Electrolysis Flashcards
Define electrolysis.
Electrolysis is the chemical decomposition of a compound brought about by a flow of an electric current through an aqueous solution of the compound or the molten compound.
An immersing bulb is connected to battery into solid NaCl. Explain if the bulb will light up.
No. Ions are held in fixed positions in the solid lattice by strong electrostatic forces of attraction between cation and anion, hence unable to move.
An immersing bulb is connected to battery into molten NaCl. Explain if the bulb will light up.
Yes. Electrostatic forces of attraction between cation and anion are overcome. Ions are free to move and acts as mobile charge carriers.
An immersing bulb is connected to battery into aqueous NaCl. Explain if the bulb will light up.
Yes. Electrostatic forces of attraction between cation and anion are overcome. Ions are free to move and acts as mobile charge carriers.
Define an electrolyte.
An electrolyte is a molten liquid or aqueous solution that contains mobile ions and can conduct electricity.
Define an electrode.
An electrode is a solid electrical conductor that carries electric current into the electrolyte. (It is often made of an inert metal such as platinum or graphite.)
Define an electrolytic cell.
An electrolytic cell refers to the entire setup where a pair of electrodes is immersed in an electrolyte and electrical energy from the battery causes a chemical reaction to take place.
Define a cathode.
A cathode is a negative electrode where reduction takes place.
Define an anode.
A anode is a positive electrode where oxidation takes place.
Choose the correct options.
Cations will be attracted to the (positive/negative) electrode where it (gains/loses) electrons. The cation is (oxidised/reduced). This electrode is the (cathode/anode).
negative, gains, reduced, cathode
Choose the correct options.
Anions will be attracted to the (positive/negative) electrode where it (gains/loses) electrons. The cation is (oxidised/reduced). This electrode is the (cathode/anode).
positive, loses, oxidised, anode
An experiment is conducted to electrolyse molten lead(II) bromide using graphite electrodes.
a) Write half equation, with state symbols, for the reactions taking place at the cathode and at the
anode respectively.
b) Describe the observation at the anode.
a)
Cathode:
Pb^2+ (l) + 2e ^- -> Pb (l)
Anode:
2Br^- (l) -> Br2 (g) + 2e ^-
b) Reddish brown fumes are formed.
What happens to ions which move to the negative electrode during electrolysis?
1: Ions lose electrons.
2: Ions increase in charge.
3: Ions are reduced.
4: Ions form molecules.
3
Water is electrically neutral. Explain how water can be oxidised and reduced at the electrodes.
There is plenty of water surrounding each electrode. The water molecules can gain or lose electrons on the surface of the electrode.
Write the half equation, with state symbols, for the reduction of water at the cathode.
H2O (l) + 2e^- -> H2 (g) + 2OH^- (aq)
Why is pure deionised water not used in electrolysis?
There are not enough ions in pure deionised water to carry much of an electric current.
Write the half equation, with state symbols, for the oxidation of water at the anode.
2H2O (l) -> O2 (g) + 4H^+ (aq) + 4e^-
What does preferential discharge of ions mean?
The charged ion that is lower in the electrochemical series being uncharged.
List the factors affecting the species that is discharged at an electrode.
1: position of the ions in the electrochemical series
2: concentration of the ions
3: nature of the electrode
List the cations in the electrochemical series, from least reactive to most reactive at the cathode.
(5 will not react when H2O present, 7 will react when H2O present)
Least reactive / most difficult to be preferentially reduced:
K^+
Ca^2+
Na^2+
Mg^2+
Al^3+
2H2O
Zn^2+
Fe^2+
Sn^2+
Pb^2+
2H^+
Cu^2+
Ag^+
Most reactive / easiest to be preferentially reduced
Write the half equation (state symbols not required) of the following cations should they be reduced.
1: potassium
2: aluminium
3: silver
4: lead
1:
K^+ + e^- ->K
2:
Al^3+ + 3e^- -> Al
3:
Ag^+ + e^- -> Ag
4:
Pb^2+ + 2e^- -> Pb
List the anions in the electrochemical series, from least reactive to most reactive at the anode.
