Chemistry 9701 · AS & A Level · States of matter

States of matter — practice question

By passing air through separate aqueous mixtures of CoCl$_2$, NH$_4$Cl and NH$_3$, a range of complex ions is formed, with the general formula $[\text{Co(NH}_3)_6{}_{-n}\text{Cl}_n]^{3-n}$.
(a(i))[1]

Find the oxidation state of the cobalt in these complex ions.

(a(ii))[2]

State the two reaction types involved when the cobalt ions form these complex ions.

(a(iii))[3]

The complex $[\text{Co(NH}_3)_4\text{Cl}_2]^+$ exhibits isomerism. Draw the three-dimensional structures of the two isomers, and indicate the type of isomerism present.

(b(i))[1]

What does the term co-ordination number mean?

(b(ii))[6]

Complete the table by predicting suitable co-ordination numbers, formulae and charges for complexes C, D, E and F.

(c(i))[3]

Write the expressions for the stability constants, $K_{\text{stab}1}$ and $K_{\text{stab}2}$, for these two equilibria. Include units in your responses.

(c(ii))[1]

An equilibrium may exist between these two complexes, as shown in equilibrium 3. Write an expression for $K_{\text{eq}3}$ in terms of $K_{\text{stab}1}$ and $K_{\text{stab}2}$.

(c(iii))[2]

The stability constants are given as follows: $K_{\text{stab}1} = 1.4 \times 10^2$, $K_{\text{stab}2} = 8.0 \times 10^{-2}$. Calculate the value of $K_{\text{eq}3}$ and include its units.

Worked solution & mark scheme

This 19-mark question has a full step-by-step worked solution and mark scheme. One marking point: Oxidation state $+3$ or $\text{Co}^{3+}$

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