Rates of Reaction Required Practicals

Explained

The rates practicals, and choosing what to measure

Every rate experiment follows one shape. Pick a factor to change, follow something that changes as the reaction proceeds, and keep everything else the same.

What varies between the practicals is only the third part, what you follow, and there are three options.

Three ways to measure a rate

Collect a gas and measure its volume, using a gas syringe or an inverted measuring cylinder over water. Record the volume every ten or fifteen seconds. This is the method for anything producing a gas, such as marble chips with acid or hydrogen peroxide decomposing.

Measure the loss of mass, standing the flask on a balance with a cotton wool plug in the neck. The gas escapes and the mass falls. This suits a reaction producing a dense gas such as carbon dioxide, since a light gas produces too small a change to measure.

Time a visible change, such as the disappearance of a cross beneath a beaker as a precipitate forms. Here the reading is a single time rather than a series of readings, and the rate is taken as one divided by that time.

The first two give you a whole curve, so you can see the reaction slowing and stopping. The third gives one number per run, which is enough to compare conditions but not enough to describe how the rate changed.

Surface area with marble chips

React calcium carbonate with dilute hydrochloric acid and collect the carbon dioxide.

Change the size of the pieces: large chips, small chips, then powder. Keep the total mass of calcium carbonate identical, along with the volume and concentration of acid and the temperature.

Equal masses matter more than they look. Using more of the powder would mean two things had changed at once, and no conclusion about surface area could be drawn from the result.

The powder reacts fastest, because breaking a solid into smaller pieces exposes more of its particles. More of the solid is in contact with the acid, so collisions happen more frequently.

Catalysts and hydrogen peroxide

Hydrogen peroxide decomposes slowly into water and oxygen on its own. Add manganese dioxide as a catalyst and it decomposes rapidly, so the oxygen can be collected and timed.

Test the gas with a glowing splint. It relights, which identifies oxygen.

To compare catalysts, use the same volume and concentration of hydrogen peroxide and the same mass of each catalyst, and collect gas for the same length of time.

The proof that a catalyst is not used up is worth building into the plan. Filter the catalyst out afterwards, dry it and weigh it. The mass is unchanged, and it can be used again.

Reading the graph

A volume of gas against time graph starts at the origin, rises steeply, then curves and levels off at a horizontal line.

The gradient at any point is the rate at that moment. The steepest part is at the start, when the reactant concentration is highest and collisions are most frequent. The curve flattens as reactant is used up, and the flat section means the reaction has finished.

The height of that flat section is the total volume of gas produced, which depends on how much reactant there was rather than on how fast it reacted. So a catalyst makes the curve steeper without raising the final level, while doubling the amount of reactant raises the level.

That distinction between the steepness and the height is what most graph questions are really testing.