Redox equations

Starter

• What is redox?

Redox – what?

Oxidation and reduction happen at the same time

There is no net gain or loss of electrons.

e- You can’t just create them or destroy them!

Example: the thermit reaction

Fe2O3 + Al Fe + Al2O3

Fe3+ Al0 Fe0 Al3+

Fe3+ + 3e- Fe Reduction

Al Al2O3 + 3e- Oxidation

Ionic half-equations

OIL - Oxidation Is Loss of electrons

RIG - Reduction Is Gain of electrons

Remember:

And often... (as a quick and simple way to tell):

Oxidation is gain in oxygen or loss of hydrogen

Reduction is loss of oxygen or gain of hydrogen

+7+6+5+4+3+2+10-1-2-3-4-5-6-7

Reduction

Oxi

datio

n

Oxidation state

Oxidation and reduction can be seen as movement up or down a scale of oxidation states

Redox in presence of acidExample: a past exam question

a) Identify, as oxidation or reduction, the formation of NO2 from NO3

- in the presence of H+ and deduce the half-equation for the reaction.

NO3- NO2

+5 +4Reduction

Steps to take1. Write a balanced equation for the species

2. Work out “before and after” oxidation states

3. Balance oxidation states with electrons

NO3- NO2

+5 +4

NO3- + e- NO2

NO3- + e- + 2H+ NO2

5. If the equation still doesn’t balance, add enough water to one side so it balances

NO3- + e- + 2H+ NO2 + H2O

4. If all the charges don’t balance, add H+ ions to one of the sides to balance them

Beautiful!

Give these a goNa Na+

Fe2+ Fe3+

I2 I¯

C2O42- CO2

H2O2 O2

H2O2 H2O

NO3- NO

NO2 NO3-

SO42- SO2

Na Na+

Fe2+ Fe3+

I2 I¯

C2O42- CO2

H2O2 O2

H2O2 H2O

NO3- NO

NO2 NO3-

SO42- SO2

Balance the half equations

All good?

Na Na+ + e-

Fe2+ Fe3+ + e-

I2 + 2e- 2I¯

C2O42- 2CO2 + 2e-

H2O2 O2 + 2H+ + 2e-

H2O2 + 2H+ + 2e- 2H2O

NO3- + 4H+ + 3e- NO + 2H2O

NO2 + H2O NO3- + 2H+ + e-

SO42- + 4H+ + 2e SO2 + 2H2O

Na Na+ + e-

Fe2+ Fe3+ + e-

I2 + 2e- 2I¯

C2O42- 2CO2 + 2e-

H2O2 O2 + 2H+ + 2e-

H2O2 + 2H+ + 2e- 2H2O

NO3- + 4H+ + 3e- NO + 2H2O

NO2 + H2O NO3- + 2H+ + e-

SO42- + 4H+ + 2e SO2 + 2H2O

Exam Question

• 1. Mercury thermometers are not used in some laboratories because of the danger of mercury vapour. This vapour is very easily absorbed through the lungs into the blood.In the blood, mercury reacts with hydrogen peroxide to form mercury(II) oxide.

• Hg + H2O2 → HgO + H2O• The mercury(II) oxide formed accumulates within

organs in the body.• Use oxidation numbers to show that the reaction

between mercury and hydrogen peroxide is an example of both oxidation and reduction.

• [Total 2 marks]

Self marking

• 1. Oxidation because oxidation state of Hg changes from 0 to +2 so oxidation (1)Reduction because oxidation number of O changes from –1 to –2 (1)

• Or• Correct identification of all the oxidation numbers (1)

Correct identification of oxidation and reduction (1) 2• Allow ecf for the identification of oxidation and reduction

from wrong oxidation numbers• [2]

Combining half equations

Just a mashing together of two half-equations!

... followed by some satisfying cancelling-out.

Back to our exam question

NO3- + e- + 2H+ NO2 + H2O

b) Deduce the overall equation for the reaction of copper with NO3

- in acidic conditions to give Cu2+.

Reduction

Cu Cu2+ + 2e- Oxidation

Now what?

1. Multiply the equations so that the number of electrons in each is the same

2. Add the two equations and cancel out the electrons on either side of the equation

3. If necessary, cancel out any other species which appear on both sides of the equation

2NO3- + 2e- + 4H+ 2NO2 + 2H2O

Cu Cu2+ + 2e-

Cu + 2NO3- + 2e- + 4H+ Cu2+

+ 2e- + 2NO2 + 2H2O

Cu + 2NO3- + 4H+ Cu2+

+ 2NO2 + 2H2O

X 2

Steps to take to combine equations

Give these a go1. Fe2+ ions are oxidised to Fe3+ ions by ClO3

- ions in acidic conditions. The ClO3

- ions are reduced to Cl- ions. Write the overall reaction.

2. Write an overall reaction for MnO4- reducing H2O2

to O2 and creating Mn2+ .

Fe2+ Fe3+ + e- ClO3- + 6e- + 6H+ Cl- + 3H2O

2MnO4¯ + 5H2O2 + 6H+ 2Mn2+ + 5O2 + 8H2O

6Fe2+ + ClO3- + 6H+ Fe3+ + Cl- + 3H2O

• 2. An unusual compound of iron has been detected on the surface of the planet Mars. This compound contains the ferrate(VI) ion.

• When chlorine is bubbled through a suspension of iron(III) oxide in concentrated aqueous sodium hydroxide, a solution of aqueous sodium ferrate(VI) forms.

• The two relevant redox systems are shown below.• Cl2(aq) + 2e– → 2Cl–(aq)

• Fe2O3(s) + 10OH–(aq) → 2FeO42–(aq) + 5H2O(I) + 6e–

• Construct the redox equation for the reaction between chlorine, iron(III) oxide and hydroxide ions.

• [Total 2 marks]

Self marking

• 2. Fe2O3 + 3Cl2 + 10OH– → 2FeO42– + 5H2O + 6Cl– (2) 2

• Allow one mark if electrons shownAllow one mark if correct reactants and products but not balanced

• [2]•

Complete the half-equationNa Na+ Na Na++ e-

Complete the half-equationPb4+ Pb2+ Pb4+ + 2e- Pb2+

Complete the half-equationH2 H+ H2 2H+ + 2e-

Complete the half-equationCr2O7

2- Cr3+ Cr2O72- + 6e- 2Cr3+

What’s wrong with this equation?Ce3+ + e- Ce4+ Electron is on the wrong side!

What’s wrong with this equation?Mg + 2H+ + e- Mg+ + H2 + e-

Should be Mg2+ Electrons should be cancelled out