Chem Unit 4 - Common Exam Q (WACE)

By referring to any intermolecular forces present, describe the dissolving process as ethanol is added to water (3 marks) (WACE 2017 SA)

- Hydrogen bonds, (dipole-dipole) and dispersion forces between water molecules are disrupted
- Hydrogen bonds, (dipole-dipole) and dispersion forces between ethanol molecules are disrupted
- New H-bonds, dipole-dipole and dispersion forces are formed between ethanol and water molecules

(Table showing the solubilities of Butane, Butanone and Butan-1-ol increasing in this order)

Explain why these organic compounds have very different solubilities in water (6 marks) (WACE 2018 SA)

- Recognition that dissolving occurs when the energy released during the forming of solute-solvent forces of attraction is sufficient to disrupt the solute-solute and solvent-solvent forces of attraction
- Recognition that force of attraction between water and butane are weak due to only dispersion forces and reference stronger dispersion and hydrogen bonding in water and so solubility is poor.
- Recognition that there is hydrogen bonding, dispersion and dipole-dipole forces of attraction between butanone and water
- that are stronger than the forces of attraction between molecules in butanone and between molecules in water, and so dissolving occurs.
- Recognition that there is hydrogen, dispersion and dipole-dipole forces of attraction between butan-1-ol and water and are stronger than the forces of attraction between the molecules in butan-1-ol and between the molecules
in water and so dissolving occurs.
- Recognition that there is more hydrogen bonding between butan-1-ol and water compared to butanone and water and so the solubility is higher

Explain how detergents are able to remove grease from a surface by referring to the intermolecular forces present. Include a labelled diagram in your answer. (7 marks) (WACE 2019 ER)

LABELLED DIAGRAM IS 2 MARKS
EXPLANATION (5 MARKS):
- Recognition that the non-polar tail of the detergent ion exhibits dispersion forces
- which are similar in strength to, and so can overcome, the dispersion forces that exist between the oil molecules and so will dissolve in them
- Recognition that the charged head of the detergent ion exhibits stronger ion-dipole forces of attraction (and hydrogen bonds) with water molecules
- and so overcoming the hydrogen bonding between the water molecules dissolves preferentially in water
- Demonstrates an understanding that the grease/oil micelles formed remain suspended in the water and with agitation, can be removed

Predict and explain the conditions that would result in the highest yield of soap in the shortest amount of time (8 marks) (WACE 2019 ER)

- PREDICTION 1: high temperature (1 mk)
- greater proportion of particles has sufficient kinetic energy to react when they collide
- higher frequency of collisions as the kinetic energy of the particles is higher
- higher yield as forward rate of reaction will increase more then the reverse rate of reaction
- PREDICTION 2: high concentration of strong base solution (1)
- more particles present in the same volume and hence greater frequency of collisions and so greater number of successful collisions
- higher yield as forward reaction will be faster then reverse reaction until equilibrium re-established
MUST MENTION AGITATION (1)
- agitation increases surface area, increasing contact/collisions between reacting particles and hence rate (no impact on yield)
- Optional: can mention that products are continually removed to increase yield, but decreasing rate of reaction

Explain why polar carbon compounds are soluble in water but proteins are not. Illustrate your answer with the aid of a labelled diagram (6 marks) (WACE 2019 ER)

DIAGRAM (1 mk): shows interactions within and/or between molecules of polar carbon compounds and water
EXPLANATION (5 mks):
- the sum of the forces of attraction (dispersion forces, dipole-dipole and H-bonding) that exist between the molecules of the polar carbon compounds and water
- are sufficient in strength to overcome
- the sum of forces of attraction (dispersion forces, dipole-dipole and H-bonding) that exist between the molecules within each of the polar carbon compounds and water (and hence become dissolved)
- being large molecules, the dispersion forces of attraction between protein molecules is large
- the sum of attractive forces (dispersion forces, dipole-dipole and H-bonding) exerted by water molecules are insufficient in strength to disrupt the dispersion forces between the protein molecules (and hence do not dissolve in water)

Explain briefly why an alteration in the strength of bonding may affect the function of the protein molecule (3 marks) (Hale 2017 SA)

- the function of the protein molecule is (often) dependent on its tertiary structure
- the tertiary structure of the protein is held in place by these intermolecular bonds
- if the strength of this bonding reduces, the tertiary structure might change (thus affecting the function of the protein)

Use green chemistry principles to explain why it is beneficial to achieve a high yield of ethanol (2 marks) (Hale 2017 ER)

- by achieving a high yield there are less wasted reactants
ANY ONE OF:
- saves money
- more sustainable use of resources
- less disposal issues
(do not accept 'better for the environment')

STIMULUS: There are 3 organic compounds: Methyl Methanoate, Propanamide, and a Ketone

