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.
Use the example of something obvious, such as reaction between two compounds, and you get 200 tons of one, and half a gram of the other one - which one is the limiting reagent, and why is it the limiting reagent? Because you run out of it before you run out of the other one. It’s something that they can imagine, something they can see. Or even in the lab, if you don't have 200 tons but one litre of one and a few milligrams of the other.
Something you can do visually in the lecture theatre is to take in some things you wish to connect and make up an item. Or in PhET, for example, there’s a little activity you can do making sandwiches and you can work out how much you need of which one and whether you’ve got something that’s there in excess or something that’s limiting. It's based on the molar ratios or the stoichiometric coefficients, which in turn are based on the number of moles reacting.
Take something into the lecture theatre, like a bag of pebbles - a whole lot that are the same size and make sure that it is visibly a kilogram (or another specified mass). Also have chickpeas or some other smaller and lighter thing. When you have a kilogram it’s really easily recognisable that there’re a lot more particles in the chick pea bag that there are in the pebble bag. You can’t show them a mole of something because it’s too many, but just use this to begin to unpack the idea that we’ve got a whole lot of tiny, tiny particles, much smaller than the ones in the demonstration.
The way back-titrations are taught is often confusing. Get them to concept map the calculation. So, instead of trying to do the calculation in their head, map it out starting from where they end up, and then relate each of those steps to where that number is coming from. Have them think about it like it’s a reaction. Because they know how to do the math, and they can understand how to do it for a reaction. An example is the dissolution of calcium carbonate and trying to get them to work out how much carbonate is in a limestone sample.
Use molecular models, simulations, Lewis diagrams, ball and stick models, space filling models. Different representations - macroscopic and microscopic. Make sure you know how to use them.
Use the Vis Chem website, which is Roy Tasker’s resource, and there are links to a Scootle site where you can download visualisations for chemical bonding and pure substances in different states. There is gaseous water and liquid water. You can see they’re close together - they’re crowded. You can talk about ice skating. You can press the ice and it becomes liquid. That’s why the ice skates slide. You can see they’re jiggling away. There’s some space between them.
