Rates of Reaction
Blogs about Chemistry
Rates of reaction is generally considered a very practical topic, and many of the questions which examine students on rates are really examining them on their understanding of how to plan practicals, what makes a good investigation and why, analysing data and evaluating results.
For this reason it is important that students carry out good quality practical work as part of the topic: this means that they know the procedures and understand why they are carrying out the practicals in the way they are.
When teaching investigative skills, it is very difficult for students to approach them all at once. The practical work needs to be scaffolded throughout the topic so that sometimes students are focused on planning, in the next practical on graph drawing and interpreting results and in a final task on evaluating procedures (which builds on their understanding of planning).
When I teach rates, I usually start of by demonstrating some visual examples of reactions with very different rates of reaction (water into wine vs. the iodine clock vs the sunset demonstration for example). The rest of the lesson is focused on ways of measuring rate for the different examples. After this I demonstrate how to determine the rate of reaction between magnesium and hydrochloric acid using a gas syringe and show students other methods for this reaction. They plot a graph and do some basic analysis. For homework they plan a practical method for determining the effect of changing surface area on the rate of reaction between calcium carbonate and hydrochloric acid. In order to scaffold this task, I provide students with an assessment grid in advance so that they can refer to it when completing their homework.
From this point on I focus each lesson on a different variable. The first is on surface area and the key investigative skill was planning it in advance. The second is concentration and the key skill is analysis. The third is on use of a catalyst which students also plan using the formative feedback they received in their first plan. Finally, as the most conceptual in terms of collision theory, the practical is a microscale investigation into temperature. For this practical the key skill is evaluation.
One this scheme is complete, I spend at least two more lessons on rates. One on past paper practice using a mixture of questions on different aspects of rates to promote students' ability to select the right approach to each question (interleaving). Finally we work on graph sketching. The RSC AfL task on Rates of Reaction is excellent and a way of drawing together students' ideas about rates graphs and checking their understanding.