Whilst the rate of reaction can be investigated in several ways there are typically three classic experiments used with students. These are the reactions between:

• calcium carbonate (marble chips) and hydrochloric acid
• magnesium and hydrochloric acid
• sodium thiosulfate and hydrochloric acid

The chemical equation for each reaction is as follows:

CaCO₃(s) + 2HCl(aq) _↓ CaCl₂(aq) + CO₂(g) + H₂O(l)

Mg(s) + 2HCl(aq) ↓  MgCl₂(aq) + H₂(g)

Na₂S₂O₃(s) + 2HCl(aq) ↓  2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)

Because a gas is evolved in all three cases, rate can be measured by mass loss or the volume of gas evolved.

What problems might you anticipate when measuring rate of reaction when carrying out these activities with students using the following?

Mass loss method.

Students usually have problems timing the addition of reagents in order to zero the balance before recording mass loss. In the case of magnesium (depending on the quantities used) the loss of mass can be too small to record accurately.

Measuring volumes of gases.

Gas volumes are often measured by displacement of water. This can cause problems with manipulation of upturned measuring cylinders in troughs of water as well as accurately measuring the volume.

Sometimes the pressure created is insufficient to displace water or the gas evolved is soluble in water.

If a gas syringe is used there is often concern about the perceived expense of the equipment.