Oxidation, reduction and redox equations (3.1.7)

Question 1

what is oxidation?

Answer 1

the process of electron loss

Question 2

what are oxidising agents?

Answer 2

electron acceptors

Question 3

what is reduction?

Answer 3

the process of electron gain

Question 4

what are reducing agents?

Answer 4

electron donors

Question 5

what do redox reactions involve?

Answer 5

a transfer of electrons from the reducing to the oxidising agent

Question 6

how do you determine the overall equation for a redox reaction?

Answer 6

by combining the separate half equations for the oxidation and reduction processes (remember to balance electrons)

Question 7

what are oxidation states?

Answer 7

the number of electrons that need to be lost/gained to form the non-charged atom eg. Li^+ would be 1+

Question 8

what is the rule for oxidation states?

Answer 8

the sum of all the oxidation states for the elements in a compound must equal the overall charge on the compound

Question 9

what are the general charges to remember for working out oxidation states?

Answer 9

group 1: +1
group 2: +2
Al: +3
F: -1 (other halogens usually -1)
O: almost always -2
H: usually +1

Question 10

what are the exceptions for the charges?

Answer 10

1. in peroxides eg H2O2, oxygen has a -1 charge (because hydrogen can’t form +2)
2. in metal hydrides eg. NaH, hydrogen has a -1 charge

Question 11

how would you work out the oxidation state for N in NO3^-?

Answer 11

each O has a charge of -2
let N be x
x + (3 x -2) = -1
x - 6 = -1
x = 5
N = +5

Question 12

what do half equations do?

Answer 12

show the change in oxidation state of a single element (they must balance in terms of atom and charge)

Question 13

what would the reduction half equation be for the overall equation H2 + S –> H2S?

Answer 13

H is +1, S is -2
S + 2H^+ + 2e^- –> H2S
(sulfur is reduced and gains two electrons, which must come from 2 1+ hydrogen ions)

Question 14

what would an example of an oxidation half equation be?

Answer 14

Cl2 + 2 H2O –> 2ClO^- + 4H^+ + 2e^-
(chlorine is oxidised and each atom loses one electron, which come from two oxygen atoms)

Question 15

what are the steps for balancing half equations?

Answer 15

1. balance the element that is changing oxidation state
2. balance O if needed using H2O
3. balance H if needed using H^+
4. balance the charge using electrons
5. check the number of electrons matches the change in oxidation state

Question 16

how do you combine half equations to get an overall or ionic equation?

Answer 16

1. multiply to get same number of electrons in each half equation
2. combine corresponding sides of both half equations
3. cancel down/out anything possible

Question 17

where do the electrons go in half equations for reduction reactions?

Answer 17

on the reactant side

Question 18

where do the electrons go in half equations for oxidation reactions?

Answer 18

on the product side