Use balloons to illustrate bonding pairs and electron pair repulsions leading to the determination of molecular shape For example, if you tie 6 balloons together, it automatically forms the octahedral shape. Then each time you pop one, they rearrange themselves to sequentially form the other shapes: trigonal bipyramidal, tetrahedral, trigonal planar and linear. It’s visually appealing, a concrete example and memorable for students. Also, popping the balloons wakes the students up!
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 Lego pieces. For example, HCl built out of Lego. Use a little blue brick and a yellow one, showing physically that on this side you’ve got one blue Lego brick and then on this side you’ve got two and so on, trying to make them see them not as chemical formulas but identities of some sort.
Try demonstrating the reaction first, to show the macroscopic changes that occur, before introducing the equation. Copper in silver nitrate solution is a standard one. Explain it in terms of particles, the ions, atoms, show a video representation (for example from YouTube) of these changes, and then explain the whole thing in terms of the symbolic chemical equation to represent the overall change.
You can use analogies to everyday objects and there are a variety of things you can use in that way, such as pebbles and peas to illustrate moles, because they have different masses. Certainly getting them to think about that and then to be comfortable in translating that into a whole lot of different elements you might be comparing.
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.
For a demonstration of concentration versus total quantity: get three 100 mL graduated cylinders. And put a bit of food colouring in one and maybe dilute to 10 mL. Put an equivalent amount of food colouring in another and make it up to 30 and 100 mLs. It’s the same amount of food colouring in 10 mL, 30 mL, 100 mL. What will they look like? Dark, medium and lighter. Look down the top, what will they look like? Identical. It’s the same amount of molecules absorbing, blue, or whatever colour’s being absorbed.
