Ask the students to look at structures and consider what charge different parts of the molecules will have when they are protonated and deprotonated (eg. COOH to COO- is neutral to negative, and NH3+ to NH2 is positive to neutral, but can have OH groups that become O- sometimes, depending on the pKa). Use a table of amino acid structures and pKa values, and get them to work out charges at the pH of interest.
Keep coming back to the curly arrow concept, in terms of reinforcing it in different contexts. For a first-year course that’s about 20 lectures, introduce the curly arrow concept in lectures four to seven, then revisit it every lecture thereafter. For 13 or 14 lectures, it would come up in some different form - different examples, different ways of using it, referring back to the original concepts, reiterating the vocabulary, the language that's being used.
Explain partial charges, with problems, then link this to bond polarity. Explain the differences in arrows 'language'. Draw organic structures as line structures and no stereochemistry (until you discuss that). Make stoichiometry explicit and link the structures to names to build on the concepts.
Do many examples, on the projector or board, from first principles. Start with partial charges and identification of the nucleophile and electrophile, then draw in arrows.
In some practical demonstrations it is very simple to see which reagent is limiting, for example if one of the reactants is liquid and you add just a tiny little bit of salt to it. Just from looking at it, which one do you think will be the limiting reagent here? We have 20ml of A, and we’re going to add half a gram of B. We know though that the number of moles is the important thing, but sometimes it works just to illustrate the concept.
If you teach a course that then goes on to other things like equilibrium, electrochemistry, intermolecular forces etc, make sure that all your equations are balanced. Because it can get very lazy, especially when you’re doing organic chemistry, you don’t bother balancing anything. Coming back to that concept each time is really key for them if they’re going to understand the stoichiometry.
This concept is something that needs to be reinforced. It’s not that you taught it in this unit for three weeks, we are over it. It’s something that keeps coming back, and you can possibly reintroduce it, with not much change to your teaching. Not every single time, but every now and then remind the students, 'Remember, you still have to think about stoichiometry and limiting reagents.'
