2.7

## National STEM Learning Centre

Skip to 0 minutes and 2 seconds[Rates of reaction; Marbles in tray analogy] Models can be useful for students to help visualise what’s happening at an atomic or particulate level. However, we do have to make students aware of the limitations of these models otherwise that concrete misconceptions that can occur later on. The marbles represents reactants. The blue marble represents one reactant, solution A and the pink marble represents a different reactant, solution B. For example calcium carbonate and acid. Every time the marbles collide it can be considered that a reaction has taken place. The more collisions in a given time the faster the rate of reaction. Count the number of collisions that occur in 10 seconds.

Skip to 1 minute and 13 seconds[Temperature] When the tray is shaken faster it can represent an increase in temperature. Count the number of collisions that occur in 10 seconds.

Skip to 1 minute and 37 secondsHow does this compare to the first model?

Skip to 1 minute and 42 seconds[Surface area] This demonstration models what happens when the surface area of one of the reactants is changed. Count the collisions that occur over 10 seconds.

Skip to 2 minutes and 10 secondsHow does this compare to the previous number?

Skip to 2 minutes and 15 seconds[Concentration] This time we have increased the concentration of solution A and represented this by increasing the quantity of the blue marbles.

Skip to 2 minutes and 29 secondsCount the number of collisions that occur in 10 seconds.

Skip to 2 minutes and 51 seconds[Volume] Finally, in this model we have reduced the volume that the marbles have. Count the number of collisions that occur in 10 seconds.

# Analogies and models

Models are used throughout science to help understand processes, particularly at the particle or atomic level.

In this video is a series of clips using different size and colour glass marbles in a tray, to demonstrate some ideas about rate. Feel free to use with your students if you find it useful.

Here we are making the assumption that every time the reagents hit each other, they will react. Students will need to pull that apart, as we know that not every collision is successful and the idea of activation energy and normal distributions of energies with a group of particles, influences the rate.

However, we can use the basic idea that more collisions is likely to lead to a faster rate of reaction.

## Comment

How effective do you think this model is in terms of supporting students’ understanding of what influences rates of reaction?