VCE Chemistry Question Thread

[sunshine98]

Do we need to know about buffers?
Thanks 

[jyce]

Do we need to know about buffers?
Thanks 

Eh, VCAA could give you a question about buffers but I'd say that they'd provide you with sufficient information to answer the question if you didn't already know about buffers. Buffers are not specifically mentioned in the study design, so I doubt you'd be required to answer questions about buffers without sufficient background information being supplied in the question stem.

[Apink!]

For Chemistry exam Unit 4 2007:


Question 7b: I thought that hydrogen gas was the stronger reductant because it was oxidised and released 2 electrons, as then why would there be a continuous supply of the gas? Why is the answer Cd?

Question 7ciii). I said 0 because the product at the half cell 1 is actually X(s) forming onto the electrode, hence its an element so the oxidation number would be zero? I know the diagram says the electron flow is going to the right, so that means the X2+ is donating electrons to become X3+ so its oxidation state would be 3, but if you read the stem of the question, it says that the molarity of X2+ was decreasing, so wouldnt that mean that X(s) is forming on the electrode as the concentration of X2+ decreases?



Also when you have something large like my previous question with the methanol and a fatty acid, how do you know how to write the chemical equation? Like i wrote OCO instead of COO because thats the configuration, like in order of the bonds on the molecule itself?

Hi Cosine!
Sorry, I actually don't know the answers to your questions. Just really happy you're back online!

YAY!
 

Someone smarter than me, please help this guy out

[jyce]

For Chemistry exam Unit 4 2007:


Question 7b: I thought that hydrogen gas was the stronger reductant because it was oxidised and released 2 electrons, as then why would there be a continuous supply of the gas? Why is the answer Cd?

Question 7ciii). I said 0 because the product at the half cell 1 is actually X(s) forming onto the electrode, hence its an element so the oxidation number would be zero? I know the diagram says the electron flow is going to the right, so that means the X2+ is donating electrons to become X3+ so its oxidation state would be 3, but if you read the stem of the question, it says that the molarity of X2+ was decreasing, so wouldnt that mean that X(s) is forming on the electrode as the concentration of X2+ decreases?



Also when you have something large like my previous question with the methanol and a fatty acid, how do you know how to write the chemical equation? Like i wrote OCO instead of COO because thats the configuration, like in order of the bonds on the molecule itself?

These questions seem like something I'll need to take a close look at, and I'm a bit short on time at the moment. I'll get back to them shortly by editing this post - that is, unless someone else answers before then 

EDIT: Okay, here goes:

Question 7b
If the pH has increased in half-cell 1, then the concentration of hydrogen ions has decreased and so the following half-reaction has occurred: 2H⁺(aq) + 2e^- --> H₂(g). This is a reduction reaction, meaning that H⁺(aq) has acted as an oxidant. Therefore, the Cd(s) is a stronger reductant than H₂(g).

Question 7ciii
If X²⁺(aq) is losing electrons, how could it possibly become X(s)? If it's losing electrons, its positive charge is going to increase, NOT decrease. In part cii, you calculate the ratio of n(X²⁺)reacted: n(e^-) to be 1:1. Therefore, one X²⁺(aq) ion loses one electron, so it is oxidised to X³⁺(aq). Hence, the answer is +3.
I'm not sure why you think the fact that the molarity of X²⁺(aq) is decreasing indicates that X(s) has formed? The molarity of X²⁺(aq) decreasing simply means that X²⁺(aq) is reacting - in this case to form X³⁺(aq), NOT X(s).

In regards to correctly representing an ester linkage as 'COO' over 'OCO'. That 'O' you're writing first in 'OCO' is kind of a side chain, and so it would come after the carbon that it is attached to.

[j0yce]

Hi
Could someone please help me with question 7 2014 VCCA exam.
In part b it says calculate the percentage purity of the lump of impure copper. So i found out the mass of the copper that had been deposited on the pure copper plate and divided that by the total mass of the lump of the impure copper and then multiplied by 100. But in the answers it says that you divide by the change in the mass of the impure electrode? Why is this the case?

Thank you

[jyce]

Hi
Could someone please help me with question 7 2014 VCCA exam.
In part b it says calculate the percentage purity of the lump of impure copper. So i found out the mass of the copper that had been deposited on the pure copper plate and divided that by the total mass of the lump of the impure copper and then multiplied by 100. But in the answers it says that you divide by the change in the mass of the impure electrode? Why is this the case?

