redox Flashcards
what is oxidation
loss of electrons
gain of oxygen
increase in oxidation state
what is reduction
gain of electrons
loss of oxygen
decrease in oxidation state
what are oxidation states
the number of electrons an atom has lost/gained/shared in a compound
what is the highest oxidation state
the max. no of electrons it could lose from its outer shell
eg nitrogen
1s22s22p3
so it could lose 5
and gain 3
therefore oxidation state is -3 - +5
what is the lowest oxidation state
the max no of electrons that could be gained
what are the rules for working out oxidation state
element are 0
ions are the same as their charge
in covalent molecules the state totals to 0 - more elctroneg = -
- less electroneg = +
in polyatomic ions state total charge on ion
which element have the same charge
hydrogen is always +1 - except in compound that are hydrides
oxygen is -2 except in peroxides
worked oxidation state example
Worked example
What are the oxidation states of the elements in the following species?
a) C b) Fe3+ c) Fe2+ d) O2- e) He f) Al3+
Answers:
a) 0 b) +3 c) +2 d) -2 e) 0 f) +3
So, in simple ions, the oxidation state of the atom is the charge on the ion:
Na+, K+, H+ all have an oxidation state of +1
Mg2+, Ca2+, Pb2+ all have an oxidation state of +2
Cl–, Br–, I– all have an oxidation state of -1
O2-, S2- all have an oxidation state of -2
what is an oxidising agent
An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
An oxidising agent itself gets reduced – it gains electrons
Therefore, the oxidation state of the oxidising agent decreases
what is a reducing agent
A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons
A reducing agent itself gets oxidised – it loses/donates electrons
Therefore, the oxidation state of the reducing agent increases
complex oxidation state example
What is the oxidation state of chromium in CrCl3?
This is a neutral compound so the sum of the oxidation states is zero. Chlorine has an oxidation state of -1. If the oxidation state of chromium is n:
n + 3(-1) = 0
n = +3 (Again, don’t forget the + sign!)
What is the oxidation state of chromium in Cr(H2O)63+?
This is an ion and so the sum of the oxidation states is equal to the charge on the ion. There is a short-cut for working out oxidation states in complex ions like this where the metal atom is surrounded by electrically neutral molecules like water or ammonia.
The sum of the oxidation states in the attached neutral molecule must be zero. That means that you can ignore them when you do the sum. This would be essentially the same as an unattached chromium ion, Cr3+. The oxidation state is +3.
What is the oxidation state of chromium in the dichromate ion, Cr2O72-?
The oxidation state of the oxygen is -2, and the sum of the oxidation states is equal to the charge on the ion. Don’t forget that there are 2 chromium atoms present.
2n + 7(-2) = -2
n = +6
what is a half equation
shows either reduction or oxidation of half of the full reaction
what do half equations contain
electrons
what are the rules for constructing half equations
atoms must balance
charge must balance
what must you add to ensure atoms on half equations balance
H2O or H+
