Unit 4 questions

[Collin Li]

To be honest, hydrochloric acid shouldn't really exist unless you've got some weird temperatures going on, because that means it's more basic than water at 25 degrees Celsius, even though the thing is nearly 100% water...

I'll withdraw this comment. It certainly can exist, but yeah, you just have to use that rule above, where you consider the protons in neutral water as well (as they are significant). The fact that it's nearly 100% water (and the rest strongly acidic) then leads to the fact you have some pH slightly less than 7 (slightly more acidic).

[bec]

I have a few questions from the TSFX exam, which was really hard (as per TSFX's tradition...)
Questions: http://tsfx.com.au/vic/Unit%204-2008-CHEMISTRY-Exam-Schools-Paper-FINAL.pdf
Solutions: http://tsfx.com.au/vic/Unit%204-2008-CHEMISTRY-Exam-Schools-SOLUTIONS-FINAL.pdf

- Is nuclear fission, by definition, the splitting of the nucleus - regardless of how it happens? For example, even if you ADD energy rather than produce it, is that nuclear fission? (answered, thanks coblin)

- I'm not sure about Question 9. It asks for K of the "dissolution of the precipitate" and then in the solutions, states that the dissolution involves both reactions that are given. But...how was I to know that? I figured that it when from a solid to aqueous in the first equation, and so I thought that A was the correct alternative...

- Question 12: 1. What's an inverted U-tube? and 2. In the solutions they say that it can't be D because OH- would react with Fe2+. But, given their positions on the electrochemical series, I thought that they wouldn't spontaneously react?

- Question 2(b) (ii) I got 95.6%, is their solution of 85% incorrect? (I know a few other people got the same answer as me)

- Question 3c(iii)...why does the forward reaction get continuously slower? My graph was opposite to theirs, because I thought that since the forward reaction was favoured, it would also get faster? Or is my logic backwards? (figured it out)

- Last thing...can you have something that's sustainable but not renewable?

Thank you!!

[roly182]

Also for 3c(iii) why is it that when concentration is decreased for both forward and back reaction species, does the rate of the back reaction immediately drop more than the forward reaction.. is it because water is a reactant??

[chewybacca]

I have a question regarding the TSSM Unit 4 trial paper...which I don't understand.

Q3a) Answer True or False for each of the following statements:
....
20.0mL of 0.1M ethanoic acid can neutralize the same vol. of 0.2M NaOH as 20.0mL of 0.1M HCl

Their answer is True, but I don't see how that can make sense.
Wouldn't you need (either) more vol. or increased conc. of acid (HCl and C₂H₅OH) in both cases (so that it can reach the equivalence point and hence neutralize the base)? Or am I missing something here...

Thanks!

[Collin Li]

Is nuclear fission, by definition, the splitting of the nucleus - regardless of how it happens? For example, even if you ADD energy rather than produce it, is that nuclear fission?

Yep.

[bec]

I have a question regarding the TSSM Unit 4 trial paper...which I don't understand.

Q3a) Answer True or False for each of the following statements:
....
20.0mL of 0.1M ethanoic acid can neutralize the same vol. of 0.2M NaOH as 20.0mL of 0.1M HCl

Their answer is True, but I don't see how that can make sense.
Wouldn't you need (either) more vol. or increased conc. of acid (HCl and C₂H₅OH) in both cases (so that it can reach the equivalence point and hence neutralize the base)? Or am I missing something here...

Thanks!

I got this one wrong too, but I think it's because if you add base to a weak acid, it's effectively removing H+ ions (since it reacts with them), and so drives the ionization reaction of the acid forward. So, if you add a strong base to a weak acid, you make it behave as like a strong acid.

[chewybacca]

I have a question regarding the TSSM Unit 4 trial paper...which I don't understand.

Q3a) Answer True or False for each of the following statements:
....
20.0mL of 0.1M ethanoic acid can neutralize the same vol. of 0.2M NaOH as 20.0mL of 0.1M HCl

Their answer is True, but I don't see how that can make sense.
Wouldn't you need (either) more vol. or increased conc. of acid (HCl and C₂H₅OH) in both cases (so that it can reach the equivalence point and hence neutralize the base)? Or am I missing something here...

Thanks!

I got this one wrong too, but I think it's because if you add base to a weak acid, it's effectively removing H+ ions (since it reacts with them), and so drives the ionization reaction of the acid forward. So, if you add a strong base to a weak acid, you make it behave as like a strong acid.

Hmm...didn't think of it that way. I see what you mean, but then, why (esp. in titrations) would you need more mol of the acid (either incr. vol. or incr. conc.) to neutralise the base (as the base is at a higher conc. in their question)?
Odd question, nonetheless...