Question: State which compound is likely to have the highest solubility in water. Explain your answer in terms of intermolecular forces (4 marks) (WATP 2024 SA)

- propanamide
- recognition that all compounds are able to form polar bonds with water (Methyl methanoate and ketone forms dipole dipole bonds, propanamide forms hydrogen bonds)
- recognition that when hydrogen bonds between propanamide and water form, this would release more energy then when hydrogen bonds form between methyl methanoate and ketone
- recognition that this would allow a greater number of attractions between molecules of propanamide and water to be disrupted (resulting in highest solubility)

Ethanol dissolves readily in water, while decan-1-ol has very limited solubility. Explain, with the aid of labelled diagrams, why ethanol is able to dissolve in water but decan-1-ol is not. (8 marks) (WACE 2021 SA)

THIS IS RESPONSE FOR ENERGY CONSIDERATION:

LABELLED DIAGRAM (2 MARKS):
- showing hydrogen bonding between ethanol and water molecules
- showing dispersion forces between decan-1-ol

FROM HERE THERE ARE 2 ACCEPTED RESPONSES:

RESPONSE FOR ENERGY (6 MARKS):
- in order for a solute to dissolve in a solvent the energy released in the formation of intermolecular forces between the solute and solvent are sufficient to overcome the existing intermolecular forces between the solute molecules and solvent molecules
- both alcohols form dispersion forces and hydrogen bonds with water
- ethanol and water both have hydrogen bonding as their predominant type of intermolecular force
- the energy required to disrupt the hydrogen bonds in the ethanol and water are comparable to the energy released during the formation of the hydrogen bonds between the ethanol and water molecules and so dissolution occurs
- the predominant type of intermolecular force in decan-1-ol is dispersion forces
- the energy released during the formation of dispersion forces with water and decan-1-ol is not sufficient to disrupt the dispersion forces between the decan-1-ol molecules and so dissolving does not occur

Ethanol dissolves readily in water, while decan-1-ol has very limited solubility. Explain, with the aid of labelled diagrams, why ethanol is able to dissolve in water but decan-1-ol is not. (8 marks) (WACE 2021 SA)

THIS IS RESPONSE FOR STRENGTH OF ATTRACTION:

LABELLED DIAGRAM (2 MARKS):
- showing hydrogen bonding between ethanol and water molecules
- showing dispersion forces between decan-1-ol

FROM HERE THERE ARE 2 ACCEPTED RESPONSES:

RESPONSE FOR STRENGTH OF ATTRACTION (6 MARKS):
- in order for a substance to dissolve, the strength of intermolecular forces formed must be sufficient to disrupt the intermolecular forces between the solute molecules and between the solvent molecules
- both alcohols form dispersion forces and hydrogen bonds with water
- when ethanol dissolves in water, hydrogen bonds are their predominant forces of attraction between the water molecules and the ethanol molecules
- the strength of the hydrogen bonds formed formed are sufficient to overcome the hydrogen bonds between water molecules and the hydrogen bonds between the ethanol molecules and so dissolution occurs
- predominant type of intermolecular force in decan-1-ol is dispersion forces
- the strength of the dispersion forces formed between the decan-1-ol molecules and water is insufficient to disrupt the dispersion forces between the (much larger) decan-1-ol molecules and so dissolving does not occur

For the following polymers, choose one and list a use of the polymer. Describe what properties the polymer has for this use (2 marks) (WACE 2016 SA)

Choose from: Polyethene, Polypropylene, Polyvinyl Chloride, Polytetrafluoroethene, Polystyrene

- Polyethene: packaging, bottles, clingwrap, insulation (strength, flexibiltiy, transparency, heat resistance)
- Polypropylene: packaging, textiles, laboratory containers (strength, flexibility)
- Polyvinyl chloride: construction, piping, flooring (strength)
- Polytetrafluoroethene/Teflon: non-stick coating for cooking, clothing (heat resistance, low friction)
- Polystyerene: containers, disposable cutlery, dinnerware, packing material (flexibility, heat resistance)

Biodiesel can be synthesised using a base catalysed method or lipase catalysed method. Outline briefly an argument to justify the use of a lipase catalysed method rather then a base catalysed method to produce biodiesel (3 marks) (WACE 2016 SA)

ONE MARK FOR EACH ARGUMENT. CONTRAST BETWEEN LIPASE/BASE CATALYST MUST BE MADE:

THE LIPASE CATAYLST METHOD:
- produces a higher yield as it does not produce the soaps that are formed using a base-catalysed method
- produces a higher purity biodiesel then when the base catalysed method is used
- is safer as it does not require the use of caustic materials such as sodium hydroxide used in base catalysed method, but also does not require high temperatures which has associated risks (as in base catalysed method)
- has less negative environmental impact as lipase catalysts are biodegradable while strong bases are not, as well as the fact that they are not caustic nor affect acidity of the environment as strong bases do
- contributes to lower overall costs because they are less subsequent processing problems then those caused by the presence of a strong base, and it does not add to the expense of purifying glycerol as in the case of when a strong base is used. Furthermore, it makes it easier for separation of biodiesel from the by-product glycerol.

