Iron(III) Thiocyanate Equilibrium

This is part of the HSC Chemistry course under the topic Factors Affecting Equilibrium

HSC Chemistry Syllabus

Conduct practical investigations to analyse the reversibility of chemical reactions, for example:

– iron (III) nitrate and potassium thiocyanate

Investigate the effects of temperature, concentration, volume and/or pressure on a system at equilibrium and explain how Le Chatelier’s principle can be used to predict such effects, for example:

– iron(III) thiocyanate and varying concentration of ions (ACSCH095)

How does the Iron(III) Nitrate & Potassium Thiocyanate equilibrium work?

In this video, we will be exploring what the mechanisms of the iron(III) nitrate and potassium thiocyanate equilibrium are.

Iron(III) Nitrate Equilibrium

When iron (III) nitrate and potassium thiocyanate are mixed in solution, an equilibrium mixture of Fe³⁺, SCN^- and the complex ion FeSCN²⁺ is formed:

$${Fe{(NO_3)}_3}_{(aq)} + \; KSCN_{(aq)} \rightleftharpoons \; {FeSCN^{2+}}_{(aq)} + \; KNO_{3(aq)} \;\;\; \Delta{H}<0$$

Net ionic equation:

$${Fe^{3+}}_{(aq)} + \; {SCN^-}_{(aq)} \rightleftharpoons \; {FeSCN^{2+}}_{(aq)} \;\;\; \Delta{H}<0$$

Figure: a similar reaction that demonstrates the colour change of Fe³⁺’s conversion to Fe(SCN)²⁺

Fe³⁺ is yellow and FeSCN²⁺ is blood red.

Potassium ion and nitrate ion are spectator ions and are not part of the chemical equilibrium.

Pressure and volume do not affect this reaction as there are no gaseous reactants nor products.

Concentration

Tube A is a reference tube – used for comparison purposes.

Tube B: Addition of Na₂HPO₄: hydrogen phosphate ions react with Fe³⁺ to form complex ions:

$${Fe^{3+}}_{(aq)} + \; {{HPO_4}^{2-}}_{(aq)} \rightarrow \; {{FeHPO_4}^+}_{(aq)}$$

This reduces [Fe³⁺] which in turn increases the rate of backward reaction and shifts the chemical equilibrium to the left. Solution turns less blood red, paler yellow or colourless.

Tube C: Addition of KSCN crystals increases concentration of reactants of forward reaction. This increases the rate of forward reaction and shifts the equilibrium to the right. Solution turns more blood red.

Tube D: Addition of Fe(NO₃)₃ increases the concentration of reactants available for the forward reaction. This will initially make the solution more yellow as [Fe³⁺] increases. However, by Le Chatelier’s principle, this change in turn increases the rate of forward reaction and shifts the equilibrium to the right. Solution turns more blood red.

Addition of SnCl₂ (or any form of Sn²⁺) reduces Fe³⁺ to Fe²⁺. Reduction of Fe³⁺ is possible because Sn²⁺is a stronger reductant.

$$Sn^{2+} + 2Fe^{3+} \rightarrow Sn^{4+} + 2Fe^{2+} $$

Reduction decreases [Fe³⁺] which in turn increases the rate of reverse reaction and shifts the chemical equilibrium to the left. Solution turns less red and more yellow.

Figure: colour analysis of Fe³⁺, SCN^- and the complex ion FeSCN²⁺ equilibrium.

Adding silver nitrate (AgNO₃): Ag⁺ forms white precipitate with SCN^- which decreases [SCN^-]:

$${Ag^+}_{(aq)} + {SCN^-}_{(aq)} \rightarrow AgSCN_{(s)}$$

This increases the rate of backward reaction and shifts the chemical equilibrium to the left. Solution turns cloudy white. Precipitate will settle to the bottom of the tube if left undisturbed for a long time.

Temperature

The formation of iron (III) thiocyanate complex ion is an exothermic reaction. This means the forward reaction is favoured at a low temperature.

Figure: effect of temperature on the dynamic equilibrium of iron thiocyanate solution.

When the solution is placed in ice-cold water, more iron thiocyanate ion is formed. This causes the solution to appear more blood red.

As the temperature of water increases, the reverse reaction becomes more favoured. This causes the solution to appear less blood red and start to adopt a yellow colour.

Previous section: Practical Investigations of Le Chatelier's Principle (Cobalt (II) Chloride Equilibrium)

Next section: Practical Investigations of Le Chatelier's Principle (Nitrogen Dioxide and Dinitrogen Tetroxide Equilibrium)