Common Test Practice Questions Flashcards
Which of the following substance cannot reduce iron(III) oxide to iron?
1: coke
2: hydrogen
3: carbon dioxide
4: carbon monoxide
3
Coke consists of carbon.
Carbon dioxide does not react with iron(III) oxide.
An electrolysis of a metal chloride is conducted. A heat source is placed below the set-up to make it molten metal chloride. When the heat source was removed and the content was allowed to cool and solidify, no electric current is observed to flow.
A student made the following comment: “An electric current will be observed if water is added into the solution.” Explain if the student is correct.
[2m]
He is correct if the metal is Group 2 metal or copper or zinc. (0.5m)
All Group 2 metals or copper(II) or zinc chloride are soluble in water (0.5m) to form mobile ions as charge carriers. (1m)
He is incorrect if the metal is lead (0.5m) as PbCl2 is insoluble, thus will not form any mobile ions and electrons to carry electrical charges. (1m)
What can be used to oxidise an aqueous solution of I^- ions to iodine?
1: sulfur dioxide
2: bromine water
3: a solution of CL^- ions
4: a solution containing Fe^2+ ions
Answer: 2
In displacement reactions between halogen and halides, the halogen acts as the oxidising agent. (i.e oxidises halide ions to halogen)
Order of reactivity: Chlorine > Bromine > Iodine
Iron(III) chloride is formed instead of iron(II) chloride when chlorine gas is passed over heated iron. Which statement best explains this observation?
1: Iron is protected by a surface layer of oxide.
2: Iron (II) chloride is decomposed when heated.
3: Chlorine is a strong oxidising agent.
4: Iron is too high in the reactivity series.
Answer: 3
Chlorine is a powerful oxidising agent, and it will oxidise Fe(II) to Fe(III).
When Fe reacts with chlorine gas, it forms iron (III) chloride which is a yellow-brown solid.
Substance X was added to iron(II) sulfate solution. When a few drops of sodium hydroxide was added to the mixture, a brown precipitate was formed. What could substance X be?
1: potassium manganate(VII)
2: carbon
3: carbon monoxide
4: hydrogen
Answer: 1
Potassium manganate(VII) is an oxidising agent whereas Fe^2+ is a reducing agent. Fe^2+ is oxidised to Fe^3+ and form a brown precipitate.
Describe a simple chemical test to show how chlorine can act as an oxidising agent.
Bubble chlorine gas through aqueous KI. Chlorine gas is an oxidising agent if colourless KI turns brown.
Describe a chemical test for sulfur dioxide gas (including observations), given that it is a reducing agent.
[2m]
Bubble the gas through acidified potassium manganate(VII) / potassium dichromate(VI).
The gas will decolourise purple acidified KMnO4^- / turn orange K2Cr2O7 green.
In an electroplating experiment, chromium is used to coat an object. The electrolyte used is aqueous chromium sulfate. Write ionic equations for the reactions that occur at both cathode and anode.
Cathode: Cr^3+ (aq) + 3e^- -> Cr (s)
Anode: Cr (s) -> Cr^3+ (aq) + 3e^-
Chromium is an active electrode.
Study the reaction below.
2KClO3 -> 2KCl + 3O2
Explain, in terms of changes in the oxidation states, why it is a redox reaction.
KClO3 is reduced as there is a decrease in oxidation state of Cl from +5 in KClO3 to -1 in KCl. KCLO3 is also oxidised as there is an increase in oxidation state of O from -2 in KCLO3 to 0 in O2. Since oxidation and reduction occur simultaneously, it is a redox reaction.
Note: K tend to form +1 cations, hence it is Cl that is reduced in KClO3.
A piece of red litmus paper was soaked in dilute sodium chloride solution. The paper was connected to a power supply. Which of the following shows the correct observations near the cathode and anode after the current had flowed for some time?
(cathode), (anode)
1: turned blue, turned white
2: turned blue, remained red
3: remained red, turned white
4: remained red, remained red
Answer: 2
An electrolysis experiment is conducted using Fe electrodes. The electrolyte is an aqueous solution of AsO3^3- and AsO4^3-. Suggest, with the aid of a half-equation, how Fe^2+ is formed in the set-up.
[2m]
Fe is an active electrode. It is preferentially oxidised to form Fe^2+ at the anode. [1m]
Fe (s) -> Fe^2+ (aq) + 2e^-
[1m]
Dry ammonia gas was passed over heated copper(II) oxide. N2 gas was collected from the experiment.
Which of the following statement about the reaction is correct?
1: Ammonia is oxidised.
2: Ammonia is neutralised by CuO.
3: CuO is oxidised.
4: No change to CuO.
Answer: 1
Pure copper can be obtained by electrolysis.
a) Name 3 suitable electrolytes for the electrolytic cell.
