Understanding the Iron(III) Thiocyanate Equilibrium System

Expert reviewed • 22 November 2024 • 5 minute read

The iron(III) thiocyanate equilibrium system represents a classic example of chemical equilibrium, demonstrating visible color changes as the system responds to various disturbances. This reaction is particularly significant in the HSC Chemistry curriculum for studying Le Chatelier's principle and equilibrium dynamics.

Chemical Equilibrium Formation

When solutions of iron(III) nitrate and potassium thiocyanate are combined, they establish a dynamic equilibrium. The reaction can be represented by the following equations:

Overall equation: $Fe(NO_3)_3(aq) + KSCN(aq) \rightleftharpoons FeSCN^{2+}(aq) + KNO_3(aq) \quad \\\Delta H < 0$

Net ionic equation: $Fe^{3+}(aq) + SCN^-(aq) \rightleftharpoons FeSCN^{2+}(aq) \quad \Delta H < 0$

The system is characterized by distinct colors:

Fe³⁺ ions appear yellow in solution
FeSCN²⁺ complex appears blood red
K⁺ and NO₃⁻ are spectator ions (colorless)

Effects of Concentration Changes

Different chemical additions can shift the equilibrium position:

1. Addition of Na₂HPO₄

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

Removes Fe³⁺ ions from the system
Shifts equilibrium left
Solution becomes paler (less red)

2. Addition of KSCN

Increases [SCN⁻]
Shifts equilibrium right
Solution becomes deeper red

3. Addition of Fe(NO₃)₃

Increases [Fe³⁺]
Shifts equilibrium right
Solution becomes more intensely red

4. Addition of SnCl₂

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

Reduces Fe³⁺ to Fe²⁺
Shifts equilibrium left
Solution becomes more yellow

5. Addition of AgNO₃

$Ag^+(aq) + SCN^-(aq) \rightarrow AgSCN(s)$

Forms white AgSCN precipitate
Removes SCN⁻ from solution
Shifts equilibrium left
Solution becomes cloudy white

Temperature Effects

The formation of FeSCN²⁺ is exothermic (ΔH < 0). Therefore:

Low temperature favors the forward reaction (more red color)
High temperature favors the reverse reaction (more yellow color)

Key Points

The equilibrium is not affected by pressure or volume changes as no gases are involved
All shifts in equilibrium can be predicted using Le Chatelier's principle
The color changes provide a visual indicator of equilibrium shifts

Understanding Color Changes in Cobalt(II) Chloride Equilibrium

Understanding the NO₂/N₂O₄ Equilibrium System

Return to Module 5: Equilibrium and Acid Reactions