St John Bosco College HSC Chemistry 2004 – 2005



St John Bosco College HSC Chemistry 2005

Mid-Year Exam Preparation Booklet

with Sample Responses

(to some questions)

Please Note

► Sample responses are given for most questions related to the Production of Materials Unit.

► For sample responses to a variety of Chemistry of Art questions check the sample responses to Assessment Task #2.

1. (3 marks)

Ethylene is an important hydrocarbon used for the production of a range of important chemicals. Explain the industrial process by which ethylene is produced.

► Ethylene is produced by the catalytic cracking of long chain hydrocarbon products derived from the fractional distillation of crude oil.

► Catalytic cracking is the process by which high molecular weight fractions from crude oil are broken into lower molecular weight substances, including ethylene (C2H4).

► The process is carried out at high temperatures (~ 500oC), in the absence of oxygen, at pressures somewhat above atmospheric pressure, and in the presence of powerful catalysts called zeolites (alumuinosilicates).

► Note: ethene and ethylene are the same thing (C2H4).

2. (6 marks)

You have carried out a first-hand investigation to compare the reactivity of an alkene with its corresponding alkene.

a. State the name of the alkene. (1)

► cyclo-hexene

b. Outline a procedure to compare the reactivity of this alkene with its corresponding alkane. (2)

1. place approximately 1ml of 1-hexene into a test tube

2. place approximately 1 ml of hexane into a test tube

3. add approximately 10 drops of bromine water, BrOH (aq), to each test tube

4. shake the test tubes and observe for any signs of a reaction

5. repeat the whole process in a dark place (or with minimal light)

Note: * conduct this experiment in a fume cupboard

* wear safety glasses and gloves

c. Describe the results obtained from this first-hand investigation and include relevant chemical equations. (3)

The 1-hexene reacted rapidly and quickly with the yellow coloured bromine water, completely decolourising it within seconds. However, the hexane did not completely decolourise the bromine water, although there was some decolourisation (some lessening of the intensity of the yellow was observed).

When the experiment was repeated in the dark (or at least in minimal light) the 1-hexene still decolourised the bromine water. There was no observable reaction between the bromine water and the hexane.

Hexane only slightly decolourised the bromine water when exposed to light. However, the 1-hexene decolourised the bromine water quickly and rapidly regardless of whether there was light present or not.

C6H12 (l) + BrOH (aq) ( BrOHC6H12

OH

H2C = CH CH2 CH2 CH2 CH3 + BrOH (aq) ( Br – C – C – CH2 CH2 CH2 CH3

Revise the notes taken in class and see below for examiner’s comments

Note the following comments about this question from the examiner’s report:

Most candidates’ answers indicated that they had carried out a first-hand investigation. However, a significant number of candidates responded with reagents such as ethene, ethane and chlorine which indicated that the candidate had not carried out a first-hand investigation. The omission of equipment used or amounts of chemicals used also indicated lack of first-hand experience. The procedure was not done well by candidates who only had knowledge of the expected results.

3. (3 marks)

Explain why alkanes and their corresponding alkenes have similar physical properties but very different chemical properties.

Alkanes and their corresponding alkenes have similar physical properties such as melting point, boiling point, and density because they have the same number of carbon atoms, similar molecular weights and both are non-polar molecules with weak dispersion forces the only forces between molecules.

However, alkanes and their corresponding alkenes have very different chemical properties because alkenes have a double bond which is very reactive. This means that alkenes react more easily with a greater range of substances than their corresponding alkanes.

An example of this is the fact that alkanes react very slowly, if at all, to decolourise bromine water and only do so in the presence of ultraviolet light. However, alkenes react very rapidly to decolourise bromine water even in the absence of ultraviolet light.

4. (6 marks)

a. Name the type of polymerisation shown in the following reaction: (1)

[pic]

► This is an example of CONDENSATION POLYMERISATION.

b. Assess current developments in the use of biopolymers (5)

► A suitable response would need to:

• Describe what biopolymers are, how they are produced and what they are used for.

• Discuss some of the advantages and disadvantages associated with their production and use

• Make a judgement about the current developments and justify that judgement.

5. (5 Marks)

Assess the suitability of biomass as a future source of energy and chemicals for industry.

► A suitable response would need to:

• Describe what biomass is, and how it might be used as a source of energy and chemicals for industry (as well as suggesting why it might be needed).

• Discuss some of the advantages and disadvantages associated with their production and use of biomass products

• Make a judgement about the suitability of biomass as a future source of energy and chemicals for industry and justify that judgement.

