AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
OCR (A) A-Level
YouTube Channel
YouTube Channel
@chemistry.student
*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

Required Practicals

1 Moles determination 2 Acid–base titration 3 Enthalpy determination 4 Qualitative analysis of ions 5 Synthesis of an organic liquid 6 Synthesis of an organic solid 7 Qualitative analysis of organic functional groups 8 Electrochemical cells 9 Rates of reaction – continuous monitoring method 10 Rates of reaction – initial rates method 11 pH measurement 12 Research skills

Core Practical 10: Initial Rates of Reaction (Iodine Clock Method)

Aim: To investigate how the rate of reaction between H2O2 and KI depends on the concentration of potassium iodide (KI) using the iodine clock method, and to determine the order of reaction and the rate constant.

Apparatus

Chemicals

Method

Iodine clock method setup with conical flask, reagents, and stopwatch.
  1. Add to a conical flask:
    • 5 cm³ potassium iodide solution
    • 2 cm³ sodium thiosulfate solution
    • 1 cm³ starch solution

    Make up with distilled water to keep the total volume constant if required.

  2. Quickly add 2 cm³ hydrogen peroxide (H₂O₂) to the flask and simultaneously start the stopwatch.
  3. Observe the flask. As iodine (I₂) is produced, it is reduced back to I⁻ by thiosulfate. When all thiosulfate has been used, iodine reacts with starch to give a blue-black colour.
  4. Stop the timer when the blue-black colour appears.
  5. Repeat using different KI concentrations, keeping all other conditions constant. Maintain a constant total volume (e.g., 10 cm³).

Example Calculation

Assume the fixed amount of iodine formed is 2.00 × 10⁻³ mol.
Time for blue-black colour to appear = 32.5 s.

Initial rate = amount ÷ time
Initial rate = (2.00 × 10⁻³ mol) ÷ (32.5 s) = 6.15 × 10⁻⁵ mol dm⁻³ s⁻¹

Repeat for other trials, then plot:

Determining Order and Rate Constant

Graphs showing how to determine order of reaction by plotting rate vs concentration and log–log plots.

Common Sources of Error

Safety

Notes on the Iodine Clock Reaction

The blue-black colour appears only once all thiosulfate ions have been consumed. The time taken is an indirect but reliable measure of iodine production, making this method suitable for determining initial rates of reaction.