[3m]
b) 0.640g of copper is coated at one of the electrodes. Describe how the concentration of the electrolyte will change during the electrolysis. Explain your answer.
[2m]
a) Aqueous copper(II) sulfate / Aqueous copper(II) nitrate / Aqueous copper(II) chloride
Note: Must write the state!
b) No change in concentration of Cu^2+.
With the passage of two electrons around the circuit, one Cu atom is oxidised at the
anode for every Cu^2+ ion reduced at the cathode.
Silver can be extracted from electrolysis by dissolving the metal to form silver ions. Silver ions can then be precipitated by adding aqueous ammonia. Silver oxides is produced which can then be electrolysed to produce silver metal.
a) Write an ionic equation to show the precipitation of the silver ion with aqueous ammonia.
[1m]
b) Name the electrolyte that is used in the electrolysis.
[1m]
c) Write a half-equation for the reaction that takes place at the cathode.
[1m]
a) 2Ag^+ (aq) + 2NH3 (g) + H2O (l) -> Ag2O (s) + 2NH4^+ (aq)
OR
2Ag^+ (aq) + 2OH^- (aq) -> Ag2O (s) + H2O (l)
b) Molten silver oxide.
c) Ag^+ (l) + e^- -> Ag (l) or Ag (s)
iPod casings are usually made of aluminium. Aluminium can be anodised by coating a layer of aluminium oxide around the metal. It is done to protect the metal and to allow the casing
to absorb paints better. The cathode is platinum and the anode is aluminium. A gas is produced at anode and it reacts with the Al anode to form aluminium oxide.
a) Write half equations for the reactions that take place at the cathode and anode.
[2m]
b) Write a balanced chemical equation to show the formation of aluminium oxide.
[1m]
c) Aluminium oxide coated on the aluminium reacts with dilute sulfuric acid to form a porous layer of aluminium oxide. Suggest how the porous layer helps to absorb paint.
[1m]
d) State the effect of the anodising of aluminium on the overall pH level of the solution. Explain your answer.
[2m]
a) Cathode:
2H^+ (aq) + 2e^- -> H2 (g)
Anode:
2H2O (l) -> O2 (g) + 4H^+ (aq) + 4e^-
b) 4Al (s) + 3O2 (g) -> 2Al2O3 (s)
c) It has a larger surface area to absorb the paint.
d) No effect on the overall pH of the solution. For 4 moles of electrons passed, 4 moles of H^+ reduced at the cathode are replenished by 4 moles of H^+ produced at the anode. Hence there is no change in concentration of H^+.
A sample of dried bird’s nest was heated in an oven and the water vapour from the sample was dissolved in ethanol (solvent) containing excess sulfur dioxide. The mixture was then titrated with iodine solution in ethanol. Explain why it is important to ensure the chemicals used in the titration must be completely dried.
[1m]
To ensure the amount of water measured from the titration comes from the dried food sample.
Which of the following is not likely to occur when molten magnesium bromide is electrolysed?
1: Magnesium liquid is collected at the cathode.
2: Magnesium ions gain electrons at the cathode.
3: Bromine liquid is collected at the anode.
4: Bromine ions lose electrons at the anode.
Answer: 3
Since a molten electrolyte is used, high temperature causes magnesium metal to melt while bromine turns into gas vapour.
An aqueous chromium salt solution is electrolysed in an experiment using platinum electrodes. 0.015 mol of chromium is deposited on the cathode and 0.090 mol of electron is passed through the electrolyte. What could be the formula of the salt?
1: CrF4
2: CrSO4
3: K2CrO4
4: CrCl3
Answer: 3
no. of moles of Cr : no.of moles of e^- = 1:6
oxidation number of Cr = +6
Which process could yield the highest amount of chlorine gas?
1: fractional distillation of liquified air
2: displacement of chloride from aqueous sodium chloride using bromine
3: electrolysis of a molten sodium chloride
4: recovering chlorine from a chlorine-containing polymer
Answer: 3
Options 1 and 4: chlorine is present in trace amounts
Option 2: bromine is unlikely to displace chloride ions from sodium chloride as it is less reactive than chlorine.
An electrolysis experiment was set up using carbon electrodes. The electrolyte used is distilled water.
Which of the following substances, when added in a small amount to water, will cause the bulb to light?
1: lead
2: ethanol
3: sugar
4: sodium
Answer: 4
A sugar and water solution does not conduct electricity. When sugar dissolves in water, it remains as molecules, not ions hence no mobile ions to carry electrical charges.
Liquid sodium conducts electricity. This is because sodium metal, in both its solid and liquid states, has a sea of delocalised electrons that are free to move and carry electric charge.
