Question

Please explain fully very confused

2a. What is the coefficient of the permanganate ion when the following equation is correctly balanced? MnO4− + Br− → Mn2+ + Br2 (acidic solution) (a) 1 (b) 2 (c) 3 (d) 5

2b. What is the coefficient of H2O when the following equation is correctly balanced?
ClO− + S2O32− → Cl− + SO42− (basic solution) (a) 1 (b) 2 (c) 3 (d) 4

2c. Which substance is serving as the reducing agent in the following reaction?
Fe2S3 + 12HNO3 → 2Fe(NO3)3 + 3S + 6NO2 + 6H2O (a) HNO3 (b) S (c) NO2 (d) Fe2S3

Answer 1

2a)

Mn in MnO4- has oxidation state of +7

Mn in Mn+2 has oxidation state of +2

So, Mn in MnO4- is reduced to Mn+2

Br in Br- has oxidation state of -1

Br in Br2 has oxidation state of 0

So, Br in Br- is oxidised to Br2

Reduction half cell:

1 MnO4- + 5e- --> 1 Mn+2

Oxidation half cell:

2 Br- --> 1 Br2 + 2e-

Balance number of electrons to be same in both half reactions

Reduction half cell:

2 MnO4- + 10e- --> 2 Mn+2

Oxidation half cell:

10 Br- --> 5 Br2 + 10e-

Lets combine both the reactions.

2 MnO4- + 10 Br- --> 2 Mn+2 + 5 Br2

Balance Oxygen by adding water

2 MnO4- + 10 Br- --> 2 Mn+2 + 5 Br2 + 8 H2O

Balance Hydrogen by adding H+

2 MnO4- + 10 Br- + 16 H+ --> 2 Mn+2 + 5 Br2 + 8 H2O

This is balanced chemical equation in acidic medium

Answer: 2

2b)

Cl in ClO- has oxidation state of +1

Cl in Cl- has oxidation state of -1

So, Cl in ClO- is reduced to Cl-

S in S2O3-2 has oxidation state of +2

S in SO4-2 has oxidation state of +6

So, S in S2O3-2 is oxidised to SO4-2

Reduction half cell:

1 ClO- + 2e- --> 1 Cl-

Oxidation half cell:

1 S2O3-2 --> 2 SO4-2 + 8e-

Balance number of electrons to be same in both half reactions

Reduction half cell:

4 ClO- + 8e- --> 4 Cl-

Oxidation half cell:

1 S2O3-2 --> 2 SO4-2 + 8e-

Lets combine both the reactions.

4 ClO- + 1 S2O3-2 --> 4 Cl- + 2 SO4-2

Balance Oxygen by adding water

4 ClO- + 1 S2O3-2 + 1 H2O --> 4 Cl- + 2 SO4-2

Balance Hydrogen by adding H+

4 ClO- + 1 S2O3-2 + 1 H2O --> 4 Cl- + 2 SO4-2 + 2 H+

Add equal number of OH- on both sides as the number of H+

4 ClO- + 1 S2O3-2 + 1 H2O + 2 OH- --> 4 Cl- + 2 SO4-2 + 2 H+ + 2 OH-

Combine H+ and OH- to form water

4 ClO- + 1 S2O3-2 + 1 H2O + 2 OH- --> 4 Cl- + 2 SO4-2 + 2 H2O

Remove common H2O from both sides

Balanced Eqn is

4 ClO- + 1 S2O3-2 + 2 OH- --> 4 Cl- + 2 SO4-2 + 1 H2O

This is balanced chemical equation in basic medium

Answer: 1

2c)

S in Fe2S3 has oxidation state of -2

S in S has oxidation state of 0

So, S in Fe2S3 is oxidised to S

Hence Fe2S3 is reducing agent

Answer: Fe2S3