Teaching Chemistry

MN1988 wrote: »

Hello, I started this thread for chemistry teachers to share their experience. As an NQT any advice is appreciated.


I wanted to know how long does it take others to teach the mole and properties of gases including the mandatory experiment to measure the relative molecular mass of a volatile liquid?
I spent way too long on both of these last year and am rewriting my scheme of work

Students find this section very difficult. Even when they get it they can lose it again. I'd be inclined to give the bones of it at first, very few definitions etc and immediately apply it in making solutions and doing titrations.

So, a mole is a big group of molecules, use the Mr to figure it out, do examples. Concentration is how close together the molecules are in a solution, either how many molecules per litre or how many grams of them there are per litre - molecules x Mr = mass. Make some solutions, try short simple experiments then titrations.

Every chance you get use these ideas over and over throughout the rest of the course. At some stage when they're well used to the concept do the rest of the section.

Do the Mr experiment whenever suits you, or while you're doing the rest of the section - not when you're introducing the concept as it's too confusing. I always found that many students didn't understand the experiment till leaving cert year, if even then.

Sorry but I can't give you a timeline - I never measured how long it took. But the above is much shorter than teaching by the book. And much more effective. And much more enjoyable ☺

There's a horrendous amount in those chapters and the students just aren't mentally ready for it as they start chemistry. It can be enough to turn a lot of them off as it takes so long to drum it in at that stage and it's boring as hell. When their maths and logic skills are more developed they'll fly through it, particularly if they're well used to the basic concepts that Mr is the weight of a molecule and the the mole is a big group of them.

As for gases, that many molecules (1 mole) take up 22.4 L of space at S.T.P. The molecules will move to fill any space you put them in but they'll be at a different temperature and pressure. They have to do work to move apart - losing energy/heat and thus temp, and since there are fewer per cm3 they will exert lower pressure. So when their volume is no longer 22.4 L then their temp and pressure are no longer 273K or 1 atm either.

Look at it another way: if you want to put that many molecules of any gas in that amount of space at that temp (temp being a measure of how fast they are moving) then you'll have to use that amount of pressure to keep them there. Which is the same pressure that they're pushing back with.

You need to be very clear that 22.4 L (or 24 L at room temp/pressure) only works with gases when molecules are free to move and the substance can expand and contract easily. Students can miss that or forget, and start using 22.4 L with liquids or even solids. They often don't think about the state of a substance when they see it in a question.