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.