i) Electrolysis
1.48 Understand that an electric current is a flow of electrons or ions
Electric current is the flow of electrons.
1.49 Understand why covalent compounds do not conduct electricity
In covalent compounds there are no free electrons or ions to move. That's why they don't conduct electricity.
1.50 Understand why ionic compounds conduct electricity only when molten or in solution
Ionic compounds normally don't have free moving electrons because they are tightly packed in three dimensional structures. But when they are molten or dissolved in solution they form ions, which allow them to conduct electricity.
1.51 Describe experiments to distinguish between electrolytes and non-electrolytes
An electrolyte is a substance that conducts electricity.
Experiment: Take a led bulb and attach the wire at the end of the substance. Now supply electricity from the other end. If the led bulb glows, that substance is an electrolyte or else its non-electrolyte.
1.52 Understand that electrolysis involves the formation of new substances when ionic compounds conduct electricity
Electrolysis is the process of using electricity to break down or decompose a compound. The compound is usually dissolved in water or molten.
1.53 Describe experiments to investigate electrolysis, using inert electrodes, of molten salts such as lead(II) bromide and predict the products
Lead bromide is heated until it becomes molten. Then it forms two ions: Li+& Br-.
The bromide ions moved to the anode and gained electrons and formed bromine molecules.
$$2Br^- \rightarrow Br2 + 2e^-$$
The lead ion is positive. So it moves to the negative electrode and by gaining two electron, it forms lead metal which is deposited on the surface of the set.
$$Pb^+ + 2e^- \rightarrow Pb(l)$$
During the electrolysis, the following observations are seemed:
i. Nothing at all happens until the lead (II) bromide melts. Then the bulb light's up showing electrons are flowing through the wire.
ii. There is bubble around the electrode (anode) as brown bromine gas is given up.
iii. After sometimes metallic red is formed which is found underneath the negative electrode. When we stop heating the lead (II) bromide solidifies again. Everything stops.
1.54 Describe experiments to investigate electrolysis, using inert electrodes, of aqueous solutions such as sodium chloride, copper(II) sulfate and dilute sulfuric acid and predict the products
Electrolysis of Sodium Chloride solution:
In an aqueous solution of sodium chloride contains four different types of ions. They are:
- Ions from sodium chloride -- Na+(aq) and Cl- (aq)
- Ions from water -- H+(aq) and OH-(aq)
When dilute sodium chloride solution is electrolysed using inert electrodes, the Na+ and H+ ions are attracted to the cathode. The Cl- and OH- ions are attracted to the anode.
At the cathode: The Na+ and H+ are attracted to the platinum cathode. H+ ions gain electrons from the cathode to form hydrogen gas.
$$2H+(aq) + 2e^- \rightarrow H2(g)$$
Na+ remains in the solution.
At the anode: Cl- and OH- ions are attracted to the platinum anode. Cl-ions give up electrons to the anode to form chlorine gas.
$$Cl-(aq) \rightarrow Cl2(g) + 2e^-$$
OH- remains in the solution.
Electrolysis of Aqueous Copper (II) Sulphate Using Inert Electrodes
During electrolysis, the cathode is coated with a layer of reddish-brown solid copper. The blue colour of the solution fades gradually as more copper is deposited. The resulting electrolyte also becomes increasingly acidic.
An aqueous solution of copper(II) sulphate contains four types of ions: Ions from copper(II) sulphate -- Cu2+ and SO4 2-
Ions from water -- H+ and OH-
At the anode: OH- ions and SO4 2- ions are attracted to the anode. OH- ions give up electrons more readily than SO4 2- ions.
$$4OH-(aq) \rightarrow 2H2O(l) + O2(g) + 4e^-$$
The SO4 2- ions remain in solution.
At the cathode: H+ ions and Cu2+ ions are attracted to the cathode. Copper is lower than hydrogen in the reactivity series. Cu2+ ions accept electrons more readily than H+ ions. As a result, Cu2+ ions are preferentially discharged as copper metal (atoms).
$$Cu2+(aq) + 2e^- \rightarrow Cu(s)$$
The H+ ions remain in the solution.
Electrolysis of dilute sulphuric acid
During electrolysis, the cathode is coated with a layer of reddish-brown solid copper. The blue colour of the solution fades gradually as more copper is deposited. The resulting electrolyte also becomes increasingly acidic.
An aqueous solution of copper (II) sulphate contains four types of ions:
Ions from copper (II) sulphate: Cu2+ and SO4 2-
Ions from water: H+ and OH-
At the anode:
OH- ions and SO4 2- ions are attracted to the anode. OH- ions give up electrons more readily than SO4 2- ions.
Consequently, OH- ions are preferentially discharged to give oxygen gas.
$$4OH- (aq) \rightarrow 2H2O (l) + O2 (g) + 4e-$$
The SO4 2 ions remain in solution.
At the cathode:
H+ ions and Cu2+ ions are attracted to the cathode. Copper is lower than hydrogen in the reactivity series. Cu2+ ions accept electrons more readily than H+ ions. As a result, Cu2+ ions are preferentially discharged as copper metal (atoms).
$$Cu2+ (aq) + 2e- \rightarrow Cu (s)$$
The H+ ions remain in the solution.
1.55 Write ionic half-equations representing the reactions at the electrodes during electrolysis
See 1.54
1.56 Recall that one faraday represents one mole of electrons
One Faraday is 96500 coulombs. That is the amount of coulombs in one mole of electrons.