2010 VCAA Chemistry U4 Suggested solutions

[monicapham93]

for Q2 3ii
about the ammonia

is it wrong to say that if theres an excess there will be no change?? cause i thought it meant, what effect does it have on the relitive proportions of the product gasses in the atmosphere (according to the eqn)

argh. i dunno i probs read it wrong

[stonecold]

But doesn't the current force a reaction which will not normally happen?

Sure, that would happen in a galvanic cell. But it wouldn't also happen during electrolosis, would it?

I think I get where you're coming from, but water will always be reduced before a sulfate ion.

Think of the hundreds of Q's you've done.  Spectator ions never get oxidised, because they are too weak of a reductant.

[m@tty]

Wait, I see the error... The cathode reaction is the reduction of water .. which is -0.83 V and then you take the closest one above.. being the other water.. :buck2: HOW COULD I MISS THAT, ARGH.

[stonecold]

Wait, I see the error... The cathode reaction is the reduction of water .. which is -0.83 V and then you take the closest one above.. being the other water.. :buck2: HOW COULD I MISS THAT, ARGH.

DW, it ranks up there with my misreading of mol ratio error in MC Q7 lol...

[ckg93]

But doesn't the current force a reaction which will not normally happen?

Sure, that would happen in a galvanic cell. But it wouldn't also happen during electrolosis, would it?

Only after the first reaction has been completed. In other words, the water would have to be used up before the second reaction could occur.

[Chavi]

ok, just asked my dad, who has a medical degree with an additional year of study in chemistry, and topped chemistry at melb university 2 years in a row, he said fuel cells ARE rechargeable,.

Ahh yes Dyeni. Echoes of Yr 9 science when you convinced the teacher to give you that 1/2 mark advantage due to some obscure finding in an ancient medicine manual. .

[becca92]

For 2.5a "iii, Provide alternative reaction pathways with lowe ractivation energy"
I wrote "It lowers the activation energy required for the reaction to proceed but did not mention alternative pathway.. will i still get the mark?

[stonecold]

ok, just asked my dad, who has a medical degree with an additional year of study in chemistry, and topped chemistry at melb university 2 years in a row, he said fuel cells ARE rechargeable,.

Ahh yes Dyeni. Echoes of Yr 9 science when you convinced the teacher to give you that 1/2 mark advantage due to some obscure finding in an ancient medicine manual. .

VCAA makes up their own rules.  As has been seen in past examiners reports, 'fuel cells are non rechargeable.'

[Souljette_93]

For 2.5a "iii, Provide alternative reaction pathways with lowe ractivation energy"
I wrote "It lowers the activation energy required for the reaction to proceed but did not mention alternative pathway.. will i still get the mark?

Yeah of course. I think the major thing you had to mention was "lowering the activation energy". There are multiple answers, so don't worry, if you have a look at the examiners report, you will see that they accept a variety of answers depending on the the question.

[m@tty]

However, you did have to explain how lowering the activation energy increased the rate.

[physics]

FOR first short answer...the K value doesnt include H2o does it?
and the concentration of H...i gpot 1.6 or something :S

[m@tty]

Unfortunately for you, it was the normal equilibrium constant, thus you needed the [H2O]..

And crap that's a high concentration... didn't you get suspicious when a weak acid produced a solution of such high pH?

[Souljette_93]

FOR first short answer...the K value doesnt include H2o does it?
and the concentration of H...i gpot 1.6 or something :S

As far as i know, you include water, this is not a Ka expression.

Also in the examiner's report (2009), they said:

The most common error was leaving [H2O] out of the equilibrium expression.

[ckg93]

for Q2 3ii
about the ammonia

is it wrong to say that if theres an excess there will be no change?? cause i thought it meant, what effect does it have on the relitive proportions of the product gasses in the atmosphere (according to the eqn)

Hey Monica, Chris from NYSF here. You were referring to section B - Question 3 - Part a ii right??? I put down what I thought would be produced, considering the yields would be different between the two parts. I also said that NH4OH would be produced in excess ammonia.

[physics]

FOR first short answer...the K value doesnt include H2o does it?
and the concentration of H...i gpot 1.6 or something :S

consequential marks atleast ?


As far as i know, you include water, this is not a Ka expression.

Also in the examiner's report (2009), they said:

The most common error was leaving [H2O] out of the equilibrium expression.