(3 will not react when H2O present, 3 will react when H2O present)
Least reactive / most difficult to be preferentially oxidised:
2F^-
2SO4^2-
2Cl^-
2H2O
2Br^-
2I^-
4OH^-
Most reactive / easiest to be preferentially oxidised
Note: SO4^2- and F^- ions have very low tendency to be oxidised at the anode.
Why do NO3^- ions not oxidise?
NO3^- ions has a lot of oxygen in itself, hence unable to be oxidised further.
The electrolysis of aqueous magnesium nitrate, Mg(NO3)2, containing universal indicator was carried out. After some time, gas were liberated and the universal indicator in both electrode compartments had changed colour.
a) Write appropriate half equations, with state symbols, for the changes that took place in the cathode and anode.
b) Write an equation for the overall reaction that took place during electrolysis.
c) Hence, explain why the gas volumes are in the ratio of 2:1.
d) State and explain the colour change observed around each electrode.
a) Cathode (negative): 2H2O (l) + 2e^- -> H2 (g) + 2OH^- (aq)
Anode (positive): 2H2O (l) -> O2 (g) + 4H^+ (aq) + 4e^-
b) 2H2O (l) -> 2H2 (g) + O2 (g)
c) For every 4 mole of electrons passed, 1 mole of O2 gas and 2 mole of H2 gas are formed. Hence, the volume of H2:O2 = 2:1.
d) At the cathode, water is preferentially reduced over Mg^2+ since it is lower than Mg^2+ in the electrochemical series. The reduction of water produces OH^- which causes the universal indicator to change from green to blue/violet. At the anode, water is preferentially oxidised over NO3^- since NO3^- cannot be oxidised. The oxidation of water produces H^+ which causes the universal indicator to change from green to red.
Electrolysis of dilute sulfuric acid using platinum electrodes.
a) State the main chemical species present in the electrolyte.
b) Write appropriate half-equations, with state symbols, for the reactions at each electrode.
c) State the observation observed at each electrode.
d) Briefly explain the pH change at each electrode.
a) H^+ (aq), SO4^2- (aq), H2O (l)
b) Cathode (negative): 2H^+ (aq) + 2e^- -> H2 (g)
Anode (positive): 2H2O (l) -> O2 (g) + 4H^+ (aq) + 4e^-
c) Cathode: Effervescence of a colourless and odourless gas that extinguishes a lighted splint with a ‘pop’ sound.
Anode: Effervescence of a colourless and odourless gas that relighted a glowing splint.
d) Cathode: pH increases since H^+ ions are consumed. Hence [H^+] < [OH^-]
Anode: pH decreases since H^+ ions are produced. Hence [H^+] > [OH^-]
Electrolysis of aqueous silver nitrate using platinum electrodes.
a) Write appropriate half-equations, with state symbols, for the reactions at each electrode.
b) State the observation observed at each electrode.
c) Briefly explain the pH change at each electrode.
a) Cathode (negative): Ag^+ (aq) + e^- -> Ag(s)
Anode (positive): 2H2O (l) -> O2 (g) + 4H^+ (aq) + 4e^-
b) Cathode: Silvery solid deposited on the cathode.
Anode: Effervescence of a colourless and odourless gas that relighted a glowing splint.
c) Cathode: No change.
Anode: pH decreases since H^+ ions are produced. Hence [H^+] > [OH^-]
State the observation if chloride ion is oxidised.
Gas turned moist blue litmus paper red then bleached it.
Suggest why electrolysis of concentrated aqueous sodium chloride produced a different product at the anode from electrolysis of dilute aqueous sodium chloride.
Higher concentration of Cl^- means Cl^- is preferentially oxidised at the anode.
Define an inert electrode and give 2 examples of it.
Inert electrodes do not usually take part in the reactions at the electrodes.
Examples: platinum, graphite (carbon)
Define an active electrode and give an example of it.
An active electrode can undergo oxidation during electrolysis.
