3AB Reactions, Equations and Stoichiometry Index plus a bit more, including absolute zero

Okay, well, I'm pretty sure I've already covered most of the stoichiometry stuff in my year 11 posts, but again, there's a little bit of new stuff to add. So first I'll put an index of everything that's carried over from last year and then add the new bits.

List of useful stoichiometry posts from Year 11:

• Formulae (naming ionic, metallic and covalent compounds, as well as some fixed names that you should know)
• Simple Chemistry Calculations (molar mass and percentage composition)
• Chemical Equations (how to write them, including balancing equations)
• Simple Calculations Involving Moles (mass to mole, mole to mole, mole to mass)
• Reactions and Equations part deux (standard reactions, ionic equations)
• Empirical Formulae and Limiting Reagents and Some Other Stuff (the "other stuff" consists of mass to volume and volume to moles calculations, as well as working out molecular formulae from empirical formulae and molar mass)

New stuff:

There's two main new things that you have to learn, and that is all the gas stuff and percentage purity.

The gas stuff

Back in 2AB, we calculated the moles of a gas at STP by using n = V/22.71. Well, you can still use that if the question says STP, but now we're going to find new questions that don't use STP.

Some geniuses used the facts that pressure is inversely proportional to volume, volume is directly proportional to temperature and pressure is inversely proportional to temperature to form the following VERY USEFUL equation:

PV = nRT

where P = pressure in kPa

V = volume in L

n = number of moles

R = 8.314 (a constant- it's on the data sheet)

and T = temperature in Kelvin

The nice thing about R is that it never changes, though some textbooks and websites might have slightly different values due to rounding (use your data sheet in the exam). Then you just need 3 of the other 4 things to work out what the 4th thing is.

If you don't already have the pressure, volume, or temperature in the units given above, then you'll have to convert. I won't talk about volume conversions because by now you'll probably feel like I'm taking you for an idiot if I do that, but I will briefly mention the conversion ratios for the other two.

I'll do temperature first, because that's easiest (well, unless you throw Fahrenheit in there, but I've never seen a question with Fahrenheit in it yet). Also, I get to cover another dot point which got put on its own under another heading, and I don't want to do a whole post for that one dot point.

I *think* that the way that zero degrees Kelvin got calculated was that someone found graphs of the average kinetic energy for different particles at different temperatures and extrapolated them all the way down to where the kinetic energy would theoretically be zero and the particles would theoretically stop moving. All of the graphs stopped at the same point, at a point estimated to be -273.15 degrees Celsius. Therefore, 0 degrees Kelvin = -273.15 degrees Celsius.

The great thing about the Kelvin scale is that it increases at the same rate as the Celsius scale. Therefore, to change from Celsius to Kelvin, just add 273.15 degrees, and vice versa.

Now for pressure! The bad news is that the pressure conversions don't appear to be on the data sheet, so if they do have a pressure conversion question then I might be stuffed because I don't remember them off the top of my head, but the good news is that in light of that, they might decide not to put one in.

From my textbook, 1 atm (atmosphere) = 101.3 kPa = 760 mmHg (millimetres of mercury- I think this measurement had something to do with some old system of measuring air pressure).

(I think that 101.3 kPa is atmospheric pressure, which is why it's 1 atmosphere.)

The PV = nRT equation is very useful for when you encounter gases in stoichiometry problems. Use and abuse it at will.

Percentage Yield

This is a bit different from percentage composition, but if you understand percentages in general, this isn't too hard to pick up. (Yes, percentage composition is still in the course. If you need a refresher, it's in my simple chemistry calculations post.)

When we calculate how much is going to be formed as a result of a reaction by using our amazing Stoichiometry skills, what we're calculating is the theoretical value. Unfortunately, life being life, some reactions form a smaller mass of products than the theoretical value, that is to say, their percentage yield is less than 100%.

If you're given the reactants and the percentage yield of the reaction and you're asked to find out what mass of one of the products is produced, you need to do two things:

1. Work out the theoretical yield of the product in question using everything else that you've learned in stoichiometry. Remember to watch for limiting reagents and other tricks that they might throw at you (but at the same time be wary of questions that provide too much information).
2. Multiply the theoretical yield of the product by the percentage yield (e.g. if the percentage yield is 91%, multiply the theoretical yield by 0.91).

(You could probably do the two steps in the reverse order as well, i.e. you could probably multiply the amount of reactant by the percentage yield and then work out the yield of products.)

If you're given the products and the percentage yield of the reaction and you're asked to find out what mass of one or more the reactants is required, you need to follow the steps below:

1. Work out how much product you were "theoretically" meant to have by first dividing by the percentage and then multiplying by 100 (e.g. if the percentage yield is 91%, divide the mass by 91 and then multiply by 100. Or you could divide by 0.91. It's the same thing really, since 0.91 = 91/100 and when you're dividing by 91/100, you're actually multiplying by 100/91.) 
2. Calculate the amount of reactants from there.

Alternatively, you could calculate the amount of reactant using the amount of product that was actually yielded, and then divide by 91 and multiply by 100.