6. (3 marks)

You performed a first-hand investigation that monitored mass changes during the fermentation of glucose to ethanol.

a. Outline the procedure you used. (2)

► Use you prac work to revise this questions. The diagram will be very useful.

b. Write a balanced chemical equation for this reaction. (1)

C6H12O6 ( 2C2H5OH (l)+ 2CO2 (g)

7. (5 marks)

The flowchart shows the production of polyethylene.

[pic]

a. Identify Compound X. (1)

► Compound X is ethanol

b. Describe Process Y. (3)

► Process Y is called addition polymerisation

► Use your notes on Production of Polyethylene to describe the process of “free radical polymerisation” – including the three steps, initiation, propagation and termination. A summary is given below.

► The production of polyethylene is an example of free radical addition polymerisation. Like all such chain reactions, it is divided into three separate stages or processes:

• Initiation: an initiator (such as a peroxide) is used to generate a free radical, which is electron deficient and attacks the double bond in the ethylene

• Propagation: the double bond on the ethylene breaks and a covalent bond is formed with the free radical. The resultant molecule is itself a free radical and will attack the double bond in another ethene molecule, resulting in the addition of another – CH2CH2 – group. This process continues and the polymer chain rapidly grows.

• Termination: At various times it is possible for two free radical chains to react to form a covalent bond. When this happens the growth of the chain is terminated.

A sample of polyethylene was produced by Process Y. The following graph shows the distribution of molecular weights of polymer molecules in the sample.

[pic]

c. Why is a range of molecular weights observed? (1)

► A range of molecular weights is observed because the free radical addition polymerisation process produces chains of varying lengths with correspondingly different molecular weights.

8. (7 marks)

Name ONE type of cell, other than the dry cell or lead–acid cell, you have studied. Evaluate it in comparison with either the dry cell or lead–acid cell, in terms of chemistry and the impact on society. Include relevant chemical equations in your answer.

► This is a 7 mark question and hence requires a substantial response. To evaluate the type of cell you choose in comparison with either the dry cell or the lead-acid cell you will need to use a scaffold something similar to the following:

|Dry cell OR lead-acid cell (choose one only) |Other type of cell |

|Describe key features of: |Describe key features of: |

|how it works (including and equation) |how it works (including and equation) |

|its energy output |its energy output |

|its cost and practicability |its cost and practicability |

|its impact upon (usefulness for) society |its impact upon (usefulness for) society |

|it’s environmental impact |it’s environmental impact |

|Judgement |

|make a judgement |

|justify your judgement |

Possible marking scheme (one possibility only)

Name of a cell (other than dry cell or lead-acid cell) 1 mark

|Cell 1 |Cell 2 |

|Description of cell (1 mark) |Description of cell (1 mark) |

|Equation (1 mark) |Equation (1 mark) |

|Overall comparison (1 mark) |

|Judgement with justification (1 mark) |

9. (3 marks)

Radioisotopes are used in industry, medicine and chemical analysis. For ONE of these fields, relate the use of a named radioisotope to its properties.

► Check page 88 of the red workbook

Note the examiners comments with regard to this question

In general, this question was answered quite well, with relatively few candidates scoring 0 or 1 marks out of three. Many candidates had difficulty recalling more than one property. Candidates who did so, usually only related one property to the radioisotope’s use. A significant number of candidates included nuclear equations unnecessarily.

[In other words, because this was a three mark question, you needed to state a use and then relate two properties to that use.]

10. (6 marks)

Students were asked to perform a first-hand investigation to determine the molar heat of combustion of ethanol.

The following extract is from the practical report of one student.

[pic]

a. After completing the calculations correctly, the student found that the answer did not agree with the value found in data books. Suggest ONE reason for this. (1)

► Any one of the following would suffice:

• Loss of heat between the spirit burner and the water

• Loss of heat to surrounding atmosphere

• Loss of heat absorbed into the wire gauze

• Loss of heat due to heating of the beaker

• Loss of heat due to heat from water surface

b. Propose TWO adjustments that could be made to the apparatus or experimental method to improve the accuracy of results. (2)

► Two legitimate suggestions are needed. Some examples are:

• Place a lid on the top of the beaker to reduce heat lost from water surface

• Use a beaker that is less heat absorbing

• Reduce the gap between the spirit burner and the water beaker

• Replace the wire gauze with a clamp to hold the water beaker

c. Calculate the molar heat of combustion of ethanol, using the student’s data. (3)

- m C ∆T

∆cH =

n

- 250 x 4.18 x 40

∆cH = = 836 kJ mol-1

0.05

11. (5 marks)

Assess the potential of ethanol as an alternative fuel, making use of data from the table.

