Chapter 7 - Oxidation, reduction and redox reactions

Página 1

7.1 - Oxidation and reduction

7.1 - Oxidation and reduction

Redox reactions:

-short for reduction-oxidation

-if one reagent is reduced, it itself is an oxidising agent

Also called electron transfer reactions.

-if a reagent has been oxidised, it has lost elections

-if a reagent has been reduced, it has gain electrons (originally removal of oxygen)

copper and oxygen -> copper oxide

Cu -> Cu(2+) + 2e-

because Cu now has a positive charge, meaning it has lose electrons, it has been OXIDISED

remember: the higher the negative number, the MORE it has been REDUCED

Oxidation

Is

Loss

Reduction

Is

Gain

oxidising and reducing agents:

~reducing agents are electron donors (give them away)

~oxidising agents are electron acceptors (get given them)

7.2 - Oxidation states

Oxidation states are used to see what has been oxidised and what has been reduced in a redox reaction.

~in an ionic compound, they tell us how many e- have been lost/gained

~in a molecule, they tell us about the distribution between elements of different electronegativity

-every element in its uncombined state = 0

-positive number = lost electrons (oxidation)

-negative number = gained electrons (reduction)

The more positive the number, the more it has been oxidised and the more negative the number, the more it has been reduced.

Rules:

1) uncombined = 0

2) some elements always have the same number, for others, sometimes it changes

3) sum of all o. numbers in a compound = 0 (they aren't charged)

4) sum of all o. states in a complex ion = the charge of the ion (NH4+ = +1)

5) the most electronegative element in a compound always has a - oxidation state

element

o. state

e.g.

H

+1 (in metal hydrides is -1)

HCl

Group 1

+1

NaCl

Group 2

+2

CaCl2

Al

+3

AlCl3

O

-2 (in peroxides and OF2 = +2)

Na2O

F

-1

NaF

Cl

-1 (except w/ F and O

working out oxidation states:

1. remember rules (e.g. H=+1)

2. using basic maths work out the values for the other elements

e.g. 1:

phosphorus pentachloride, PCL5

1. there are 5 Cl, each with a charge of -1

2. if Cl5=-5, then P has to equal +5

e.g. 2:

red copper oxide, Cu2O

1. oxygen=-2

2. Cu2 = +2

(3. divide Cu2 by 2 to get the value for EACH Cu element)

+2/2= +1, Cu=+1

nb: if an element's oxidation state has not changed, it is a spectator ion.

7.3 - Redox equations

when an element is reduced, it GAINS electrons and its o. state goes down

make sure you balance equations for not only ATOMS but CHARGE as well

1. write oxidation states

2. balance atoms and charge

3. make sure it's balanced correctly