SQA National 5 Chemistry

## Following a reaction The progress of a reaction is followed by measuring a quantity that changes as the reactants turn into products. Which quantity is used depends on what the reaction does. - A reaction that gives off a gas can be followed by the **volume** of gas collected in a gas syringe or an upturned measuring cylinder of water, or by the **loss in mass** as the gas escapes from a flask on a balance. - A reaction that makes a solid or a cloudy mixture can be followed by the time taken for a cross viewed through the flask to disappear. - A reaction in which a coloured substance is used up or made can be followed by the time taken for the colour to change. The measurements are taken at regular intervals and plotted as a graph of quantity against time. The graph has the same general shape for a product being made and for a reactant being used up, except that the product curve rises and the reactant curve falls. ### Reading a graph For a product being made, the curve is steepest at the start, because the reactants are at their highest concentration, and it becomes less steep as they are used up. It then goes flat. The time at which it first goes flat is the **end-point** of the reaction, because nothing more is being made. The height at which the curve goes flat gives the **quantity of product** made in total. If the quantity of the limiting reactant is the same in two experiments, the curves level off at the same height whatever the conditions, and only the time…