[pic]

► The data in the table suggests that

o It will be a liquid at room temperature (with a boiling point of 78.3oC it will be able to be safely stored as a liquid without major storage and safety concerns that might be associated with having to deal with a gas).

o Ethanol is the only fuel that can be derived from sources other then fossil fuels – it is the only fuel (out of those listed) that can come from a renewable source

o The heat of combustion information is given in terms of kJ g-1. This does not take into account the mass of the fuels. A more accurate measure for comparing heat of combustion is kJ mol-1. To obtain this information multiply the given heat of combustion by the molar mass of the fuel. The approximate results would be:

|Fuel |Heat of Combustion |Molar Mass (M) |Heat of Combustion |

| |(kJ g-1) | |(kJ mol-1) |

|Methane |55.6 |16 |889.6 |

|Propane |50.3 |44 |2213.2 |

|Octane |47.3 |114 |5392.2 |

|Ethanol |29.7 |46 |1366.2 |

o The heat of combustion of ethanol is less than some other fuels, however, it is still an efficient fuel, and has other advantages including being a liquid at room temperature.

Some pro’s and con’s of using ethanol as an alternative fuel

|ADVANTAGES |DISADVANTAGES |

|Oxygenated biofuels, such a ethanol, reduced toxic emissions to the |Existing car engines would need modifications to run on |

|environment. Ethanol burns completely to form water and carbon dioxide. |hydrous ethanol, due to potential corrosion caused by |

|Ethanol can be produced from a renewable source, unlike fossil fuels, which |the presence of water |

|are non-renewable. (Note that ethanol can also be made from fossil fuel |Greater engine wear occurs with high ethanol-petrol |

|sources.) |blends due to the need for higher compression ratios |

|Ethanol burns more cleanly than most fossil fuels, with less carbon |Ethanol has a higher flash point than petrol and so |

|deposited in the car engine, allowing spark plugs to last longer. |combustible fuel-oxygen mixtures are hard to obtain in |

|Mixtures of ethanol in petrol boost the octane rating, meaning that the fuel|cold areas of the world |

|burns more completely and reduces engine knock |Large areas of land are needed to grow crops that can be|

|Because ethanol is an excellent solvent, deposits that have built up in the |harvested for ethanol production. There is a scarcity |

|engine can dissolve, producing a cleaner system |of land with availability of water and sunshine for such|

|10% - 24% of anhydrous ethanol can be added to existing petrol engines |crops. Food production would also not be possible in |

|without modification to produce a fuel called a gasohol, which can act as a |such areas. Removal of forests to create such farming |

|petrol extender |land would be ecologically unacceptable. (This problem |

|Ethanol has a lower ignition temperature than petrol, so fuel-air |is reduced if ethanol is produced from waste sources of |

|combustible mixtures ignite more readily |glucose such as sugar cane pulp.) |

|Ethanol vapour-air mixtures have a greater flammability range than petrol |At present, it is more expensive to produce 1 litre of |

|vapour-air mixtures. Thus, rich ethanol-air mixtures will ignite and |ethanol than 1 litre of super petrol, although there may|

|combust more readily |be more equity in the price in the future as crude oil |

|Ethanol-petrol blends reduce CO emissions by 25% - 30% as well as reducing |becomes more expensive due to increasing scarcity or |

|ozone formation |other factors (including disruption to production and |

|Ethanol can be relatively cheaply and safely stored and transported as it |war). |

|has a boiling point of 78.3oC and is therefore a liquid at most temperature | |

|ranges in which it is likely to be used. | |

This question came from the 2004 HSC Chemistry paper – note the examiner’s comments and the marking guidelines.

The greater majority of candidates were very familiar with the actual content required in the question. Many candidates directed their responses towards listing the benefits and problems associated with the use of ethanol, but did not make any judgement as to the potential use of the chemical as a fuel.

There was considerable confusion over the use of the data and the units in the table. Many candidates assumed that ethanol was a renewable fuel, even when formed from a chemical derived from fossil fuels. The wording of the question required candidates to use the table. Many responses revealed rote learning of prepared answers, and some candidates presented very long and detailed responses, containing little information relevant to the question.

Lessons to learn from this question:

← Answer the question

← Don’t just use a rote response – adapt your knowledge to respond to the question asked.

← If the question says to use data provided, then do so.

