculations you are actually training to make mistakes. Despite your 'hard work' you are actually worse off than the (lazy) person who does nothing. You have no choice but to do the work in this class, so make it count - work toward the goal of success in everything you do. It is not easy to do things properly, but doing things properly leads to long term success.
If Near Enough Is Good Henuff (NEIGH) does not work for you, perhaps you should make this famous quote your guide to how to approach Chemistry:
'Practice doesn't make perfect. Perfect practice makes perfect'
So what to do?
* NEVER use the Internet to find a definition for a Chemistry term unless specifically asked - this is a shortcut to thinking and is not a successful strategy. There is no excuse for not being bothered to look up your class notes or search your textbook. You are given these resources for a reason.
* NEVER leave a question incomplete or partially done. You will always get an answer to an emailed question within 24h and you will not be turned away if you come to seek help in person.
* NEVER use a 'common sense' answer (that you would expect everyone to know) to a Chemistry question. You need to demonstrate your understand your understanding of Chemistry in all your answers, but not just that - because you are competing with the state you need to show that you are a better Chemist who has a greater understanding of the Chemistry syllabus than the next person. Eg
"Describe advantages and disadvantages of ethanol"
ANSWER 1) Ethanol is a renewable fuel as it is made from sugar (from sugar cane) and is theoretically carbon-neutral. However, it means that farming land and food is being used for fuel which could drive up prices of food and lead to food shortages. In addition, at present it cannot be made as cheaply or in comparable volumes to petroleum.
ANSWER 2) Ethanol is renewable as it is made by fermentation of photosynthetic glucose. The presence of oxygen in the molecule means it has more complete combustion than petrol meaning less CO and C (soot) pollution. However, ethanol has a lower heat of combustion per gram than petrol meaning more frequent refilling is required. Distillation of the 15% ethanol mixture from fermentation requires large amounts of energy due to its hydrogen bonding with water.
Both have 'answered the question' but which answer do you think demonstrates a greater understanding of Chemistry?...
Q1) Ethanol when it is made in fermentation is in an aqueous solution (15% max). However, this is distilled to ~100% liquid ethanol before it is combusted. Thus in your combustion equations ethanol should be a liquid. NB Balancing these equations should NOT be a problem at this stage.
Q1) Saying that 'ethanol is renewable because it is made from photosynthesis' is unbelievably wrong. Think about it. Also ANSWER the question. If the question asks you to explain why ethanol is renewable but petrol is non-renewable you have to do both. Just explaining why ethanol is renewable does not explain why petrol is non-renewable. Be thorough. For 2b see the examples above.
Q3) Remember that fermentation produces two products. Ingredients means the chemicals/organisms required to start the reaction. Conditions includes things that need to be excluded. For the equation, remember that fermentation requires the glucose to be in aqueous solution, yeast to be added, 36 degrees (approx) and exclusion of air (oxygen). You should be able to balance this equation, with states, and with the details above the arrow with no problems at this stage.
Q4) See the fourth point of the surfing tips above- use our notes not a random internet search for definitions.
Q5) Two major errors occurred because students did not read the question:
i) 30% heat loss to the environment means only 70% reaches the water. So figure out the theoretical heat released by burning the ethanol, then multiply by 0.7 before you calculate the temperature change
ii) It asked for the final temperature. DT = Tf - Ti, so Tf = Ti + DT - you don't have to use the maths equations - just think about it - it is common sense that if something was 10oC and the temperature change was + 15oC then the final temperature would be 25oC.
Q6) The question asked you to find the moles of carbon dioxide per kilojoule. You ere given the molar heat of combustion (the kilojoules per one mole of fuel) so you needed to find the moles of CO2 produced when combusting one mole of fuel (which you could do with an equation, but if you think about it - all the carbon in the CO2 has to come from the fuel, eg if there are 3 carbons in the fuel you will get three moles of CO2 produced per mole of fuel...)
Once this value is determined it is a simple matter of realising that 'moles of CO2 per kilojoule' = #moles of CO2 / DcH (ie for octane = 8/5460)
Then you just need to rank them from smallest to largest.
Q7) Again - the m in the equation q = mCDT is the mass of water! In addition, whenever discussing the errors in heat of combustion experiments you MUST say incomplete combustion and heat loss to the environment (resolved by using a high oxygen atmosphere and by enclosing the reaction in water in a bomb calorimeter (or just by insulating the sides of the 'can' and using a lid).
Q8) You can't assume that 1ml = 1g unless the liquid is water. You were given the densities (d) so you could calculate the mas from d = m/V. And because the density was in g/ml you need to convert the volume from L to mL. Many students added in an extra unnecessary step. q = n*(molar heat) so as soon as you find the mass you can calculate q from the molar heat GIVEN in the data. When calculating don't forget to write down your units as you go. Lots of mistakes were made when converting (or not converting) between J and kJ.
Q9) The biggest errors were reading from the graph & J to kJ. If you use the q=mCDT formula you MUST use Joules NOT kJ! If we did our heat of combustion experiment and measured the final temperature of the water 5 minutes after the experiment to calculate DT would it be valid? Didn't we use the Max temperature the water reached to figure out the actual heat given to the water? So which value of Tf should we use from the graph?
Summary Feedback
This was probably the most difficult summary to date and the results reflected this. A mark of 8/12 or better was very good.. But do not despair – there are not many harder than this!
