Topic 3: Redox

Question 1

define Oxidation number

Answer 1

the oxidation number of an atom shows the number of electrons which it has lost or gained as a result of forming a compound.

Question 2

what formula will the chlorate (V) ion have

Answer 2

ClO₃⁻. in anions that end in -ate the quantity of oxygen will b the umber of combined oxygen atoms to give the anion the smallest negative charge

Question 3

Do oxidising agents gain or lose electrons

Answer 3

oxidising agents gain electrons.

oxidising agent is reduced.

Question 4

Do reducing agents gain or lose electrons

Answer 4

reducing agents lose electrons.

reducing agent is oxidised

Question 5

what is a disproportionation reaction

Answer 5

when a species undergoes simultaneous oxidation and reduction

Question 6

steps to write a half equation:

Answer 6

balance for key atoms
balance for electrons
balance for hydrogen and oxygen by adding H⁺ and H₂O (acidic conditions)
H₂O and OH⁻ (alkali conditions)

Question 7

write the half equation of the conversion of hydrogen peroxide, H₂O₂, to oxygen, O₂

Answer 7

H₂O₂ -> O₂ + 2H⁺ + 2e⁻

Question 8

find the formula of chromate (VI)

Answer 8

CrO₄^2-

Question 9

The chlorate(V) anion has the formula ClO₃⁻.
write the formula of the following ions:
a) Chlorate (I)
b) Chlorate(VII)
C) Name the compound whose formula is Ba(ClO₂)₂

Answer 9

a) Chlorate (I) ClO⁻
b) Chlorate (VII) ClO₄⁻

c) Barium Chlorate (III)

Question 10

assign the oxidation numbers of oxygen in the following potassium oxides

a) Potassium peroxide (K₂O₂)
b) Potassium superoxide (KO₂)

Answer 10

a) -1

b) -0.5

Question 11

1. Iron(II) can be oxidised by some substances, including bromine, to iron(III) according to the following half-equation: Fe2+ → Fe3+ + e—
   a. Write a half-equation representing what happens to bromine (Br2) when it is used to oxidise iron(II). [2]
   b. Combine both half-equations to form a chemical equation for the oxidation of iron(II) by bromine. [2]
   c. Identify, giving a reason, the following species in the reaction:
   i. The reducing agent. [2]
   ii. The species whose oxidation number decreases. [2]
   d. Iron(III) can be reduced back to iron(II) by iodide ions. Use appropriate half-equations to create a chemical equation for this redox reaction. [4]

Answer 11

1. a. Br2 + 2e— → 2Br— Br2 and 2Br— [1] 2e— [1].
   b. 2Fe2+ + Br2 → 2Fe3+ + 2Br— 2Fe2+ and 2Fe3+ [1] Br2 and 2Br— [1]
   c.
   i. Fe2+ / iron(II) [1] Loses electrons / increases oxidation number / gives electrons away [1]
   ii. Br2 / bromine [1] Oxidation number changes from 0 to -1 [1]
   d. 2Fe3+ + 2I— → 2Fe2+ + I2 Fe half-equation: Fe3+ + e— → Fe2+ [1] I— half-equation: 2I— → I2 + 2e— [1] Doubling of Fe half-equation for equal electrons on both sides [1] Correct addition of half-equations [1]

Question 12

in this reaction;
Cl2+ 2Br-→2Cl-+ Br2
what safety precautions should be taken

Answer 12

An answer that makes reference to the following points:
chlorine/bromine toxic/poisonous
(Carry out the experiment in a) fume cupboard
OR
bromine corrosive
wear gloves