Rates of reactions

The concept of speed or rate as the change in a measured quantity per unit of time is readily understood by most children. For example, if a car travels at 50 km h^-1, it is simply stating that in 1 hour, a distance of 50 km is travelled.

In science lessons, most students encounter the idea of rate when exploring the rate of dissolving in simple practical activities. They invariably use a clock of some description and make observations regarding the ‘disappearance’ of a material. They may be encouraged to consider the effect of temperature on the rate of dissolving or the effect of particle size. This enables the collection of data and allows students to demonstrate a quantitative as well as a qualitative approach to this practical activity.

However, in chemical reactions, the challenge for most students is first to understand what is meant by chemical change. One typical reaction to explore at secondary level is the rate of the reaction between magnesium ribbon and hydrochloric acid. The instructions on a worksheet may state ‘time how long it takes for the magnesium to disappear’. This is unhelpful if we are trying to teach the concept that a chemical reaction is taking place. The language we use to describe chemical change is vitally important and even in a previous issue of Education in Chemistry, copper has been described as dissolving in nitric acid rather than reacting with nitric acid. Asked to state what is happening at the surface of the magnesium, many students will struggle as the concept of particle collisions is not well understood.

Steve Hacker discusses four key issues students may have with this subject and how best to support these, providing links to suitable videos, demonstrations and summaries.

Offering a suggested teaching sequence, resources and ways to develop students' math skills, this article breaks down how the concept of reaction rates could be introduced to students.