Chem - Unit 3 - Common Exam Q (WACE)

Question: Table showing boiling points, in increasing order: pentane, pentanal, pentanoic acid

Account for the different in boiling points of the 3 compounds (5 marks) (WACE 2016, SA)

- higher boiling points result from more energy being required to overcome the total intermolecular forces in the substance/higher BP due to higher strength of IMF
- all substances have similar dispersion forces strength (due to similar size/structure)
- pentane exhibits only dispersion forces
- pentanal has dipole-dipole forces in addition to dispersion forces
- pentanoic acid has hydrogen bonding in addition to dipole dipole and dispersion forces

Consider the reaction: 2SO2 + O2 <--> 2SO3 (all are gases btw)

Use collision theory to explain the effect of raising the total pressure on yield (5 marks) (WACE 2016, ER)

- when pressure increased, reactant and product particles are forced closer together, increasing frequency of collisions
- Increasing the rate of both forward and reverse reactions
- Ratio of gaseous particles of LHS to RHS is 3:2. Forward reaction as written has greater proportion of gaseous particles and hence
- forward reaction proceeds faster then reverse reaction until equilibrium has been re-established.
- results in a greater proportion of products of forward reaction (increasing the yield)

Other then having too low a molar mass, state two reasons why the concentration of sodium hydroxide solution cannot be reliably determined by weighing out an amount of solid sodium hydroxide and dissolving it in a known volume of distilled water? (2 marks) (WACE 2018 SA)

ANY TWO OF THE FOLLOWING:
- hygroscopic: it absorbs atmospheric moisture which changes the mass of the NaOH sample
- deliquescent: it dissolves in the water it absorbs, so no mass of solid can be weighed
- absorbs carbon dioxide gas to form Sodium carbonate and water
- not known purity (% purity not known)

DO NOT ACCEPT:
- 'not available in pure form'

Outline the difference between random error and systematic errors (2 marks) (WACE 2018 ER)

- Random errors are fluctuations in measured data due to limitations of the measurement device/technique, they occur unpredictably and to varying extents throughout the experiment. Non-directional effect on results (e.g different people doing titrant readings)
- Systematic errors are repeatable errors that occur due to faulty equipment or poor experiment design, they occur consistently to the same extent throughout the experiment. Results are skewed in one direction (e.g faulty timer)

Explain the function of the electrolyte in a fuel cell (2 marks) (WACE 2017 ER)

ANY TWO OF THE FOLLOWING:
- will allow for movement of ions
- maintains electrical neutrality in the cell
- completes the circuit

Explain why a salt bridge is required in a galvanic cell (2 marks) (WACE 2019 SA)

ANY TWO OF THE FOLLOWING:
- maintain electrical neutrality in a cell
- complete the circuit/connect the two cells
- required for ion flow
- electrons are forced through an external circuit

General Answer for effect on equilibrium when a reactant is removed (5 marks) (WACE 2020 SA)

- reduced concentration/pressure of ___ means a decrease in collisions between the reactant particles (good idea to name them)
- this decreases the rate of the forward reaction
- the rate of the reverse reaction is initially not affected
- the rate of the reverse reaction, therefore, is greater then the rate of the forward reaction
- the equilibrium position therefore shifts left/equilibrium favours the reverse reaction

Explain how a HPO4 2-/H2PO4- solution prevented any significant pH change when H+ ions were added into the solution (4 marks) (WACE 2020 SA)

- Recognition that HPO4 2-/H2PO4- is a buffer (because it is a weak base/weak acid combination)
- H+ reacts with the HPO4 2- base in the buffer
- Recognition that this consumes the majority of the added H+ in the solution (and therefore minimal increase in H+ conc.)
- recognition of how conc of H+ links to pH

State 2 consequences of ocean acidification on marine organisms with shells (2 marks) (WACE 2020 SA)

- building shells becomes harder
- weaker/thinner/poor quality shells
- makes organisms more vulnerable to predators
- less likely to grow large
- less likely to reproduce

Explain why potassium hydrogensulfite, KHSO3 produces an acidic solution when dissolved in water, while potassium hydrogencarbonate, KHCO3, produces a basic solution when dissolved in water. Use equations to illustrate your answer (7 marks) (WACE 2022 SA)

WRITTEN: 5 marks
- Recognition that K+ ions in solution are neutral/do not react with water
- Recognition that HSO3- and HCO3- ions undergo hydrolysis reactions
- Recognition that for the hydrolysis reactions for HSO3- the reaction that produces H3O+ occurs to a greater extent then the reaction that produces OH- (therefore the solution will be acidic)
- Recognition that for the hydrolysis reactions for HCO3- the reaction that produces OH- occurs to a greater extent then the reaction that produces H3O+ (therefore the solution is basic)
- Recognition that a basic solution has a greater concentration of OH- ions then H3O+ ions/an acidic solution has a greater concentration of H3O+ ions then OH- ions

