Monitoring and controlling chemical reactions Study Guide

The rate of a chemical reaction tells us how quickly reactants are used up or products are formed. To determine this rate practically, we need to monitor a change that occurs during the reaction over time. The choice of method depends on the specific characteristics of the reaction being studied.

One common approach is to measure the production of a gas. If a reaction produces a gas, we can collect this gas in a gas syringe and record its volume at regular time intervals. Alternatively, if the gas is being released to the atmosphere, we can place the reaction flask on a balance and monitor the decrease in mass as the gas escapes. Both methods allow us to plot a graph of gas volume or mass change against time, from which the reaction rate can be calculated (the steeper the gradient, the faster the rate).

Another practical method involves reactions where a precipitate forms, making the solution cloudy or opaque. We can place the reaction mixture over a marked cross on a piece of paper and measure the time it takes for the cross to disappear from view. This 'disappearing cross' experiment gives a measure of the reaction rate; a shorter time indicates a faster reaction. Variables such as temperature, concentration, and surface area significantly impact the rate of reaction. For accurate results, it is crucial to control other factors that might affect the reaction rate, such as temperature, by using a water bath.

A catalyst is a substance that speeds up the rate of a chemical reaction without being used up itself. This means that a catalyst can be recovered unchanged at the end of the reaction and used again. Catalysts work by providing an alternative reaction pathway with a lower activation energy. Activation energy is the minimum amount of energy required for reactants to be converted into products. By lowering this energy barrier, more reactant particles have sufficient energy to react effectively upon collision, thus increasing the reaction rate. It's important to recognise that catalysts do not change the products of a reaction, nor do they affect the overall energy change (enthalpy change) of the reaction. Many industrial processes rely on catalysts to make reactions more efficient, faster, and more economical, as they can operate at lower temperatures and pressures, saving energy. Enzymes are biological catalysts that control reactions in living organisms.