2 Using mental models to generalise and predict
One of the wonderful things about carbon chemistry is the myriad of compounds that are possible. The huge number of different compounds would be very difficult to study if we had to learn about each compound individually. Fortunately, it is possible to organise compounds into groups sharing common structural features and common chemical properties. The simplest example of these is the alkanes, which can all be represented by a common formula CnH2n+2. Once we recognise common features we can start to make predictions about how compounds in a particular family will behave based on what we know about the behaviour of one or two members of the family and about the effect of differences such as the mass of the molecule.
In order to generalise and make predictions about carbon compounds, your students will learn to apply the concepts of a homologous series and functional groups.
The first mental model they will have to build is that every member of a homologous series shares the same general molecular formula, such as CnH2n+2 for alkanes or CnH2n for alkenes. For each of these series, adding a –CH2 unit to a molecule will produce the next molecule in the series.
The next mental model that your students need, is that you can replace one of the hydrogen atoms attached to a carbon atom by something else called a functional group such as a chlorine or bromine atom, or one of the other groups shown in Tables 4.3 or 4.4 in the textbook. The chemical properties of the new compound will depend on what functional group you use to replace a hydrogen atom.
Combining these two models allows your students to produce a homologous series of alcohols, carboxylic acids or other chemical families, such as those shown in Table 4.4. Every compound in a homologous series will have similar chemical properties to all the other compounds in the series.
These can be difficult concepts for students to understand, it is wise to try to introduce the content in small ‘chunks’, a small amount of new information in each lesson so that students have time to process the information and see patterns.
Case Study 2: Teaching homologous series through diagrams and mental models
Mr Bulsara used diagrams and mental models to teach about homologous series.
I decided to introduce the idea of homologous series to my students as a ‘spot the connection’ game. I asked everyone to close their textbook and I began by writing the molecular formula for methane, ethane and propane on the blackboard:
CH4 C2H6 C3H8
I asked my students, ‘Predict the next two items in the sequence and tell me how you worked it out.’
They told me that the next two items would be C4H10 and C5H12, and that each time you had to add one C and two Hs.
Then I asked, ‘Work out what the answer should be if it started with C10, and to explain how they arrived at the answer.’ I asked them to discuss the answer in pairs, before I chose someone to reply.
Ravi told me that the answer should be C10H22, and one of my students said that whatever number you had for C, you should double it and add 2 to get the number for H. I wrote this down as CnH2n+2, then told them that this was a general formula for the family known as the alkanes. A family like this is also called a homologous series.
I also wanted my students to relate what they had just discussed to the structural diagrams for molecules in these two series, as they would need to recognise both ways of representing the same molecule.
I provided the structural diagrams for CH4 and C2H6 and asked everyone to draw diagrams for C4H10 and C5H12. As they already understood the rules of the homologous series, everyone managed this easily.
I suggested that next time they visited an internet café, they should find some images of carbon molecules so they could see some of the different ways of representing them.
Activity 2: Extending the models with functional groups
This activity will help you to develop your planning and in-class teaching.
Look at the list of functional groups in Table 4.4 in the Class X textbook. It shows the effect of substituting the functional group for one or more hydrogen atoms on a propane molecule.
- If you were to use the approach in Case Study 1 for teaching the homologous series for alkenes, what would be the key feature to look out for when you start using structural diagrams? How will you know if your students have the correct metal model for this series?
- Why is it helpful to use propane as the starting structure to be modified in all the examples in the table?
- Another way of helping your students to become familiar with homologous series for different functional groups is to provide a pack of cards, each with a different diagram of molecular structure drawn on it, and tell your students to sort the cards into different homologous series as quickly as possible. If you wanted to work on recognition of alcohols, aldehydes, ketones and carboxylic acids, you would include examples with different chain lengths for each of these functional groups. What mental model would you expect your students to have to distinguish:
- alcohols from carboxylic acids?
- aldehydes from ketones?
- aldehydes from carboxylic acids?
Read Resource 1, ‘Planning lessons’, for more information.