VCE Chemistry Question Thread

[mary1911997]

Hi there! Happy to help, but please specify what exactly you're not getting with the above questions. Once you've described that, happy to help out indeed

i do not know which one is the right answer for both questions and need explanation of why is it so

[smiley123]

How does the type of electrode affect the reaction that occurs? I thought that to work out what reaction occurs at each electrode you have to split the solution into ions and compare with water on the electrochemical series. Is that not the case?

[zsteve]

need help please

5. A galvanic cell consists of one half cell that is made up of a copper electrode in a solution containing 1 M Cu2+ (aq) at 25 C.
Which one of the following could be used as the second half cell so that the polarity of the electrode in this second half cell is negative
   • A lead electrode in a solution of 1.0M Pb2+ (aq)
   • A silver electrode in a solution of 1.0M Ag+ (aq)
   • An inert electrode in a solution of 1.0M Fe2+ (aq) and 1M Fe 3+ (aq)
   • An inert graphite electrode in a solution of 1.0M Sn4+ (aq)
The second half cell has to be negative so it needs to be the anode. Looking at the electrochemical series, the only chemical on the list that is oxidised by Cu2+ is Pb(s)/Pb2+(aq)

6. The rechargeable lithium iron phosphate cell is used to power small appliances such as portable computers. When the cell is being charged, the electrode reactions are
   LiFePO4 ----->FePO4   +    Li+    +   e-
   Li+   +   e-   +   6C  ----->  LiC6
   Which one of the following occur
   I. LiC6 is produced at the positive electrode
   II. The concentration of Li+ in the electrolyte increases
   III. The direction of electron flow in the external circuit is from the anode to the cathode

Discharge reactions are the reverse of the given reactions; so the top reaction occurs at the positive terminal, whilst the bottom reaction at the negative terminal. Hence I is false, LiC6 is produced at the negative electrode.

Concentration of Li+ doesn't change, as Li+ produced by the top reaction is consumed by the bottom reaction.

Correct - anode and cathode switch during recharging (polarities remain the same)

[klippo]

Hi guys, would greatly appreciate if someone could answer these:
1. other than ionisation, dissociation and hydrolysis are there any other words that mean the same that could come up in the exam?
2. do we not consider equilibrium for strong acids? (ie single arrow?)
3. Find the pH of a 0.20 micromolar solution of HCl. Here do we assume that HCl completely ionises and hence [HCl]=[H₃O⁺]? Hence it's just a normal pH question rather than an acid-base equilibria question?
4. In most cases, why do we assume that [acid ion]=[H⁺]?
5. In what cases do we not make the assumption that [acid ion]=[H⁺] and that [acid initial]=[acid @ equilibrium]?
Thanks so much!

[vox nihili]

Hi guys, would greatly appreciate if someone could answer these:
1. other than ionisation, dissociation and hydrolysis are there any other words that mean the same that could come up in the exam?
2. do we not consider equilibrium for strong acids? (ie single arrow?)
3. Find the pH of a 0.20 micromolar solution of HCl. Here do we assume that HCl completely ionises and hence [HCl]=[H₃O⁺]? Hence it's just a normal pH question rather than an acid-base equilibria question?
4. In most cases, why do we assume that [acid ion]=[H⁺]?
5. In what cases do we not make the assumption that [acid ion]=[H⁺] and that [acid initial]=[acid @ equilibrium]?
Thanks so much!

1. Will leave this to someone else better acquainted with the course. Suffice to say hydrolysis, dissociation and ionisation are not all the same thing.
2. Correct
3. Yes, that's also correct. HCl is a strong acid.
4. Pretty sure that in VCE you'd always say this. The amount of conjugate base is equivalent to the amount of acid, because they are produced in equal proportions when an acid dissociates.
5. We assume that strong acids dissociate completely. Strictly speaking this is not actually true, but in VCE it is the assumption with which we work.

[Sine]

What do we need to know about primary and secondary cells? How do relate them to Daniell (Galvanic)Cells?

What sort of questions come up for primary and secondary cells?

[jyce]

What do we need to know about primary and secondary cells? How do relate them to Daniell (Galvanic)Cells?

What sort of questions come up for primary and secondary cells?

You should be able to:
- define and compare primary and secondary cells (it's a simple matter of non-rechargeable vs. rechargeable, respectively)
- explain what enables secondary cells to be recharged (your textbook should give a pretty good - and simple - explanation)
- apply your knowledge of the electrochemical series, anodes and cathodes, etc. to real-life examples of these cells (take a look at some VCAA exams)

How do you relate primary and secondary cells to galvanic cells? Primary and secondary cells are types of galvanic cells. They transform chemical energy directly to electrical energy via a spontaneous redox reaction. When a secondary cell is being recharged, though, the discharge reaction is reversed, becoming a non-spontaneous reaction, and so a secondary cell being recharged can be thought of as acting as an electrolytic cell. Electrolytic cells are in many ways the opposite of galvanic cells.

I can't give you any more specific advice, as your questions are fairly broad. Taking a look at some VCAA exams will give you an idea of what's expected of you, and you should definitely also have a read of the relevant section of the study design.

