Electrolysis

Electrolysis

1.47 understand an electric current as a flow of electrons or ions

Current is basically a flow of charged particles and in a metal wire, it is a flow of electrons. These have a negative 1 charge. Ions are charged particles too as they have lost or gained electrons. The movements of these ions are responsible for the conduction of electricity. 

1.48 understand why covalent compounds do not conduct electricity

Covalent compounds do not have spare free electrons that can move and carry the charge; and neither do they contain ions-it’s a covalent compound not an ionic compound.

1.49 understand why ionic compounds conduct electricity only when molten or in solution

When they are a solid the ions are not free to move and carry the charge. When they are molten-as in it is in melted form-the ions are free to move. Remember in liquids the particles are able to slide over each other and move whereas in solids the particles can only vibrate around a fixed position. When the ionic compound is dissolved in a solvent to form a solution the ions are also made free to move.

1.50 describe simple experiments to distinguish between electrolytes and non-electrolytes

http://www.dummies.com/how-to/content/how-to-distinguish-electrolytes-from-nonelectrolyt.html

1.51 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity

Passing an electric current through a compound which is either molten or in solution causes chemical changes, the chemical reactions produce new products—new substances.

1.52 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide

Right hand electrode: Cathode-attracts cations-is the negative electrode, as it is attracting positive ions.

Left hand electrode: Anode-attracts anions-is the positive electrode, as it is attracting negative ions.

[Opposite charges attract]

Nothing happens until the lead (II) bromide melts.

Lead (II) bromide is an ionic compound. The solid consists of a giant structure of lead (II) ions and bromide ions packed regularly in a crystal lattice. It doesn’t have any mobile electrons, and the ions are locked tightly in the lattice and aren’t free to move. The solid lead (II) bromide doesn’t conduct electricity.

As soon as the solid melts, the ions do become free to move around, and it is this movement that enables the electrons to flow in the external circuit.

Electrodes are made out of carbon-which is inert (unreactive).

As soon as you connect the power source, it pumps any mobile electrons away from the left-hand electrode towards the right-hand one. The excess of electrons in the right-hand electrode makes it negatively charged-called the cathode. The left-hand electrode is positively charged because it is short of electrons. There is a limit to how many electrons can squeeze into the negative electrode (Cathode) because of the repulsion by the electrons already there.

The positive lead (II) ions are attracted to the cathode, which is the negative electrode. When they get there, they gain 2 electrons each from the electrode and forms neutral lead atoms. These fall to the bottom of the container as molten lead.

Pb²⁺ (l) + 2e^- à Pb (l)

This leaves spaces in the cathode that more electrons can move into. The power source pumps new electrons along the wire to fill those spaces.

Bromide ions are attracted to the positive anode. When they get there, the extra electron which makes the bromide ion negatively charged moves onto the anode because this electrode is short of electrons.

The loss of the extra electron turns each bromide ion into a bromine atom. These join in pairs (bond covalently) to form bromine molecules. Overall:

2Br^-(l) à Br₂ (g) + 2e^-

The new electrons on the anode are pumped away by the power source to help fill the spaces being created at the cathode.

The ions are discharged at the electrodes. Discharging an ion simply means that it loses its charge-either giving up electron(s) to the electrode or receiving electron(s) from it.

Redox reaction

Look back at the ionic equations, see that the lead (II) ions gain electrons at the cathode. Gain of electrons is reduction. The lead (II) ions are reduced to lead atoms.

The bromide ions lose electrons at the anode. Loss of electrons is oxidation. The bromide ions are oxidized to bromine molecules.

1.54 write ionic half-equations representing the reactions at the electrodes during electrolysis

For the electrolysis of lead (II) bromide, PbBr₂

Cathode:

Pb²⁺ (l) + 2e^- à Pb (l)

Anode:

2Br^-(l) à Br₂ (g) + 2e^-