[shinny]

I have a question regarding the TSSM Unit 4 trial paper...which I don't understand.

Q3a) Answer True or False for each of the following statements:
....
20.0mL of 0.1M ethanoic acid can neutralize the same vol. of 0.2M NaOH as 20.0mL of 0.1M HCl

Their answer is True, but I don't see how that can make sense.
Wouldn't you need (either) more vol. or increased conc. of acid (HCl and C₂H₅OH) in both cases (so that it can reach the equivalence point and hence neutralize the base)? Or am I missing something here...

Thanks!

NaOH is a strong base, so regardless of whether you react it with a weak acid or a strong acid, it'll react to completion. Therefore since it'll ionise to the same extent, it doesn't matter whether it's a strong acid or weak acid that you're reacting against; it's just the amount of H+ that matters, and both acids have the same volume and concentration, hence same amount of H+.

- I'm not sure about Question 9. It asks for K of the "dissolution of the precipitate" and then in the solutions, states that the dissolution involves both reactions that are given. But...how was I to know that? I figured that it when from a solid to aqueous in the first equation, and so I thought that A was the correct alternative...

- Question 12: 1. What's an inverted U-tube? and 2. In the solutions they say that it can't be D because OH- would react with Fe2+. But, given their positions on the electrochemical series, I thought that they wouldn't spontaneously react?

- Question 2(b) (ii) I got 95.6%, is their solution of 85% incorrect? (I know a few other people got the same answer as me)

- Question 3c(iii)...why does the forward reaction get continuously slower? My graph was opposite to theirs, because I thought that since the forward reaction was favoured, it would also get faster? Or is my logic backwards? (figured it out)

- Last thing...can you have something that's sustainable but not renewable?

9. I second this question; anyone got any ideas? I figured I had to combine the two since the first reaction in itself doesn't seem like a dissolution given its negligible K value, but where'd the [AgCl] go in the final concentration fraction?

12. No idea about the inverted U-tube; don't think its really important. They stated its a salt bridge already. As for the OH and Fe reacting, they won't have a spontaneous redox reaction as you said, but they'll have a non-redox reaction to form a precipitate. I'm just not sure whether its (too lazy to latex) Fe+OH=FeOH2 or Fe+2OH=FeO2+H2. Someone clarify?

2b. Yeh I got 95.6%, they screwed up I'm pretty sure. Looks like their answers took the wrong figures.

Last question: None of our current sources of energy would be one without the other, but I'm pretty sure if we made some sort of synthetic source of energy (somehow), it could be sustainable but not renewable. That's just my guess.

[chewybacca]

I have a question regarding the TSSM Unit 4 trial paper...which I don't understand.

Q3a) Answer True or False for each of the following statements:
....
20.0mL of 0.1M ethanoic acid can neutralize the same vol. of 0.2M NaOH as 20.0mL of 0.1M HCl

Their answer is True, but I don't see how that can make sense.
Wouldn't you need (either) more vol. or increased conc. of acid (HCl and C₂H₅OH) in both cases (so that it can reach the equivalence point and hence neutralize the base)? Or am I missing something here...

Thanks!

NaOH is a strong base, so regardless of whether you react it with a weak acid or a strong acid, it'll react to completion. Therefore since it'll ionise to the same extent, it doesn't matter whether it's a strong acid or weak acid that you're reacting against; it's just the amount of H+ that matters, and both acids have the same volume and concentration, hence same amount of H+.

Yeah I know that, but you've still got more mol of OH- compared to H+ (regardless of strength of acid used), and therefore not neutralised (i.e. reaching equivalence point). Hence why I think that's false?
I could be wrong though...

[shinny]

It's not saying completely neutralise though. It's just asking if it can neutralise the same amount, so it doesn't matter which is in excess.

[Mao]

TSFX Question 9:

I see it as, since AgCl is a solid, it can't have a concentration. I'd think equilibrium involving solids + aqueous solutions would  more or less has to do with surface area than density?
Anyhow, C is the one that is "correct" and doesn't include [AgCl] (not A/D), and B doesn't deal with the dissolution at all.


TSFX Question 12 - inverted U tube is a glass-tube in the shape of a U, used in galvanic cells as a salt bridge [none of those dodgy paper stuff ]
basically, it's a tube filled with electrolyte, placed up-side down in the beakers. because there is no holes to let air in, it doesn't empty itself out into both beakers, and acts pretty much as a "liquid salt bridge" that supplies ions to both sides.
OH- is sometimes avoided in salt-bridge set-ups as they can form precipitates. Iron hydroxide is one i think.

2b ii, i got 95.6 as well.

[bec]

Thanks mao and shinjitsuzx...good luck for tomorrow everyone