Monday, September 13, 2010

REDOX - electrolysis as aqueous of molten?

Only copper ions, silver ions and gold ions can undergo electrolysis as an aqueous solution.
Every other metal ion needs to be in its molten state.

Because in an aqueous solution there is also H+ and OH- ions present H2O --> H+ + OH-
and so the H+ ions compete for the cathode with the metals ions.
Because all metals except Cu, Ag, Au are more reactive than H, the H+ ions will be reduced to H2 before the others.
(remember the more reactive the metal is the more stable the metal ions is, and so the harder it is to reduce.)

REDOX - electrolysis

This is used to make a chemical take back electrons when it rather wouldn't.
Cu + Cl2 --> 2CuCl2 This is spontaneous. It is a REDOX.

Cu --> Cu2+ + 2e- (LEO-oxidation)
Cl2 + 2e --> 2Cl- (GER- reduction)


CuCl2 --> Cu + Cl2 is not spontaneous but can be achieved via electrolysis.

REDOX -terms

When answersing questions on REDOX, you must always think about what is happening to the electrons, what chemical is gaining and what chemical is losing electrons.

Oxidised = has lost electrons (LEO)
Reduced = has gained electrons (GER)

Oxidising agent/ oxidant = the chemical that is itself reduced, so it has gained electrons.
Reducing agent/ reductant = the chemical that is itself oxidised, so it has lost electrons.

REDOX -Halogens as reducing agents

Colours of the halogens:

All halogen ions (Cl-, Br-, I-) are colourless.

Fluorine (F2) - Pale yellow
Chlorine (Cl2) - Green
Bromine (Br2) - Orange
Iodine (I2) - Brown

F2 is the strongest reducing agent because it is the most reactive and 'wants' to be F- ions more than the other halogens.
F2 is able to displace any other halide ion from solution.

F2 + 2NaBr --> 2NaF + Br2
(F atoms) (Br- ions) (F- ions) (Br atoms)
Pale yellow Orange

REDOX -Balancing equations

REDOX - Oxidation numbers

There are some things you MUST put into your answers to questions about Oxidation numbers.

1. If the oxidation number goes up - the chemical has been oxidised and so has lost electrons.

2. If the oxidation number goes down - then the chemical has been reduced and so has gained elections.

Assigning Oxidation numbers

Monday, September 6, 2010