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Initial Rates Method - Clock Reactions

Dr. Davinder Bhachu

Teacher

Dr. Davinder Bhachu

Contents

Introduction to the Initial Rates Method

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Iodine Clock Reaction - Key Reactions

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Determining Reaction Order Using the Initial Rates Method

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Example Data & Calculation

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Initial Rates Clock Reactions Questions

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Introduction to the Initial Rates Method

  • The initial rates method is used to determine the order of reaction with respect to a reactant.

  • The clock reaction is a practical way to measure the initial rate by timing how long it takes for a sudden visible change to occur.

  • The iodine clock reaction is commonly used, where a fixed amount of thiosulfate and starch are added to a reaction mixture.

  • When all the thiosulfate is consumed, iodine () reacts with starch to form a blue-black complex, indicating the reaction has reached a specific point.

Iodine Clock Reaction - Key Reactions

Step 1: Formation of Iodine

Hydrogen peroxide reacts with iodide ions:

Step 2: Reaction with Sodium Thiosulfate

Initially, iodine is reduced back to iodide by sodium thiosulfate: This delays the colour change.

Step 3: Starch Complex Formation

Once the thiosulfate is exhausted, free  reacts with starch, forming a blue-black complex.

Determining Reaction Order Using the Initial Rates Method

To determine the order with respect to the concentration of a specific species, the experiment is carried out multiple times using different concentrations of that species while keeping the concentrations of all others constant.

Practical details are covered in the RP7 – Iodine clock note.

To find the order with respect to

Diagram of an iodine clock experiment showing timing from mixing colourless solutions until a sudden blue-black colour change appears, indicating the reaction endpoint.

1. Measure fixed volumes of hydrogen peroxide, sulfuric acid, potassium iodide, sodium thiosulfate into separate containers

2. Combine all solutions and a few drops of starch, adding the hydrogen peroxide last. Start the timer.

3. Measure the time (t) for the appearance of the blue-black colour

4. The reaction is repeated with different concentrations of iodide while keeping other reactant volumes and concentrations constant.

5. The initial rate is approximated as 1/t

6. A graph of 1/t vs. [I⁻] is plotted to determine the order with respect to I⁻.

  • Horizontal line → Zero order

  • Straight line through the origin → First order

  • Curve of form → Second order

Graphs comparing reaction orders showing concentration vs time: zero order as a horizontal line, first order as a straight decreasing line, and second order as a curved decreasing line.

Example Data & Calculation

Example Data for Varying Concentrations

Experiment

 

Time for Color Change

Graph of iodide concentration versus initial rate (1/t) showing a straight-line relationship through the origin, indicating first-order dependence on [I⁻].

Determining the Order of Reaction

  • The shape of the graph above (straight line through the origin) indicates a first order relationship.

  • The order could also be determined directly from the data, when [I⁻] doubles , 1/t doubles .

  • Since the initial rate is proportional to [I⁻], the reaction is first order with respect to I⁻.

Initial Rates Clock Reactions Questions

Key Tips and Reminders

  • The iodine clock reaction is a useful method for determining reaction order via initial rates.

  • By analysing how 1/t changes with concentration, we can establish the reaction order efficiently.


Exam Tip: Ensure that temperature and volumes of other reactants remain constant to get accurate results!

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