TM: Oxidation States + Titrations

Question 1

What type of reaction occurs when a transition metal changes oxidation states?

Answer 1

redox

Question 2

State the four different ions of vanadium, their oxidation state and their colour.

Answer 2

V²⁺ → 2+ (violet)
V³⁺ → 3+ (green)
VO²⁺ → 4+ (blue)
VO₂⁺ → 5+ (yellow)

Question 3

How is vanadium (V) ions (VO₂⁺) reduced?

Answer 3

using zinc in an acidic solution

Question 4

State the colour change when vanadium (v) is changed to (iv)

Answer 4

yellow to blue

Question 5

State the colour change when vanadium (iv) is changed to (iii)

Answer 5

blue to green

Question 6

State the colour change when vanadium (iii) is changed to (ii)

Answer 6

green to violet

Question 7

Which ions have the largest redox potential?

Answer 7

least stable ions

Question 8

What does the redox potential of a transition metal depend on?

Answer 8

• ligand type
• pH (more acidic = larger potential)

Question 9

What is meant by redox potential?

Answer 9

a value that predicts how easily an atom can be reduced to a lower oxidation state

Question 10

State which type of solution is required for the reduction and oxidation of a transition metal to take place.

Answer 10

reduction → acidic solution
oxidation → alkaline solution

Question 11

In terms of reactions and ions, explain what happens during a Tollen’s test.

Answer 11

the Ag⁺ in [Ag(NH₃)₂]⁺ is reduced to Ag metal by the aldehyde

Question 12

Give the equation for the reduction of manganate (vii) ions in a redox titration.

Answer 12

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Question 13

What reagent is used for manganate titrations and why?

Answer 13

→ dilute sulfuric acid
→ provides sufficient H+ ions to allow the reduction of oxidising agent

Question 14

State and explain what is observed when there is an insufficient volume of sulfuric acid during a manganate titration.

Answer 14

→ solution is not acidic enough
→ MnO₂ will be produced instead of Mn²⁺
→ which has a brown colour that will mask the colour change (so more manganate is used)

Question 15

Explain why you cannot use concentrated HCL for a titration between Fe²⁺ with MnO₄⁻.

Answer 15

→ Cl⁻ ions would be oxidised to Cl₂ by MnO₄
→ leads to greater volume of manganate being used

Question 16

Explain why you cannot use ethanoic acid for a titration between Fe²⁺ with MnO₄⁻.

Answer 16

→ too weak (does not provide enough H+ ions)

Question 17

Give the half equation for the oxidation of Fe during its titration with MnO₄⁻ ions.

Answer 17

Fe²⁺ (aq) → Fe³⁺ (aq) + e⁻

Question 18

State the colour change during a titration of Fe²⁺ with MnO₄⁻ (if MnO₄⁻ is in the burette).

Answer 18

colourless to purple/pink

Question 19

Explain why you cannot use nitric acid for a titration between Fe²⁺ with MnO₄⁻.

Answer 19

→ it is also an oxidising agent
→ oxidises Fe²⁺ to Fe³⁺
→ leads to smaller volume of manganate being used

Question 20

Give the half equations for the titration between manganate (vii) and ethanedioate ions.

Answer 20

C₂O₄²⁻ → 2CO₂ + 2e⁻
MnO₄⁻ + 8H⁺ +5e⁻ → Mn²⁺ + 4H₂O

Question 21

What reaction conditions are needed for the titration between manganate (vii) and ethanedioate ions and why?

Answer 21

→ solution needs to be heated to about 60°C
→ initial reaction has high activation energy as both ions are negative

Question 22

Write an equation between iron and dilute sulphuric acid.

Answer 22

Fe + H₂SO₄ → FeSO₄ + H₂