Chemistry 2020 ONLY CLUTCH Exam

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STUDENT NUMBER

2020 ONLY VCE CHEMISTRY 3/4

CLUTCH Practice Examination No.1 (2020 Adjusted VCAA Study Design)

Reading time: 15 minutes Writing time: 2 hours and 30 minutes

QUESTION AND ANSWER BOOK

Structure of book

Section Number of questions

Number of questions to be

answered

Number of Marks

A 30 30 30

B 10 10 90

SECTION A Question 1 (6 marks)

Letter

• Students are permitted to bring into the examination room: pens, pencils, highlighters, erasers, sharpeners and rulers.

• Students are NOT permitted to bring into the examination room: blank sheets of paper and/or correction fluid/tape.

• No calculator is allowed in this examination. Materials supplied

• Question and answer book of 34 pages • Additional space is available at the end of the book if you need extra paper

to complete an answer. Instructions

• Write your student number in the space provided above on this page. • All written responses must be in English.

Students are NOT permitted to bring mobile phones or any other unauthorized electronic devices into the examination.

2020 ONLY CLUTCH Academy Chemistry Exam 1

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Question 8 (7 marks) Stearic acid is a saturated fatty acid which is a major component of cocoa butter and shea butter. Stearic acid undergoes combustion in excess oxygen according to the following reaction.

C17H35COOH(s) + 26O2(g) à 18CO2(g) + 18H2O(l)

a) Determine the total volume of greenhouse gases produced if 3.20kg of stearic acid undergoes complete combustion at SLC.

4 marks

____________________________________________________________________________________________________________________________________________________________________________________________________________________________

b) Compare the viscosity of stearic acid and linoleic acid in liquid form. Which fatty acid is more viscous? Explain your answer by referring to intermolecular forces.

2 marks

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Question 9 (13 marks) A student designed an experiment to investigate the energy density of biodiesel and petrodiesel. The student decided to use bomb calorimetry to determine the heat of combustion of the two fuels as a measure of energy density.

The incomplete results for the biodiesel tests are shown in the table below.

TRIAL 1 TRIAL 2 TRIAL 3

SAMPLE MASS (g)

0.498 0.500 0.504

INITIAL TEMPERATURE (°C)

24.2 24.0 24.1

CHANGE IN TEMPERATURE (°C)

1.74 1.84 1.86

HEAT OF COMBUSTION (kJ/g)

39.1

a) Using the table, determine the calibration factor of the calorimeter.

2 marks

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b) Use your answer from part a) to complete the table.

2 marks

____________________________________________________________________________________________________________________________________________________________________________________________________________________________

The bomb calorimeter was first calibrated using a sample of benzoic acid to determine the calibration factor of the calorimeter. Three samples of biodiesel were then combusted using the calorimeter and the change in temperature was recorded. The student then combusted three samples of petrodiesel in the calorimeter. The calorimeter was returned to its original conditions in between each combustion.


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The results for the petrodiesel tests are shown in the table below:

TRIAL 1 TRIAL 2 TRIAL 3

SAMPLE MASS (g) 0.491 0.516 0.494

INITIAL TEMPERATURE (°C)

22.3 22.7 21.9


2.15 2.22 2.11


49.0 48.1 47.8

c) Comment on the precision of the heat of combustion results for petrodiesel.

1 mark

_______________________________________________________

d) Name two variables that should be kept controlled during the experiment. 2 mark

_______________________________________________________

_______________________________________________________ e) What conclusion can be made about the energy densities of petrodiesel and

biodiesel? Use the results to support your answer. 2 marks

_______________________________________________________

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_______________________________________________________ f) Referring to the structure of biodiesel and petrodiesel, explain the theoretical

reasoning behind your conclusion. 2 marks

_______________________________________________________

______________________________________________________________________________________________________________

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g) Critique the validity of the experiment. 2 marks

_______________________________________________________

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2020 ONLY VCE 3/4 CHEMISTRY

CLUTCH Practice Examination No.1 (2020 ONLY Adjusted VCAA Study Design)

