Rate Practicals: RP7 & the Iodine Clock (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about Required Practical 7 and the iodine clock reaction.

Both are ways of measuring how fast a reaction goes. Required Practical 7 covers an initial rate method and a continuous monitoring method, and the iodine clock times a sharp colour change to find the order of reaction with respect to a reactant.

Required Practical 7 is the practical on measuring the rate of a reaction, and it sets out two ways of doing it. The first is an initial rate method and the second is a continuous monitoring method. The iodine clock reaction is a separate experiment that does the same job by a different route, using a sudden colour change as the reaction timer and giving you the order of reaction for a reactant. What joins all of them is that each one turns something you can observe in the lab into a rate.

We'll start with Required Practical 7 and the two methods it names. Then we'll look at the two experiments the notes use for it, and the apparatus each one needs. Last, we'll go to the iodine clock reaction and what its results tell you about order.

Required Practical 7 is measuring the rate of a reaction. The notes give two ways to do it: an initial rate method, and a continuous monitoring method.

Whichever one you use, the finish is the same. You plot the results as a graph, draw a tangent, and the gradient of that tangent is the rate of reaction. For an initial rate the tangent is drawn at time equals zero. To find the rate part-way through a reaction, you draw the tangent at that point on the curve.

The notes work Required Practical 7 through two experiments. The first is the iodination of propanone, carried out with a catalyst of dilute sulfuric acid. The iodine decolourises as it turns into iodopropanone and hydrogen iodide, and a colorimeter measures colour absorbance, which is proportional to the concentration of the coloured species. Before you start, you need a calibration curve from standard solutions of iodine, so that colorimeter readings can be converted into concentrations.

The second is magnesium reacting with dilute hydrochloric acid, which gives off hydrogen. Hydrogen is too low in density for the mass change to register on a laboratory balance, so you measure the volume of gas instead, either by displacing water into an inverted measuring cylinder or by using a gas syringe.

In both, the concentration is what gets varied: in the propanone experiment by changing the volumes of the solutions while keeping the total volume constant with distilled water, and in the magnesium experiment by diluting the acid while the temperature is held constant.

The iodine clock reaction is a clock reaction, which means it shows a sharp, dramatic colour change after a period of time has elapsed. That makes it ideal for studying kinetics. The version in the notes uses hydrogen peroxide, iodide ions and acid, which react together to produce iodine.

Sodium thiosulfate is added to the mixture and uses up the iodine as it forms, and that is what acts as the reaction timer. The amounts are chosen so that the iodine produced ends up in excess compared with the other reagents, and as soon as it is in excess, the blue-black colour of iodine in starch appears and the timer is stopped.

To turn those times into rates you take the reciprocal, so the rate is one over the time. Plotting rate against the concentration of potassium iodide gives a straight line: as the concentration doubles, the rate of reaction also doubles. That tells you the reaction is first order with respect to potassium iodide, which is the same job Required Practical 7 does, reached through a colour change rather than a tangent.

Whichever rate experiment you carry out, the examiners want you to be able to describe the steps in the procedure and name all the apparatus used. Your data tables need headings and units, and your graphs need labels, units and best-fit lines. They also expect you to be able to determine an initial gradient, or the gradient at any point on the curve.

Required Practical 7 is measuring the rate of a reaction, by an initial rate method or by a continuous monitoring method.

The notes use the examples of the iodination of propanone followed with a colorimeter, and magnesium and hydrochloric acid with the hydrogen collected in a gas syringe. In both, the rate comes from the gradient of a tangent.

The iodine clock reaction reaches a rate a different way by timing a sudden blue-black colour change and taking the reciprocal. Its rate-concentration graph shows the reaction is first order with respect to potassium iodide.

All of them are the same task: turning something you can measure in the lab into a rate of reaction.