EQUATIONS: Minimum of two appropriate equations which include at least one equation for HSO3- and one for HCO3-.
(they must be the hydrolysis of both to produce H3O+ or OH-)

In order for a reaction to occur between Iron(III) oxide and Aluminium, they are mixed as powders and a heat source, such as burning magnesium, is used to ignite the mixture. Using your understanding of collision theory, explain these observations (4 marks) (WACE 2022 ER)

- Recognition that a high activation energy requires additional energy source for reaction to occur
- for successful reactions, particles must collide with sufficient energy
- recognition that powders have a large surface area, and so there are more particles available for reaction
- increased availability of particles increases likelihood or collisions (frequency of collisions) and so reaction more likely to occur

Hydrochloric acid must be standardised against a primary standard before it can be used in titrations. List 3 properties of substances suitable for use as primary standards (3 marks) (WACE 2022 ER)

ANY 3 OF:
- high molar mass
- not hygroscopic
- not deliquescent
- available with a known purity
- does not react with substances in the atmosphere (e.g CO2)
- highly soluble in water
- predictive reactivity

Outline the difference between systematic and random errors. Use an example for each to support your answer (4 marks) (WACE 2022 ER)

- recognition that systematic errors produce consistently high or consistently low measurements compared to the true value
- recognises that random errors produce measurements that can be either high or low/fluctuate around the true value
- example of systematic error includes (one of the following): using an inappropriate proportion of reactants, errors in calibration of equipment, inappropriate rinsing of glassware
- example of random error includes (one of the following): parallax (reading of meniscus), judging the end point, use of stopwatch, reading thermometer, not using the same measuring equipment during the reaction, using measuring cylinder rather then pipette.

Explain, in terms of reaction rates, how this buffer:
SiO4 4- + H2O <--> HSiO4 3- + OH-
would respond to the addition of H3O+ caused by rainfall. Include a relevant chemical equation in your answer. (6 marks) (WATP 2022 SA)

- An increase in H3O+ would neutralise/remove the OH- ions from the system.
- Both forward and reverse reaction rates decrease
- but the forward reaction rate decreases less then the reverse reaction rate
- this results in the position of equilibrium shifting right
- resulting in the production of more OH- ions and therefore pH is maintained

EQUATION: equation should show H3O+ reaction with conjugate base species:
H3O+ + SiO4 4- <--> H2O + HSiO4 3-

State what a sacrificial anode is and explain how it is effective in preventing corrosion of the toolboxes. You should state which metal could be used for a sacrificial anode in your answer. (4 marks) (WACE 2023 SA)

- Recognition that the sacrificial anode is a more reactive metal then the tool boxes (cathode)
- Metals could include any one of: zinc, magnesium, chromium, manganese, aluminium
- Recognition that the two metals are touching/connected electrically
- Recognition that the E nought value of the anode is more negative compared to that of iron/more readily oxidised/stronger reductant

NOTE: Do not accept reactive group 1 metals or Ca/Ba

Explain what will happen in the blood when there is an elevated concentration of carbon dioxide. Predict how blood pH is affected. Include relevant chemical equations in your answer. (5 marks) (WACE 2023 SA)

Equation of Buffer of Blood: H2CO3 + H2O ←→ HCO3- + H3O+

- Recognition that elevated concentration of carbon dioxide in the blood increases concentration of carbonic acid in the blood
- Equation: CO2 + H2O <--> H2CO3
- Recognition that the increase in blood carbonic acid concentration shifts the blood buffer equilibrium to the right, increasing H3O+ concentration
- Recognition that the HCO3- will react with additional H3O+, or equation: HCO3- + H3O+ --> H2CO3 + H2O
- recognition that there is a small decrease in pH

Identify the most appropriate indicator for a titration between Barium Hydroxide and Ammonium Chloride, using an equation to support your answer.

Question had a table with
Methyl Orange: pH change range 3.1-4.4
Bromothymol Blue: pH change range 6.2-7.6
Phenolphthalein: pH change range 8.3-10

(5 marks) (WACE 2023 SA)

- phenolphthalein
- recognition that (at equivalence point) the ammonia hydrolyses to produce OH-
- recognition that at (equivalence point) [OH] > [H3O+] (and solution is basic)
- Recognition that indicator changes in the basic range/indicator colour change/end point is at a similar pH to the equivalence point
- Appropriate Equation: NH3 + H2O <--> NH4+ + OH-