Let us know if you have any more questions 

Hi guys, would greatly appreciate if someone could answer these:
1. other than ionisation, dissociation and hydrolysis are there any other words that mean the same that could come up in the exam?
2. do we not consider equilibrium for strong acids? (ie single arrow?)
3. Find the pH of a 0.20 micromolar solution of HCl. Here do we assume that HCl completely ionises and hence [HCl]=[H₃O⁺]? Hence it's just a normal pH question rather than an acid-base equilibria question?
4. In most cases, why do we assume that [acid ion]=[H⁺]?
5. In what cases do we not make the assumption that [acid ion]=[H⁺] and that [acid initial]=[acid @ equilibrium]?
Thanks so much!

Just wanted to add on vox's response:

1. The reaction of a weak acid with water can be thought of as an ionisation reaction, a dissociation reaction and a hydrolysis reaction. But this does not mean that all these reactions types are the same thing! Sometimes a reaction can be classified as one or two of these things, but not all three. An ionisation reaction generally refers to when an electrically neutral compound reacts to form ions (e.g., the ionisation of an acid or base in water); a dissociation reaction is when a compound breaks apart in some way (e.g., the ionisation of an acid, but also the dissolution of a salt); a hydrolysis reaction refers to a reaction in which water is added to a substance, which often causes that substance to break apart into smaller components (e.g., the ionisation of an acid, but also the catabolism of biopolymers such as carbohydrates, proteins and nucleic acids).

4. We often assume that [conjugate base]_equilbrium = [H₃O⁺]_equilibrium for two reasons: (1) these two products are produced in a 1:1 mole ratio; (2) the self-ionisation of water occurs only to a very small extent. In reality, the concentration of hydronium ions is greater due to the self-ionisation of water, but this is so small that it can be considered negligible (i.e., can be ignored).

5.
If you want an example of when the assumption that [conjugate base]_equilibrium = [H₃O⁺]_equilibrium is not valid, take a look at VCAA 2011, Exam 2, Section B, Question 6bi. The concentration of methanoate ions becomes greater due to the addition of sodium methanoate.

If you want an example of when the assumption that [acid]_equilibrium = [acid]_initial is not valid, take a look at VCAA 2012 Exam 2, Section B, Question 3d - a notoriously difficult question. The assumption is not valid in this case because sorbic acid has a relatively high acidity constant for a weak acid, ionising closer to 50% as opposed to 0%, and because VCAA provided you with enough information to actually calculate [sorbic acid]_initial. VCAA explained that the fact that [sorbic acid]_initial was not already given in the question should have signalled to students not to make the assumption.

EDIT: Apparently I'm one of those people who can't use "your" and "you're" correctly 

[blacksanta62]

Yo, if we increase the pressure of the reaction vessel the system will try to oppose this by decreasing the pressure. But what happens if all reactants and products are (aq)?

Do I treat it the same way I would if all the reactants/products were (g)? I.e. system favours reaction which produces fewer particles.
Thank you

[jyce]

Yo, if we increase the pressure of the reaction vessel the system will try to oppose this by decreasing the pressure. But what happens if all reactants and products are (aq)?

Do I treat it the same way I would if all the reactants/products were (g)? I.e. system favours reaction which produces fewer particles.
Thank you

Aqueous species don't have a pressure; they have a concentration. Dilution results in a decrease in the overall solution concentration, which leads to a net reaction in the direction that produces more particles, in order to (partially) oppose the dilution.

[blacksanta62]

But in my Chen SAC there was a question, 2 actually which wanted us to write "forward, backward and no change" for 3 reactions, 2 of which were (AQ)
What is correct then?
Thanks

[jyce]

But in my Chen SAC there was a question, 2 actually which wanted us to write "forward, backward and no change" for 3 reactions, 2 of which were (AQ)
What is correct then?
Thanks

Um, then I guess an increase in "pressure" would lead to a net reaction in the direction with fewer particles, as with a gaseous system.

[blacksanta62]

Thanks for the reply jyce, it really stumped me and it's been on my mind all day. What's your opinion on this and have you ever seen this on a VCAA exam?
Thank you

[sweetiepi]

But in my Chen SAC there was a question, 2 actually which wanted us to write "forward, backward and no change" for 3 reactions, 2 of which were (AQ)
What is correct then?
Thanks

Um, then I guess an increase in "pressure" would lead to a net reaction in the direction with fewer particles, as with a gaseous system.

In my assessment, aqueous solutions had no change when the pressure was altered. Maybe this is the same the sac? (Assuming your teacher got the QATs SAC?)

[Elizawei]

Hey guys, for a MSDS for a school SAC, does it have to be as detailed as the ones used professionally? I've looked up a few online and they have 16 sub sections with 5k + words O.o


and on a side note, congratz on 200 posts insanipi <3

[sweetiepi]

Hey guys, for a MSDS for a school SAC, does it have to be as detailed as the ones used professionally? I've looked up a few online and they have 16 sub sections with 5k + words O.o

Hey! As far as I'm aware, most schools use a simple table with chemical names and you have to use a MSDS to find the safety stuff/risks and waste management.
and on a side note, congratz on 200 posts insanipi <3