Chapter 24 - continued

Question 1

What is redox reaction involving the oxidation of iron and what are the colour changes?

Answer 1

a. The redox reaction occurs in acidic conditions.
b. Fe2+ is oxidised to Fe3+
c. MnO4- is reduced to Mn2+
d. The solution containing MnO4- ions is purple and is decolourised to form a colourless solution containing Mn2+ ions.

Question 2

What is the redox reaction involving the reduction of iron and what are the colour changes?

Answer 2

a. Fe3+ is reduced to Fe2+
b. I- is oxidised to I2
c. The equation is, 2Fe3+ + 2I- => 2Fe2+ + I2.
d. The orange brown Fe3+ ions are reduced to pale green Fe2+ ions. However, this colour change is obscured by the oxidation of iodide ions to form iodine, which has a brown colour.

Question 3

What is the redox reaction involving the reduction of chromium and what are the colour changes?

Answer 3

a. The redox reaction occurs in acidic conditions.
b. Cr2O72- is reduced to Cr3+
c. Zn is oxidised to Zn2+
d. Colour change is orange (Cr2O72-) to green (Cr3+).

Question 4

What happens when an excess of zinc is added to the redox reaction involving the reduction of chromium ions?

Answer 4

With an excess of zinc, chromium (III) ions (green) are reduced further to chromium (II), which is a pale blue colour.

Question 5

What is the redox reaction involving the oxidation of chromium and what are the colour changes?

Answer 5

a. The redox reaction occurs in alkaline conditions.
b. Cr3+ is oxidised to CrO42-
c. Oxygen is reduced from H2O2 to CrO42-
d. The colour change is green (Cr3+) to yellow (CrO42-).
e. Hot alkaline H2O2 is a powerful oxidising agent.

Question 6

What is the redox reaction involving the reduction of copper and what are the colour changes?

Answer 6

a. When aqueous copper (II) ions, Cu2+, react with excess iodide ions I-, a redox reaction occurs.
b. I- is oxidised to I2
c. Cu2+ is reduced to Cu+
d. The colour changes from pale blue in Cu2+ to a white precipitate of CuI and brown iodine I2.
e. The reaction is, 2Cu2+ + 4I- => 2CuI + I2.
f. The white precipitate of Copper iodide deposits at the bottom of the brown solution that forms. The white colour is somewhat masked by the brown iodine colour.

Question 7

What is the disproportionation reaction of Cu+ ions?

Answer 7

a. When solid copper (I) oxide, Cu2O, reacts with hot dilute sulfuric acid, a brown precipitate of copper forms with a blue solution of copper (II) sulfate.
b. The reaction is, Cu2O(s) + H2SO4(aq) => Cu(s) + CuSO4(aq) + H2O(l).
c. Cu is a brown solid.
d. CuSO4 is a blue solution.