Rates of reaction

The rate of a chemical reaction can be found by measuring the quantity of a reactant used, or the quantity of product formed, over time:

mean rate of reaction = quantity of reactant used ÷ time taken mean rate of reaction = quantity of product formed ÷ time taken

The quantity of reactant or product can be measured by the mass in grams or by a volume in cm³, so the units of rate of reaction may be given as g/s or cm³/s. For the Higher tier you are also required to use quantity of reactants in terms of moles, and units for rate of reaction in mol/s.

You should be able to draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time.

These graphs have a characteristic shape. They are steepest at the start, when the reactants are most concentrated, and flatten as the reaction proceeds. When the line becomes horizontal the reaction has finished — no more product is being formed — and the height of that flat section shows the total amount of product made.

You should be able to draw tangents to the curves on these graphs and use the slope of the tangent as a measure of the rate of reaction.

The difference between the two rates is worth being clear about. The mean rate over a period is the total quantity divided by the total time — a single number for the whole run. The rate at a specific time is given by the slope of the tangent at that point on the curve, and it changes throughout the reaction: steep at the start, shallow near the end.

Higher tier students are also required to use quantity of reactants in terms of moles, and units for rate of reaction in mol/s, and should be able to calculate the gradient of a tangent to the curve on these graphs as a measure of rate of reaction at a specific time.

Calculating the gradient, rather than only describing the slope, is what separates the Higher tier requirement from the Foundation one. Draw the tangent so that it just touches the curve at the time asked for, extend it well across the graph, then take a large triangle from the tangent itself.

Collision theory explains how various factors affect rates of reactions. According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.

Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions, and so increases the rate of reaction.

Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.

You should be able to predict and explain, using collision theory, the effects of changing conditions of concentration, pressure and temperature on the rate of a reaction, and to predict and explain the effects of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio.

Breaking a solid into smaller pieces does not change how much there is, but it increases the surface area to volume ratio — more of the substance is exposed where collisions can happen — so powdered marble reacts far faster with acid than a single lump of the same mass.

You should also be able to use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction. If doubling the concentration doubles the frequency of collisions, then it doubles the rate — so a graph of rate against concentration is a straight line through the origin. Being able to say "twice as many particles, so twice as many collisions per second, so twice the rate" is what proportionality means here.

Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature, and the presence of catalysts.

You should be able to recall how changing these factors affects the rate of chemical reactions. Increasing any of the first four increases the rate, and adding a catalyst increases it too.

Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. Enzymes act as catalysts in biological systems.

Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.

You should be able to explain catalytic action in terms of activation energy. The chain of reasoning is: the catalyst provides a different pathway with a lower activation energy, so a greater proportion of the colliding particles have enough energy to react, so more collisions are successful and the rate increases.

You should also be able to identify catalysts in reactions from their effect on the rate of reaction, and because they are not included in the chemical equation for the reaction. A substance that speeds the reaction up but appears on neither side of the equation, and can be recovered unchanged at the end, is a catalyst. You do not need to know the names of catalysts other than those specified in the subject content.