Important Compounds of the d-Block: Quick Revision

A brief note on Important Compounds of the d-Block. Read it before a test. Try each question at the end before you open its answer.

Oxides

• For one metal, a higher oxidation state gives a more acidic oxide: basic at $+2$, amphoteric in the middle, acidic at the top.
• Chromium: $\text{CrO}$ is basic, $\text{Cr}_2\text{O}_3$ is amphoteric, $\text{CrO}_3$ is acidic. Manganese: $\text{MnO}$ is basic, $\text{Mn}_2\text{O}_7$ is acidic and gives $\text{HMnO}_4$ with water.
• Oxygen brings out the highest states, because it can form double bonds with the metal.

Potassium dichromate

• $2\text{CrO}_4^{2-} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}$. Acid gives orange, alkali gives yellow, and chromium stays $+6$.
• Made from chromite: roast with sodium carbonate and air, acidify the chromate, then add potassium chloride.
• In acid: $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$. Orange turns green.
• It is a primary standard. Cold dilute hydrochloric acid can be used with it.

Potassium permanganate

• Made from $\text{MnO}_2$: fuse with KOH and air to get green $\text{K}_2\text{MnO}_4$. Then oxidise by electrolysis in alkali. With acid, a third of the manganese is lost as $\text{MnO}_2$.
• Heated, $2\text{KMnO}_4 \rightarrow \text{K}_2\text{MnO}_4 + \text{MnO}_2 + \text{O}_2$.
• Permanganate is $d^0$ and diamagnetic. Manganate is $d^1$, with $\mu = 1.73\ \text{BM}$.
• Acid: to $\text{Mn}^{2+}$, 5 electrons. Neutral: to brown $\text{MnO}_2$, 3 electrons. Strong alkali: to green $\text{MnO}_4^{2-}$, 1 electron.
• Hydrochloric acid is never used with permanganate: the chloride is oxidised.

Catalysts, alloys and interstitial compounds

• Catalysts: several oxidation states ($\text{V}_2\text{O}_5$ goes between $+5$ and $+4$) and a surface that holds the reactants. A catalyst does not move the equilibrium.
• Alloys: a mixture of metals with similar atomic radii, differing by no more than 15 per cent. Brass is copper and zinc. Bronze is copper and tin.
• Interstitial compounds: small atoms (H, C, N) in the gaps of a metal lattice. Hard, high melting point, metallic conduction, inert and not stoichiometric.

A worked example

A $10.0\ \text{mL}$ portion of sodium nitrite solution, $0.0750\ \text{mol/L}$, is acidified and titrated with permanganate of concentration $0.0300\ \text{mol/L}$.

1. Nitrite. $0.0750 \times 10.0 = 0.750\ \text{mmol}$.
2. Permanganate. The ratio is 2 to 5, so $\dfrac{2}{5} \times 0.750 = 0.300\ \text{mmol}$.
3. Volume. $\dfrac{0.300\ \text{mmol}}{0.0300\ \text{mol/L}} = 10.0\ \text{mL}$.

Practice with new numbers

A program to try

The program finds the mass of each oxidising agent that takes up the same amount of electrons.

Mistakes to avoid

• Chromate to dichromate is not redox. Chromium is $+6$ on both sides.
• The n-factor of permanganate is 5, 3 or 1, by the medium.
• Catalysts speed up a reaction. They do not change the equilibrium and they are not used up.
• The size rule: divide by the smaller radius.

Check yourself

1. What colour change shows that dichromate has been reduced in acid, and which ion forms?

2. What is the n-factor of $\text{KMnO}_4$ when it forms brown $\text{MnO}_2$?

3. Why is hydrochloric acid not used in a permanganate titration?

4. Name the catalyst of the contact process and the two oxidation states of its metal.

5. Give two properties of an interstitial compound.