Unit 4 questions

[Mao]

* Mao refrains from making another sexist joke

[bec]

...ignoring that comment....

How do you work this out?

Passage of 0.10 faradays of electricity through an aqueous tin chloride solution deposited 5.94g of tin at one electrode.
What was the oxidation number of tin in the chloride salt likely to be?

[shinny]

Therefore oxidation number is +2

[bec]

Thanks shinjitsuzx - I always get confused with faradays and what they actually are...

[Collin Li]

Don't be confused by Faraday's constant: and a Faraday: .

One is a property, one is an amount of charge.

[ben4386]

hi just after a bit of confirmation

there was this multiple choice i disagree with

A particular reaction gives out heat, this means that:

A) The bonds made are stronger than the bonds broken
B) The bonds broken were stronger than the bonds made
C) delta H is positive and heat is evolved
D) delta H is negative an heat is absorbed

obviously the last two are wrong, the answers say B because "the energy required to break the bonds in the reactants is greater than the energy in the bond of the product", I disagree with this
i think its A

if you have an exothermic reaction the activation energy is clearly less than the activation energy of the reverse reaction. Since the reverse reaction is going from products to reactants that would imply that the products have stronger bonds and need more energy to break these bonds. Isn't chemical energy greater in those unstable chemicals that have weaker bonds?

[Collin Li]

Their reasoning is wrong because the activation energy (the energy required to break the bonds in the reactants) has nothing to do with it.

If you are giving out heat, you are releasing chemical potential energy. That means you are going from a more energetic state (less stable, weaker bonds), to a less energetic state (more stable, stronger bonds).

So the answer is A.

[bec]

A couple of Qs from the TSSM exam:

1. I thought the value of K NEVER changed except when temperature changed? The solns have K decreasing when the pressure is increased in the reaction ...

2. In a vanadium redox cell, the half equations during discharge are:



The cell runs for 8.0 hours with a current of 3.5 amps, what mass of VO would react in this time?

The solutions state that , but I thought ?
My solution was 35g, theirs was 69.9g. Why does n(VO)=n(e)?

[Mao]

nfi about the first one, i thought the same as you did.

2. look at your first equation

[bec]

ARGHH READ THE QUESTION GIRL
I would be so much better at chem and methods if I actually did...

thanks mao, for teaching me to read

[orsel]

random guess for q1:
forward reaction is endo/exothermic, so when pressure increases(has to be volume decrease) the resultant reaction causes a change in temp, which causes a change in K.

[bec]

Nah I'm pretty sure actually that K only changes with temperature, since it's temp-specific (and since I spent about a month trying to learn that fact before a SAC)

in this case, you have added M of HCl, which completely ionises so [1]

Why doesn't [H+]=[HCl] since it ionises completely?

In all acid-base calculations, we usually ignore the fact that of protons sit around in neutral solutions. However, if we add something of a similar magnitude to that, we shouldn't ignore it.

We can definitely ignore it if we add say, hydrochloric acid, because if we accounted for the , it would only be seen at the 7th decimal place or so.

I understand the explanation for why [dilute acid] [H+] but how do you work out the actual concentration of H+?

[Collin Li]

Or, in general:

[Collin Li]

Or, in general:

Actually, we should be working on a molar basis, so we should really figure out how many litres of water we have, and then using , find out how much moles of protons we have.

On another note, do not confuse dilute strong acids with a "weak acid." A dilute strong acid will still fully deprotonate, while a weak acid, no matter how concentrated, will achieve some kind of equilibrium disassociation.

[bec]

Thanks Coblin. That's what I thought it was but I stuffed up my calculation and said haha and that's why i should use my calculator