SUGGESTED SOLUTIONS AND ADVICE

Author’s General Comments: This practice chemistry exam aims to replicate the difficulty and style of a VCAA end-of-year exam. The exam consists of 30 multiple choice questions and 90 marks worth of short answer questions, and each of these questions is assigned a difficulty rating from 1-5. These ratings may give some insight into the level of difficulty of these questions in comparison to that of the VCAA exams. When completing this practice exam, and when completing the VCAA chemistry exam towards the end of the year, students should remember and take on the following advice. All calculations questions should be structurally approached, a clear and consistent set of steps in your working out will allow the examiner to identify correct working out and this may see you picking up extra marks even when your answer is not correct. Students should also try to be more concise in their answers for questions worth 2 or 3 marks. By limiting the number of words in your answers, the point you are trying to make becomes clearer to examiners and will also save you more time during your exam. Try to develop some short and concise answers to common exam questions. Familiarise yourself with experimental design and the elements of experimental design. Often this is not addressed enough in class, however, a 10-15 mark question regarding an experiment appears in exams every year and therefore being familiar with all elements of experimental design can be crucial to achieving a higher chemistry study score.

Good luck with your preparation!

Written by CLUTCH Academy’s Chemistry expert, Kishan Gupta


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Question 8 a) Worked solution: n(stearic acid) = (1000 x 3.20)/284.0 = 11.3mol (1 mark) n(stearic acid) : n(CO2) à 1 : 18 Therefore, n(CO2) = 18 x 11.3 = 203mol (1 mark) n(stearic acid) : n(CO2) à 1 : 18 Therefore, n(H2O) = 18 x 11.3 = 203mol (1 mark) We can use the molar volume equation to determine the total volume of gas since one mol of any gas occupies the same volume. Total volume of greenhouse gases = (203+203) x 24.8 = 1.01 x 104 L (1 mark) Difficulty: b) Sample response: As a liquid, stearic acid would be more viscous than linoleic acid. (1 mark). Stearic acid is saturated and therefore contains no carbon-carbon double bonds whereas linoleic acid contains two carbon-carbon double bonds. The presence of these bonds result in kinks in the fatty acid chain (1 mark) and therefore the fatty acid molecules of linoleic acid cannot be packed as tightly together. The molecules of stearic acid are able to be packed more tightly together and therefore the strength of the intermolecular forces between the molecules will be higher (1 mark) causing a greater resistance to flow. Difficulty: Question 9 a) Worked solution: 39.1 x 0.498 = 19.5 (1 mark) 19.5 / 1.74 = 11.2kJ/°C (1 mark) Difficulty: b)

Difficulty:

TRIAL 1 TRIAL 2 TRIAL 3 SAMPLE MASS (g) 0.498 0.500 0.504 INITIAL TEMPERATURE (°C)

24.2 24.0 24.1


1.74 1.84 1.86


39.1 41.2 (1 mark) 41.3 (1 mark)


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c) Sample response: The heat of combustion results are precise as there is only a difference ±0.7kJ/g from the average. (1 mark) – must provide mathematical reasoning. Difficulty: d) Mark allocation:

1 mark for each of the following up to 2 marks

- The initial temperature inside the calorimeter - The mass of the fuels - Pressure of oxygen inside the calorimeter - The temperature outside the calorimeter

Difficulty: e) Sample response: According to the experimental results, the average heat of combustion of the biodiesel was 40.5kJ/g and the average heat of combustion of the petrodiesel was 48.3kJ/g (1 mark) The heat of combustion of petrodiesel is much higher than the heat of combustion of biodiesel and therefore it can be concluded from this experiment that petrodiesel has a higher energy density. (1 mark) Difficulty: f) Sample response: The ester linkage groups in biodiesel molecules allow for biodiesel molecules to form dipole-dipole bonds with other biodiesel molecules. (1 mark). These dipole dipole bonds are much stronger than the weak dispersion forces between petrodiesel molecules and therefore it takes more heat energy to break the dipole dipole bonds. (1 mark). Therefore the net energy release from biodiesel will be less than petrodiesel. Difficulty: g) Sample response: A small difference in initial temperature can be seen when comparing the petrodiesel results to the biodiesel results and this could have been controlled better to improve validity. However, since the initial heat is factored into calculations and all other variables, apart from the independent variable, have been controlled, the experiment can still be considered valid. Difficulty:


3

h) Mark allocation:

1 mark for one of the following - Incomplete reaction - The calorimeter may heat the fuels unevenly.

Difficulty:

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Chemistry 2020 ONLY CLUTCH Exam