A) The heat per gram of a fuel will ALWAYS be less than the heat per mole of that fuel (as there is always more than 1 gram per mole of a substance, even a mole of hydrogen atoms has a mass of 1.008g!) BUT the hypothesis you are making is whether the heat per gram will increase or decrease as the molecular weight of the fuel increases (i.e. will propan-1-ol have a greater heat/gram than ethanol, and will ethanol have a greater heat/gram then methanol etc)
B) You do not need to write the method beyond step 11 on the instruction sheet
C) Remember – diagrams need to be drawn in simple scientific style with a pencil & ruler and must be labelled. They must include all the essential apparatus (at minimum: retort, clamp, thermometer, can, spirit burner – NO Bunsen burner!)
D) Observations must include the change in appearance/colour/soot of the flame and difference in amount of soot produced as the molecular weight increases. You do not need to include the entire data table.
E) You need to justify EACH of the specified steps in relation to how they enabled us to achieve the aim (measuring heat of combustion) and/or increasing validity and reliability. Eg 1: You need to show (with an equation) where some of the measurements were used in calculations (mass of water [m] was needed to calculate heat absorbed by the water, q = mCDT). Eg 2: The aluminium can was used as it is a good conductor of heat, this allowed more of the heat released by the combustion to be transmitted to the water. We weighed the can so we could calculate how much heat the can absorbed – incorporating this into our calculation increased the validity of our results.
F) Account for – means you need to give a ‘chemistry-based reason why the longer chain alkanols had less-complete combustion. HINT - look how much oxygen is required for the complete combustion of each alkanol. Why might the longer alkanols have less complete combustion?...In addition, it is not always easy balancing complex equations with lots of oxygen containing molecules. To make it easier to balance, when writing an incomplete combustion reaction make the products solid carbon (soot) and water only.
G) Assessing means you need to give in intro, positives, negatives and a judgement. For this question you should start off with a very simple introduction (perhaps why we need alternative fuels OR the trends in heat of combustion and completeness of combustion as alkanol molecular weight increases). Your positives and negatives should relate to the extent that ethanol achieved ALLthe criteria (don't forget toxicity - it is an important criteria in ruling out one of the alkanols) supplied in the question (and should incorporate the provided clues saying why these factors are important). Your judgement should then say whether ethanol is/isn’t, can/can’t be used as an alternative fuel.
H) Because you were given support to draw the graph you need to make sure that it is perfect. For guidance see this video: https://www.youtube.com/watch?v=2ITHezUiLmI
Also the data to check to make sure your calculation are right are:
2013/14 Data check
- first methanol water mass: 100.2g
- second methanol temperature change: 11oC
- first ethanol fuel mass: 2.24g
- second ethanol heat released: 8.08kJ
- first propan-1-ol HPG: 14.3 kJ/g
- second propan-1-ol molar heat: 536.86 kJ/mol
J) Assessing is a high level verb. i) Intro (define). ii)Pros and cons (when assessing reliability you need to say what aspects of the results were and weren’t reliable – did we repeat, did we repeat enough times and were the results consistent?) and then iii) make a judgement.
K) The ‘error’ in our results is the difference between the theoretical values and the experimentally calculated values. Clearly, our results were lower (you must say this) – but did the amount of error change as the molecular weight increased? – what source of error would cause that?
L) In an assess Q you need to introduce the topic (ie define validity) - you need to note errors (ie In all heat of combustion experiments the TWO main sources of error are HEAT LOSS TO THE ENVIRONMENT and INCOMPLETE COMBUSTION) Then you need to consider: did these errors mean our findings were quantitatively valid or invalid? Could we still make valid qualitative generalisations? NB this also relates to (K) above – you would expect the longer chains to have less complete combustion thus the amount of error for the longer chain alkanols should be…
M) The conclusions should be IMPERSONAL. Just state what was found (did increasing molecular weight increase or decrease completeness of combustion, heat per gram and heat per mole) and then outline the error that were found in the results. Remember hypotheses cannot be proven, they can only be supported.
N) Hypotheses are predictions based on scientific principles. Your hypotheses do not need to be correct but for this activity the justification must use correct chemistry.
O) This was mentioned in the method (and in the corrected experiment #5 summary instructions) but part (d) of Q5 required you to use the equation of your line of best fit from your experiment data to extrapolate the molar heat of combustion of methanol and hexan-1-ol. These values need correct units & working.
P) It is good to summarise similarities between chemicals eg all fuels used are highly flammable so… But you need to actually mention the chemicals names & you must be specific – saying they are toxic is insufficient – ie say by what route they are toxic (this then relates to the safety precautions you take). Remember that methanol burns with a nearly invisible flame - hence the need to extinguish with the cap to ensure it is extinguished.
Q) One reason why correct units are needed is because the equations we use rely on correct units – if correct units are not used any calculated values will be incorrect.
R) You need to show full working for your fuel AND All calculated values need to show correct units. Ie having magnitudes (numbers) only is not sufficient. for 7d you needed to calculate the error for EACH fuel.
S) The symbol k for ‘kilo’ must be lowercase. It means 1000 or 1000x, The symbol J for joules is uppercase.
T) Equations must be correctly balanced
U) Remember, we used propan-1-ol and butan-1-ol (need to be specific).
V) States are important. Fermentation produces aqueous ethanol but it must be distilled before combusting- ie when we combust alkanols they are liquids. While the water produces from combustion is released as steam, it would cool back to a liquid at room temp so we include the state as liquid.
Completing a thorough scientific investigation is not easy and there are lots of factors to consider - hence the difficulty of this expeirment.
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