Aim: To show the change in colour of universal indicator as the pH changes in a chemical reaction.

Materials:

• 2 x 500mL beakers
• 250mL measuring cylinder
• Glass stirring rod
• White cardboard (background)
• 0.5g Na₂S₂O₃.5H₂O
• 18.5g KI
• 6mL of 0.1M ethanoic acid
• Universal indicator
• 5ml 20% H₂O₂

Method:
Prepare:

• 500mL solution containing 0.5g Na₂S₂O₃.5H₂O, 18.5g KI and 3mL of 0.1M ethanoic acid. Add enough universal indicator to get a strong red colour and let settle for 6 hours.
• 500mL solution containing 3mL of 0.1M ethanoic acid and 5mL of 20% H₂O₂. Add enough universal indicator to produce a strong red colour.

For demo:

• Pour equal volumes of the two solutions above and observe the colour changes.

Reactions:

H₂O₂(aq) + 2I^-(aq) --> 2OH^-(aq) + I₂(aq)

I₂(aq) + 2S₂O₃^2-(aq)--> 2I^-(aq) + S₄O₆^2-(aq)

As the OH^- is formed it reacts with the acetic acid, which causes the indicator to change colour.
CH₃COOH(aq) + OH^-(aq) --> CH₃COO^-(aq) + H₂O(l)

Using this transition from red to purple the relationship of temperature with speed of reaction can be investigated. Consider the two reaction mixtures, shown on the right.

Which one is at room temperature and which one is at 5 ^oC? Explain why.