To illustrate Le Chatelier’s principle by demonstrating the effects of both concentration changes and temperature changes on the reaction

Theory

• When yellow iron III (Fe3+)ions combine with colourless thiocyanate (CNS-) ions, an equilibrium reaction is set up forming red ferrothiocyanate Fe(CNS)2+ ion as follows:

• The effect of a) concentration b) temperature on this reaction are examined

Equation

Fe3+ + CNS– → Fe(CNS)2+

Fe3+: yellow

CNS: colourless

Fe(CNS)2+: red

Procedure: i) effect on concentration

1. Using a dropper, add one drop of potassium thiocyanate solution to iron (III) chloride solution in a test tube

Initially

- A red colour forms due to formation of Fe(CNS)+2

- Equilibrium lies on right hand side of equation

2. Add dilute hydrochloric acid (HCl) – source of Cl– ions

Result

- Concentration of Fe3+ decreases - they react with Cl– ions to form iron chloride (FeCl3)

- Equilibrium shifts to increase concentration of Fe3+ to PRODUCE MORE of it

- Reverse reaction favoured to the left

- Yellow colour forms due to formation of Fe3+

3. Add potassium thiocyanate (KCNS) – source of CNS- ions

Result

- Concentration of CNS– increases

- Equilibrium shifts to decrease concentration of CNS– to USE IT UP

- Forward reaction favoured to the right

- Red colour forms due to formation of Fe(CNS)+2

Procedure: ii) effect on temperature

1. Using a dropper, add one drop of potassium thiocyanate solution to iron (III) chloride solution in a test tube

Initially

- A red colour forms due to formation of Fe(CNS)+2

- Equilibrium lies on right hand side of equation

2. Add some solution to a new test tube in a beaker of hot water

Result

- Temperature increases

- Equilibrium shifts to decrease temperature in the ENDOTHERMIC DIRECTION TO USE UP HEAT

- Reverse reaction favoured to the left

- Yellow colour forms due to formation of Fe3+

3. Add some solution to a new test tube in a beaker of ice water

Result

• Temperature decreases

• Equilibrium shifts to increase temperature in the EXOTHERMIC DIRECRTION TO PRODUCE HEAT

• Forward reaction favoured to the right

• Red colour forms due to formation of Fe(CNS)+

What is the significance of the initial red colour in this equilibrium mixture?

The equilibrium lies on the right i.e. the concentration of the products is greater than the concentration of the reactants

Explain why changing the pressure has no effect on the equilibrium in this reaction

• All of the reactants and products are in solution (liquids)

• Pressure only has an effect on equilibrium when the reactants and products are gases