Example: Cu
Electrolysis of aqueous copper(II) sulfate using copper electrode.
a) With the aid of a half-equation, explain the observation at the cathode.
b) Rank in decreasing ease of oxidation of Cu and the competing species/ion at the anode.
c) If graphite electrode was used instead of copper electrodes, how will the observation at the cathode and at the anode differ?
a) Cu^2+ (aq) -> Cu(s) + 2e^-
Cu^2+ is lower than H2O in the electrochemical series and hence Cu^2+ is preferentially reduced over H2O to form copper metal that is deposited on the cathode.
b) Cu, H2O, SO4^2-
c) No change at cathode. At the anode, H2O is lower than SO4^2- in the electrochemical series and is preferentially oxidised. Effervescence will be observed at the anode and the gas relights a glowing splint.
Note: for part (c), if O2 is produced at the graphite electrode, it can react with carbon under high temperatures to form CO and/or CO2
An electrolysis of an aqueous solution of calcium hydroxide was conducted at high temperature using graphite electrodes.
a) Write half-equations, with state symbols, for the reactions happening at the cathode and anode.
b) During the electrolysis, the anode decreases in mass and a white precipitate is seen forming in the solution. Explain these observations.
a)
Cathode: 2H2O (l) + 2e^- -> H2 (g) + 2OH^- (aq)
Anode: 4OH^- (aq) -> O2 (g) + 2H2O (l) + 4e^-
b) The oxygen gas produced at the anode reacts with the graphite anode at high temperature to form CO2. Hence, the anode decreases in mass.
The CO2 produced reacts with Ca(OH)2 to form insoluble CaCO3, which is a white precipitate in the solution.
Three experiments of electrolysis were conducted.
Experiment 1: electrolysis of concentrated aqueous NaCl using platinum electrodes
Experiment 2: electrolysis of aqueous CuSO4 using copper electrodes
Experiment 3: electrolysis of molten CaBr2 using platinum electrodes
Which experiment(s) produce(s) a gas at the negative electrode?
Answer: Experiment 1 only
Product in Experiment 1: hydrogen gas
Product in Experiment 2: copper solid
Product in Experiment 3: molten calcium (only ions are present in molten solutions, no water present)
Name the 4 types of applications of electrolysis.
1: Extraction of aluminium from aluminium oxide
2: Purification of copper
3: Electrolysis of brine
4: Electroplating
In the extraction of aluminium from aluminium oxide via electrolysis, the aluminium oxide is dissolved at 1000°C in molten synthetic cryolite (Na3AlF3).
a) Write appropriate half-reactions, with state symbols, occurring at the cathode and anode.
b) Suggest what might happen to the liberated oxygen at the anode.
a) Half-reaction at cathode: Al^3+ (l) + 3e^- -> Al (l)
Half-reaction at anode: O^2- (l) -> O2 (g) + 2e^-
b) Due to the high temperatures, the liberated oxygen will react with the carbon of the anode to form carbon dioxide or carbon monoxide.
Electrolysis is used to extract metals whose ions are lower in the electrochemical series.
True/False?
False
Electrolysis is used to extract metals whose ions are higher up in the electrochemical series as the ions cannot be reduced easily.
Metals whose ions are lower in the electrochemical series can be extracted using suitable reducing agents.
Write chemical equations, with state symbols, of how iron is obtained from iron(III) oxide in the blast furnace where iron(III) oxide is reduced by
a) coke (carbon)
b) carbon monoxide
a) Fe2O3 (s) + 3C (s) -> 2Fe (l) + 3CO (g)
b) Fe2O3 (s) + 3CO (s) -> 2Fe (l) + 3CO2 (g)
An impure anode containing zinc, iron and silver is used in an experiment of electrolysis. Zinc is in the middle of the anode surrounded by other ions. Will zinc react with the electrolyte?
No. Zinc is not exposed, only ions at the surface of the anode will be reacted.
In an electrolysis of aqueous copper(II) sulfate used for purification of copper, the anode is made of impure copper, containing traces of Zn, Fe, Au and Ag.
a) Explain why Au and Ag are not oxidised and predict what will happen to the two metal impurities.
b) The cathode is made of a thin sheet of pure copper metal. Briefly describe what will happen at the cathode.
a) Au and Ag are lower than Cu in the electrochemical series and have a lower tendency to be oxidised than Cu. They will fall off the anode and be collected below the anode as anode sludge.
b) The Cu^2+ ions present in the electrolyte are reduced to Cu, depositing onto the cathode.