←

[pic]

12. (3 marks)

[pic]

Some theory as it applies to this question.

► OIL oxidation is loss of electrons AN OX oxidation occurs at anode

RIG reduction is gain of electrons RED CAT reduction occurs at cathode

► Lead is more reactive than silver. Therefore, lead will more readily form ions than silver. Hence, the lead electrode will dissolve to form lead ions. The silver ions will precipitate on the silver electrode.

► Hence, some equations can be constructed:

Pb (s) ( Pb2+ (aq) + 2e- loss of electrons ( oxidation ( anode

Ag+ (aq) + e- ( Ag (s) gain of electrons ( reduction ( cathode

a. Identify the cathode in the diagram. (1)

► The cathode is the silver electrode (this is where reduction is occurring)

b. Write the net redox equation for the cell reaction, and calculate the cell potential (Eo) (2)

Pb (s) ( Pb2+ (aq) + 2e- Eo = +1.3

2Ag+ (aq) + 2e- ( 2Ag (s) Eo = 2 x 0.8 = 1.6

Pb (s) + 2Ag+ (aq) ( Pb2+ (aq) + 2Ag (s) Eo = 1.6 + 1.3 = 1.9 volts

13. (6 marks)

A galvanic cell was made by connecting two half-cells. One half-cell was made by putting a copper electrode in a copper (II) nitrate solution. The other half-cell was made by putting a silver electrode in a silver nitrate solution. The electrodes were connected to a voltmeter as shown in the diagram.

[pic]

Some theory as it applies to this question (not all relevant to the question)

► OIL oxidation is loss of electrons AN OX oxidation occurs at anode

RIG reduction is gain of electrons RED CAT reduction occurs at cathode

► Copper is more reactive than silver. Therefore, copper will more readily form ions than silver. Hence, the copper electrode will dissolve to form copper ions. The silver ions will precipitate on the silver electrode.

► Hence, some equations can be constructed:

Cu (s) ( Cu2+ (aq) + 2e- loss of electrons ( oxidation ( anode

Ag+ (aq) + e- ( Ag (s) gain of electrons ( reduction ( cathode

a. Complete the above diagram by drawing a salt bridge. (1)

► Draw in the salt bridge which connects the two beakers – showing clearly that it dips into both beakers. (An example of this can be seen in the question above.)

b. Using the standard potentials table in the data sheet, calculate the theoretical voltage of this galvanic cell. (2)

Cu (s) ( Cu2+ (aq) + 2e- Eo = (0.34

2Ag+ (aq) + 2e- ( 2Ag (s) Eo = 2 x 0.8 = 1.6

Cu (s) + 2Ag+ (aq) ( Cu2+ (aq) + Ag (s) Eo = 1.6 ( 0.34 = 1.26 volts

c. A student removes the voltmeter from the circuit and replaces it with an electrical generator. The generator causes the copper electrode to increase in mass.

Explain, using an equation, why the copper electrode will increase in mass. (3)

► Replacing the voltmeter with an electrical generator, means that an electrical current is now being applied to the cell. Hence, it is no longer a galvanic cell. Rather it is now an electrolytic cell. This means that the cathode and anode will be reversed. Hence, the copper electrode now acts the cathode, where reduction occurs.

► Cu2+ (aq) + 2e- ( Cu (s)

► Thus, copper ions from the copper solution form copper metal and are deposited upon the copper electrode, thus increasing its mass.

14. (5 marks)

a. Describe the conditions under which the nucleus is unstable. (2)

► The nucleus of an atom is composed on neutrons and protons. The stability of the nucleus depends upon the ratio of neutrons and protons. The nucleus of lighter elements (Z < 20) likely to be unstable if the ratio of neutrons to protons is not close to 1:1.

► The nucleus of heavy elements (Z > 80) are likely to be unstable if the ration of neutrons to protons is not close to 1.5:1.

► The nucleus of elements with atomic numbers greater than 83 are unstable as their neutron to proton ratio is greater than 1.5:1.

b. The following is a flow diagram showing the sequence of products released during the decay of uranium.

[pic]

Use examples from the flow diagram to describe the processes by which an unstable isotope undergoes radioactive decay. (3)

► Unstable nuclei have an excess of energy and release energy by means of decay, emitting alpha, beta or gamma particles, in order to achieve greater energy stability. The unstable radioactive nuclei of heavier elements are often unable to achieve stability through a single emission. The products of the decay process are often themselves unstable and undergo further decay. This process continues until a stable nucleus is achieved.

