RP7: A

Measuring the rate of reaction by an initial rate method.

List the materials you need for this reaction.

• Dilute sulfuric acid.

• Potassium iodide solution.

• Sodium thiosulfate solution.

• Hydrogen peroxide solution.

• Starch solution.

• Distilled water.

List the equipment you need for this reaction.

• Burette.

• White tile.

• Funnel.

• Stand with burette clamp.

• Pipette.

• Stop clock.

• Measuring cylinder.

• Small beaker.

• Large beaker.

• Glass rod.

What is a clock reaction?

Reactions in which two solutions are mixed, with no change for a bit, but after a time delay give a sudden observable change.

Give the balanced symbol equation for the reaction between hydrogen peroxide and iodide ions.

H₂O_{2 (aq)} + 2H⁺_(aq) + 2I^-_(aq) → I_{2 (aq)} + 2H₂O _(l)

Give the balanced symbol equation for the reaction between thiosulfate ions and iodine.

2S₂O₃^2-_(aq) + I_{2 (aq)} → 2I^- _(aq) + S₄O₆^2- _(aq)

What is the initial rate of reaction?

The rate of reaction when time is 0.

Why is initial rates used to determine the order of reactants?

You know the concentrations of all the reactants at 0 time (before any of them have reacted).

What is the sudden observable change in the iodine clock reaction?

Colour change from colourless to blue-black.

What does it mean when the reaction mixture turns blue-black in the iodine clock reaction?

It means that all the thiosulfate ions have reacted.

Give the main source of error in the procedure for this reaction.

Addition of starch increases the total volume of the mixture slightly, which can alter known concentrations.

What is the purpose of the white tile?

You place it under the conical flask so you can more clearly see the colourless to blue-black change (so more easily determine the end point).

How can you decrease uncertainty in measurements?

Use more precise measuring instruments.

Why is starch added to the reaction mixture?

Starch reacts with excess iodine to form the blue-black solution, this gives us the very clear colour change.

Why do you need to stir the reaction mixture immediately after starting the stopclock?

To evenly mix all of the reactants so that they will properly react.