CAIE A-Level Chemistry 28.5 Stability Constants, Kstab

Practise defining Kstab, writing formation expressions and using stability data in equilibrium calculations.

Exam points

write the complex-formation equilibrium before constructing its Kstab expression and concentration powers
interpret a larger Kstab as stronger equilibrium favouring of that complex under comparable conditions
combine or rearrange stability constants to calculate concentrations or ligand-exchange equilibrium constants

Covered syllabus points

28.5.1—Stability constant, Kstab, of a complex 28.5.2—Write an expression for a Kstab of a complex 28.5.3—Kstab expressions to perform calculations 28.5.4—Ligand exchanges

[Maximum number: 2]

(a)

（f）Table 1.1 shows values for the stability constants， $K_{\text {stab }}$ ，of some silver（I）complexes．

Table 1.1

[ 2 ]

(i)

（i）Define the stability constant of a complex．

[ 1 ]

(ii)

（ii）Use the information in Table 1.1 to identify the most stable silver（I）complex．
Explain your answer．
most stable
explanation

[ 1 ]

[Ag(CN)2]-(aq) AND largest (value of) $K_{\text {stab }}$

[Maximum number: 2]

Copper is a transition element and has atomic number 29.

(a)

The following equilibrium exists between two complex ions of copper in the +2 oxidation state.

\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons\left[\mathrm{CuCl}_{4}\right]^{2-}+6 \mathrm{H}_{2} \mathrm{O}

[ 1 ]

(i)

Write the expression for the stability constant, $K_{\text {stab }}$ , for this equilibrium.

K_{\text {stab }}=

[ 1 ]

$K_{\text {stab }}=\left[\mathrm{CuCl}_{4}{ }^{2-}\right] /\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}{ }^{2+}\right][\mathrm{C} /]^{4}$

1

1

(b)

Copper also forms the complex ions $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}$ and $\left[\mathrm{Cu}(e n)\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}$ where en is the bidentate ligand ethane-1,2-diamine, $\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}$ .

\begin{gathered} {\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+2 \mathrm{NH}_{3} \rightleftharpoons\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}+2 \mathrm{H}_{2} \mathrm{O}} \\ {\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+e n \rightleftharpoons\left[\mathrm{Cu}(e n)\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}+2 \mathrm{H}_{2} \mathrm{O}} \end{gathered}

equilibrium 1
equilibrium 2

[ 1 ]

(i)

The table lists the values of stability constants for these two complexes.

Table for Question (i) — CAIE A-Level Chemistry A2

What do these $K_{\text {stab }}$ values tell us about the relative positions of equilibria 1 and 2?

[ 1 ]

[Maximum number: 5]

An aqueous solution of copper(II) sulfate is a blue colour due to the presence of $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ complex ions.

(a)

The numerical value of the stability constant, $K_{\text {stab }}$ , of the $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$ complex ion is $1.40 \times 10^{13}$ .

[ 5 ]

(i)

Define stability constant.

[ 1 ]

(ii)

Compare the stabilities of the $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ and $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$ complex ions. Explain your answer.

[ 1 ]

$\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$ AND as $K_{\text {stab }}$ is large OWTTE

(iii)

Write an expression for the stability constant, $K_{\text {stab }}$ , of the $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$ complex ion. State the units of the stability constant.
$K_{\text {stab }}=$
units =

[ 2 ]

$\begin{aligned} K_{\text {stab }}=\left[\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}\right] /\left[\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\right]\left[\mathrm{NH}_{3}\right]^{4}[1] \mathrm{mol}^{-4} \mathrm{dm}^{12}[1] \text { ecf M1 } \end{aligned}$

(iv)

In a particular solution the concentration of the $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$ complex ion is $0.0074 \mathrm{~mol} \mathrm{dm}{ }^{-3}$ and the concentration of $\mathrm{NH}_{3}$ is $0.57 \mathrm{~mol} \mathrm{dm}^{-3}$ .

Use your expression in (c)(iii) and the $K_{\text {stab }}$ value $1.40 \times 10^{13}$ to calculate the concentration of the $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ complex ion in this solution.
concentration of $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}=$ $\mathrm{mol} \mathrm{dm}^{-3}$

[ 1 ]

$\begin{aligned} \left.\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\right]=(0.0074) \div\left(1.4 \times 10^{13} \times 0.57^{4}\right) \left.\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\right]=\mathbf{5 . 0 1} \times \mathbf{1 0}^{-\mathbf{1 5}} \text { min } 2 \text { sf ecf } \mathbf{6}(\mathbf{c})(\text { iii }) \end{aligned}$

All question bank results loaded

Covered syllabus points

28.5 Stability constants, K stab question 1

Question (a)

Question (i)

Question (ii)

28.5 Stability constants, K stab question 2

Question (a)

Question (i)

Question (b)

Question (i)

28.5 Stability constants, K stab question 3

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)