VERY SIMPLE ANSWER WITH NO DEPTH FOR USE OF CATALYSTS IN TRANESTERIFICATION:

Which of the catalysts, strong base or lipase is more likely to be the industrially preferred catalyst when using waste vegetable oil to make biodiesel? Justify your answer. (3 marks) (2020 WACE ER)

NO MARK AWARDED FOR CHOICE OF CATALYST

ARGUMENT FOR SODIUM HYDROXIDE:
Any 3 of the following:
- can operate at high temperatures making conversion quicker
- cost effective
- sodium hydroxide is cheaper
- higher yield
- more readily available
- not pH or temperature sensitive

ARGUMENT FOR LIPASE CATALYST:
Any 3 of the following:
- avoids contaminating the biodiesel with soap
- requires less energy
- can be reused many times
- operates at a lower temperature
- catalyst less harmful

IF ANSWER JUST SAYS 'ENVIRONMENTALLY FRIENDLY' NO MK AWARDED

Define 'protein tertiary structure' and describe how it is formed (3 marks) (WACE 2022 SA)

- 3D shape of a protein/folding of secondary structure
- due to interactions of alpha amino acid side chains (in the polypeptide)
- such as hydrogen bonding/dipole-dipole interactions/dispersion forces/ionic interactions/disulfide bridges

Capsaicin (it is a long hydrocarbon chain with an OH at the end) is the compound that makes chillies taste hot in the tongue. Drinking milk is effective in reducing the 'hotness' of chillies, by dissolving the capsaicin due to the presence of fats in the milk and removing it from the tongue. Capsaicin does not dissolve in water, and so drinking water does not reduce the effect of the compound when eaten. Explain this observation, using your understanding of intermolecular forces (5 marks) (WACE 2022 SA)

- Recognition of the forces of attraction present between the molecules in capsaicin are predominantly dispersion forces, and in fats are also predominantly dispersion forces
- Recognition of the forces of attraction present between the molecules in water are predominantly hydrogen bonds
- Recognition that the forces of attraction formed between water and capsaicin are relatively weak compared to the forces of attraction formed between capsaicin and fat
- recognition that the energy produced by the forces of attraction formed between capsaicin and fats are sufficient to overcome the existing forces of attraction between the molecules of fats and between the molecules of capsaicin (and so capsaicin dissolves in milk)
- recognition that the energy produced between the forces of attraction formed between capsaicin and water is insufficient to overcome the existing forces of attraction between the molecules of capsaicin and between the molecules of water (and so capsaicin does not dissolve in water)

State the conditions used in both the fermentation of glucose and hydrolysis of ethene method for production of ethanol (4 marks) (WACE 2022 SA)

PROCESS: FERMENTATION OF GLUCOSE:
- zymase/yeast used as a catalyst
Any one of the following:
- temperature stated (range 25C to 37C)
- pH range 3-5
- atmospheric pressure

PROCESS: HYDROLYSIS OF ETHENE:
- acid catalyst (H+/H3PO4/H2SO4)
Any one of the following:
- temperature of around 300C/moderate temperature
- pressure of 6000-7000 kPa/high pressure
- water in the gaseous state

OTHER RELEVANT ANSWERS WERE ACCEPTED BUT BE WITHIN REASON

Compare soaps and detergents in terms of structure (2 marks) (WACE 2022 ER)

- recognition that soaps and detergents both contain a long (non-polar) hydrocarbon chain
- recognition that soaps contain a carboxylate group whereas detergents contain a sulfonate group

Compare soaps and detergents in terms of cleaning action, with the aid of a labelled diagram (7 marks) (WACE 2022 ER)

- Recognition that soaps and detergents have the same cleaning action
LABELLED DIAGRAM: 2 marks (all parts of micelle correctly labelled)
- recognition that charged groups are soluble in water through ion-dipole attraction
- recognition that chains are non-polar (and aggregate together)
- recognition that non-polar grease/dirt aggregates with the hydrocarbon chains within the micelle through dispersion forces
- recognition that grease/dirt becomes 'soluble'/suspended in water and washed away via agitation

Compare soaps and detergents in terms of properties in hard water (3 marks) (WACE 2022)

- recognition that cleaning action of soap is diminished in hard water whereas detergents are effective
- recognition that hard water contains (divalent) cations such as Ca2+/Mg2+
- recognition that the Ca2+ salts of soaps are insoluble in water and precipitate to form 'soap scum' and the Ca2+ salts of detergents are soluble in water