What is brine?
Brine is concentrated aqueous sodium chloride.
A diaphragm cell is used for the electrolysis of brine.
a) Write appropriate half-equations, with state symbols, for the changes that took place in the cathode and anode.
b) Sodium hydroxide made in the diaphragm cell is formed in aqueous solution. With reference to answer in (a), explain how sodium hydroxide is formed.
c) Steel is an alloy made of iron. Suggest why steel is never used for the anode.
a) Cathode: 2H2O (l) + 2e^- -> H2 (g) + 2OH^- (aq)
Anode: 2Cl^- (aq) -> Cl2 (g) + 2e^-
b) Na^+ is attracted to cathode but not reduced at the cathode since H2O is lower in the electrochemical series than Na^+ and H2O is preferentially reduced. OH^- is produced at the cathode due to reduction of H2O. OH^- reacts with Na^+ present to form NaOH.
c) Cl2 produced at the anode can react with iron and hence Cl2 cannot be obtained.
OR
Iron present in the anode may be oxidised instead of Cl^- and hence Cl2 cannot be obtained.
The products of electrolysis of brine are hydrogen gas (at cathode) and chlorine gas (at anode). State one large-scale use of this 2 products.
Use of hydrogen gas at cathode: Manufacture NH3 in Haber process from N2 and H2.
Use of chlorine gas at anode: Manufacture bleach (NaClO).
Electroplating involves coating an object with a metal. Identify the negative electrode, positive electrode, and the electrolyte.
The negative electrode should be the object to be electroplated.
The positive electrode should be the metal that the object is coated with.
The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate.
In an experiment of electroplating with copper, the object to be plated is a metal pan which is connected to the cathode. A piece of copper is connected to the anode, and the electrolyte is copper sulfate. Write the half-equation, with state symbols, for the process taking place at the cathode and anode.
Cathode: Cu^2+ (aq) + 2e^- -> Cu (s)
Anode: Cu (s) -> Cu^2+ (aq) + 2e^-
State 2 ways to aid in electroplating onto non-conductors, such as shoes.
1: Soak the shoe in a concentrated electrolyte solution to make it conductive.
2: Coat the shoe with a layer of graphite.
A metal spoon is electroplated with silver. Which row correctly describes the electrolytic cell used?
(electrolyte in aqueous solution), (positive electrode), (negative electrode)
1: silver nitrate, object to be plated, silver
2: silver nitrate, silver, object to be plated
3: silver iodide, object to be plated, silver
4: silver iodide, silver, object to be plated
Answer: 2
Silver iodide is insoluble.
In an electrolysis experiment, the same amount of charge deposited on 6 grams of titanium and 16 grams of copper. The charge on the copper ion is 2+. What is the charge on the titanium ion?
(molar mass of Ti = 48, molar mass of Cu = 64)
Answer: 4+
Let the charge on titanium ion be x.
Cu^2+ + 2e^- -> Cu
Ti^x+ + xe^- -> Ti
no. of moles of Ti
= 6g / 48gmol^-1
= 0.125 mol
no. of moles of Cu
= 16g / 64gmol^-1
= 0.25 mol
0.125 / 0.25 = 1/4
x = 4
Four different solutions were electrolysed in separate experiments. In one of the experiments, element X2 was formed at the positive electrode.
For which combination of solution and electrodes can the reaction for the formation of X2 be represented by the equation shown below?
2X^- -> X2 + 2e^-
1: dilute aqueous sodium chloride with inert electrodes
2: aqueous copper(II) sulfate with copper electrodes
3: aqueous copper(II) sulfate with inert electrodes
4: concentrated aqueous sodium chloride with inert electrodes
Answer: 4
Since X2 is formed, X is a halogen (ie Group 17).
Options 2 and 3 are wrong as sulfate ions typically do not react.
