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Module 1: Practical chemistry for exams›PAG 10.3 - Rates – activation energy

PAG 10.3 - Rates – activation energy

An overview of PAG 10.3 - Rates – activation energy

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In OCR Chemistry Required Practical 10 (PAG 10.3), the rate of the reaction between potassium manganate(VII) solution and a solution of ethanedioic acid at different temperatures is investigated, by measuring the time taken for a light brown colour to be produced.

We can use this data with the Arrhenius equation to obtain a value for the activation energy, given that the rate equation is:

In OCR PAG 10.3: Rates – Activation energy, normal lab practice such as wearing goggles is adequate when carrying out the practical steps.

Experimental method for OCR Chemistry Required Practical 10 (PAG 10.3)

A step-by-step experimental method labeled 'OCR PAG 10.3' is illustrated. Step 1: Measure out the solutions. Using a burette, measure exactly 20 cm³ of 0.5 mol dm⁻³ ethanedioic acid and add to a 100 cm³ conical flask. Measure 10 cm³ of 0.02 mol dm⁻³ KMnO₄(aq) into a boiling tube. Images show burettes with labels '0.02 mol dm⁻³ KMnO₄(aq)' and '0.5 mol dm⁻³ H₂C₂O₄', and bottles labeled '0.02 M KMnO₄' and 'Ethanedioic acid'. Step 2: Set up a water bath at about 10°C (283 K) for 5 minutes to maintain a constant temperature. An image shows a water bath with a flask and boiling tube. Step 3: Draw up a results table with columns T/K, 1/T/K⁻¹, Time 1/s, Time 2/s, Mean time/s, Initial rate /mol dm⁻³ s⁻¹, k / dm³ mol⁻¹ s⁻¹, ln k. Temperatures listed are 273, 295, 302, 311, and 322 K. Step 4: Pour potassium manganate(VII) solution into the conical flask, swirl, and start the stopwatch. Stop when the solution changes from purple to brown. Record time and temperature. An image shows a stopwatch and color change. Step 5: Repeat the experiment to obtain a second reaction time, increasing the water bath temperature by 10 K/°C. Images show a repetition of pouring and color change. © Medify.

Clean, dry glassware should always be used in any experiment where concentration is important.

An illustration showing a graduated beaker with a blue stirring rod, a round-bottom flask, and a conical flask.

Do

Ensure glassware is clean and dry before use to prevent dilution or unwanted reactions.

Four laboratory glassware pieces, including two conical flasks, one round-bottom flask, and one beaker, all featuring orange stains.

Don't

Use wet or contaminated glassware – this can affect concentrations and skew results. Rinsing with distilled water directly prior to use is not appropriate here.

Accurately measuring time is essential in rate investigations.

A round yellow stopwatch displaying the time 00:00:20 on a green digital screen.

Do

Start timing immediately after mixing reagents to ensure an accurate measurement of the reaction rate.

A round yellow stopwatch displaying the word 'ERROR' in large green letters on its screen.

Don't

Stop the timer to take measurements. The reaction hasn’t stopped, so neither should the timer.

Calculate the initial rate of the reaction using:

Then calculate the rate constant using:

Calculate ln and ( must be in kelvin), and then plot a graph of ln ln against

The image is a graph with the x-axis labeled as '1/T' and the y-axis labeled as 'ln k'. The graph features a downward sloping blue line with several plotted blue markers along it. There are four visible blue markers approximately evenly spaced along the line. Red arrows and text labels indicate changes along the axes: 'Δy' is marked vertically and 'Δx' is marked horizontally. The line and markers represent a trend, but no specific numeric values or tick marks are visible on the axes. The graph is attributed to '© Medify' in the lower right corner.

From the gradient is so calculate the activation energy using: