AQA CHEM5 A2 Chemistry - 19th June 2013

Yep I got 1.93 but I don't understand the explanation part.

For part c, I have been doing the RHS-LHS rule. How do you know which one is positive and which one is negative?

Thanks btw!

It's okay well with the conventional cell diagram, the one with the most positive E is always on the RHS (with some exceptions but they're not important) and the one with the least positive/most negative E is always on the LHS.

So RHS = +E = a reduction reaction
and LHS = -E = an oxidation reaction

Oh sorry, I didn't look at the question properly!! Okay so Mg is on the LHS so it is being oxidised:

Mg <-> Mg2+ +2e-

By decreasing the concentration of Mg2+, can you see that the equilibrium above would shift to the RHS to produce more Mg2+ to oppose this change in the equilibrium. Shifting to the RHS will therefore also result in more electrons being lost and this means the electrode potential of Mg is made more negative. So say it goes from being -2.37 to -2.40 for example.

Now the overall emf = (Fe electrode potential) - (Mg electrode potential)
= (-0.44) - (-2.40)
= +1.96V

So it increases, I hope that makes sense haha

Nah just alkaline and acidic Fuel cells.

Yep that's absolutely fantastic.
Just one more thing though, if RHS= +ve and LHS= -ve then surely RHS-LHS= +ve - -ve?
So why is it better to use the RHS- LHS formula..?

Oh haha no solid reason really, just when I did some homework on this topic a while ago I did (+ve) - (-ve) on one cell diagram and it wasn't right so my teacher just said to use (RHS) - (LHS) from now on because that would have given me the right answer instead somehow!

Ahhhh! Okay. Thanks so much for taking the time out to explain that, much appreciated

No problem at all and best of luck!!

You too! I'll probably have to ask you something before the exam anyway, hopeless at this unit!

I thought we just had to know hyrdogen fuel cell equation?

Hey guys I'm doing the Jan 10 paper and it asks why the theoretical val of lattice dissociation would be greater for AgCl than for AgBr, saying that cl is smaller therefore attracted more strongly.

I thought the argument for Br being a bigger ion meant it is easier to be distorted by the Ag so has some added covalent character, don't understand why this argument isn't applicable to this question?
(As in the textbook it says that the Selenide is a big ion and therefore plenty of electrons to distort so I assumed with Br having an extra shell this would be the answer? So very confused, if anyone has an idea please quote me!

thanks for the help!

Nah just alkaline and acidic Fuel cells.

any good ways to learn all the definitions?

Well maybe cockup is the wrong word. I just know that there is no mention in the specification of having to combine three half equations. It wasn't especially hard but obviously a lot of people got really confused so the mark scheme allowed a wide range of answers and you only lose a mark for getting the ratio itself wrong

any good ways to learn all the definitions?

I think it's because the question is concerned with the theoretical value and so the theoretical model of a lattice, which takes a lattice to be made up of perfectly equal spherical ions with 100% ionic bonding.

As you've said, Br is the bigger negative ion and so it wouldn't fit the theoretical model of a lattice as well as the smaller Cl ion would and so it would give a a smaller value.

Is your problem that they didn't accept your argument in the mark scheme? Maybe I'm just getting tired haha but could you rephrase your last bit

What table?

ooohh okay awesome, my head was too into this whole covalent character that i didnt realise it's just concerned with theoretical (not taking into account adding covalent character, thanks you so much!