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 taken differs. If one experiment uses twice as much of the limiting reactant, it makes twice as much product and levels off at twice the height. The quantity of reactant used can be read from a graph in the same way: for a reactant whose quantity falls, the amount used up is the starting value minus the value at the end-point.

A steeper curve means a faster reaction. Comparing two curves on the same axes shows the effect of changing a condition: the curve for the faster reaction is steeper and reaches its end-point sooner.

Calculating the average rate

The average rate over a period is the change in the quantity divided by the time taken.

The units come from the quantity and the time: $\pu{cm3 s-1}$ for a volume in cm³ and a time in seconds, $\pu{g s-1}$ or $\pu{g min-1}$ for a mass, and $\pu{mol l-1 s-1}$ for a change in concentration. Sometimes the time for a fixed change, such as a cross disappearing, is measured, and then a shorter time means a faster rate. Since the same change happens each time, $1/t$ is proportional to the rate.

Worked example

Magnesium reacts with excess dilute hydrochloric acid, and the hydrogen is collected in a gas syringe.

The reaction has finished by 60 s, since the volume stops changing, and the total made is $\pu{60 cm3}$.

The rate falls over time, which is what the decreasing slope of the graph shows. It falls because the concentration of the acid, and the amount of magnesium left, both decrease as the reaction goes on. Calculating the average rate over different stages of a reaction is the way to show that it slows down.

Changing the rate

A reaction happens when particles collide, so anything that makes collisions more frequent, or makes more of them successful, increases the rate.

Temperature

Raising the temperature makes the particles move faster, so they collide more often, and a greater proportion of the collisions have enough energy to react. A reaction at 40 °C is faster than the same reaction at 20 °C.

Concentration

A higher concentration of a reactant means more particles in the same volume, so the particles are closer together and collide more often. For a gas, increasing the pressure has the same effect, because it squeezes the same number of particles into a smaller volume.

Surface area and particle size

Only the particles at the surface of a solid can collide with the other reactant. Cutting a solid into smaller pieces exposes more of its particles, so the surface area is greater for the same mass and there are more collisions each second. Powdered calcium carbonate reacts with acid much faster than the same mass of marble chips, and the powder is still the same substance and makes the same total quantity of carbon dioxide.

Catalysts

A catalyst is a substance that speeds up a chemical reaction but can be recovered chemically unchanged at the end. It takes part in the reaction and is reformed, so it is not used up, and a small amount can speed up the conversion of a large amount of reactant. The amount of product made does not change, only how fast it is made.

Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide solution, which is slow on its own:

The oxide can be filtered off, dried and weighed afterwards, and its mass is the same as at the start. Industry uses catalysts to make reactions economical at lower temperatures, such as iron in the Haber process and platinum in the Ostwald process, and a catalyst is chosen for the particular reaction it speeds up.

Planning a fair comparison

To test the effect of one factor, everything else is kept the same. To compare the effect of temperature on the reaction of a metal with acid, the mass of metal, its form, the volume and concentration of acid are all fixed, and only the temperature changes. The measurement of quantity or time is repeated and an average taken, since one result could be unreliable.