► The decay series shown in the diagram begins with uranium-238, and continues until lead-206 is formed.

► The diagram shows a series of steps in which there are two types of transitions. The first is represented by the diagonal arrows, is the emission of two protons and two neutrons from the nucleus. This is called alpha particle emission and effectively corresponds to the emission of He2+ ions from the nucleus. This is seen in first transition of the series where U-238 becomes Th-234 with the emission of 4He2.

► The second, represented by the horizontal arrows, is the emission of beta particles (effectively, electrons) from the nucleus. This occurs when a neutrons changes to a proton and an electron. This can be seen in the transition of Pb-214 to Bi-214, as a neutron splits to produce a proton and an electron.

15. (5 marks)

Analyse the relationship between the chemical composition and properties of, including colour, of pigments used in traditional art by Aboriginal people.

16. (7 marks)

Assess the potential health risks associated with chemicals found in cosmetics that were used in an ancient culture.

17. (5 marks)

Outline the processes used and the chemistry involved to prepare and attach pigments to surfaces in a named example of medieval or earlier artwork

18. (4 marks)

a. Identify the metal ion that will produce a yellow colour in a flame test. (1)

b. Explain how some metal ions produce a characteristic colour in a flame. (3)

19. (5 marks)

Describe the Bohr model of the atom, and identify ONE merit and ONE limitation of the model.

20. (5 marks)

a. Explain what is meant by a reflectance spectrum.

b. Outline how infrared light and ultraviolet light are used to determine the chemical composition of pigments.

21. (4 marks)

During your practical work you performed a first-hand investigation to observe the flame colour of different metal ions.

a. Explain the precautions you took during your investigation. (2)

b. Outline the procedure you used to observe the flame colour of different metal ions. (2)

22. (6 marks)

Evaluate the usefulness of the range of technologies used by chemists to identify pigments.

23. (6 marks)

The diagram shows the emission spectrum of sodium seen through a spectroscope.

[pic]

a. Draw an energy level diagram to represent the origin of these spectral lines.

b. Explain how the Bohr model of the atom was developed using the emission spectrum of hydrogen, and outline its limitations.

24. (6 marks)

a. Name a transition element. (1)

b. (ii) Outline a first-hand investigation to demonstrate the colour changes of a named transition element as it changes in oxidation state. (2)

c. Describe a safety risk associated with the procedure outlined in part (b), and suggest a same work practice to minimise the risk. (3)

25. (7 marks)

With reference to two pigments, explain how the metallic components produce colour.

26. (4 marks)

a. what is the maximum number of electrons found in an atomic orbital? (1)

b. The graph below shows the first five ionisation energies for sodium, aluminium and magnesium. (3)

[pic]

Explain how the data can be used to provide information about the arrangements of electrons around the atoms.

27. (6 marks)

During your practical work you performed a first hand investigation to determine the oxidising strength of potassium permanganate.

a. State the electronic structure of manganese in terms of subshells. (1)

b. Outline the procedure used to determine the oxidising strength of potassium permanganate. (2)

c. Using the results from the procedure in part (b), justify a conclusion about the oxidising strength of potassium permanganate. Use half equations in your answer. (3)

28. (8 marks)

During your practical work you performed a first hand investigation to observe the colour changes of a named transition element as it changed in oxidation number.

a. Define the Term transition element. (1)

b. Outline the procedure used in your investigation, and describe the results obtained. (3)

c. Explain why transition elements may have more than one oxidation state. (4)

29. (4 marks)

The formula for three coordination complexes containing chromium (III) are shown below.

a. What is the electron configuration of chromium

metal in the ground state? (1)

b. Draw a Lewis structure of ONE of these

coordination complexes, and explain the

bonding in this molecule. (3)

-----------------------

m = 250g m 2.3

n = = = 0.05

C = 4.18 M 46

"T = 40

[pic]∆T = 40

[pic]

Use your notes from doing this question in class as a means of revising for a question like this.

Use an “assess” scaffold to respond to this question.

Ensure you refer to the data in the table.

Net redox reaction is the sum of the two half reactions

Cell potential is the sum of the potential of each of the half reactions

The silver reaction has to be doubled to balance the total number of electrons.

The potential of the silver reaction is doubled because twice that equation is needed to balance the electrons.

The potential of the lead reaction is positive because it is going in reverse to the information given in the table of standard potentials.

Please Note:

The information in the boxes is for your reference only and as an aid to how to respond to the question. It does not need to be included in responding to this question – rather it is assumed knowledge.

This method can be applied to the following question as well.

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