Thank you

- 10.30 - 0.855 = 9.445 kg of material was removed from the impure lump (i.e. the anode).
- Of this 9.445 kg, 8.25 kg was deposited on the cathode.
- This 8.25 kg is the copper that was removed from the lump.
- Therefore, in 9.445 kg of the lump, 8.25 kg of it is copper.
- Therefore, the percentage purity of the lump = 8.25/9.445 x 100 = 87.3%

I think you mean the 2013 VCAA exam by the way 

[j0yce]

- 10.30 - 0.855 = 9.445 kg of material was removed from the impure lump (i.e. the anode).
- Of this 9.445 kg, 8.25 kg was deposited on the cathode.
- This 8.25 kg is the copper that was removed from the lump.
- Therefore, in 9.445 kg of the lump, 8.25 kg of it is copper.
- Therefore, the percentage purity of the lump = 8.25/9.445 x 100 = 87.3%

I think you mean the 2013 VCAA exam by the way 

haha yep sorry meant 2013!
Thanks for the explanation i get it now! 
Hopefully wont get confused on things like this on the real exam!

[zsteve]

Quick terminology question:
When we refer to a system, exactly WHAT are we referring to?
Can I say a system is exothermic or endothermic? (Does it have those properties like a reaction) And can I say that a REACTION shifts left/right or should I say the SYSTEM shifts left/right?
Does a 'reaction' mean the entire reversible reaction comprised of both significant forwards and reverse reactions? Or can I only use it to specifically refer to the forwards and reverse reactions?
And how strict is VCAA on correct terminology (of this sort)?
Thanks!

[jyce]

Quick terminology question:
When we refer to a system, exactly WHAT are we referring to?
Can I say a system is exothermic or endothermic? (Does it have those properties like a reaction) And can I say that a REACTION shifts left/right or should I say the SYSTEM shifts left/right?
Does a 'reaction' mean the entire reversible reaction comprised of both significant forwards and reverse reactions? Or can I only use it to specifically refer to the forwards and reverse reactions?
And how strict is VCAA on correct terminology (of this sort)?
Thanks!

I wouldn't worry about ANY of this. They're practically interchangeable terms, although we tend to use 'system' when discussing a reaction that reaches an equilibrium. If you really wanted to make the distinction though, I suppose an equilibrium system involves both a forward reaction and backward reaction, one of which is exothermic and the other of which is endothermic.

[sunshine98]

So I thought I understood the whole energy profile thingo , because I get the whole explanation with bonds and stuff . But then there was a question on my SAC that looked at it from a kinetic and chemical energy perceptive. Obviously  got it wrong. Asked the teacher , she doesn't know how to explain it (which probs explains why she didn't teach it in the first place ) . Watched a vid on it , still don't understand. Can someone pls explain it?
Thanks 

[jyce]

So I thought I understood the whole energy profile thingo , because I get the whole explanation with bonds and stuff . But then there was a question on my SAC that looked at it from a kinetic and chemical energy perceptive. Obviously  got it wrong. Asked the teacher , she doesn't know how to explain it (which probs explains why she didn't teach it in the first place ) . Watched a vid on it , still don't understand. Can someone pls explain it?
Thanks 

I'm fairly certain an energy profile represents only how chemical energy changes over the course of a chemical reaction.
- The horizontal line at the beginning represents the stable chemical energy of the reactants.
- The rise up to the transition complex is where energy is being absorbed from the environment by the reactants to break bonds, so here chemical energy is being gained
- The peak is where chemical energy is greatest, it is where there are no bonds at all, and it is the least stable point
- The drop down represents energy being released to the environment as bonds form within the products, so here chemical energy is being lost
- The horizontal line at the end is the stable chemical energy of the products, once all the new bonds have formed.

[zsteve]

I'm fairly certain an energy profile represents only how chemical energy changes over the course of a chemical reaction.
- The horizontal line at the beginning represents the stable chemical energy of the reactants.
- The rise up to the transition complex is energy being absorbed from the environment by the reactants to break bonds, so chemical energy is being gained here
- The peak is where chemical energy is greatest, it is where there are no bonds at all, and it is the least stable point
- The fall down is energy being released to the environment as bonds form within the products, so chemical energy is being lost here
- The horizontal line at the end is the stable chemical energy of the products, once all the new bonds have formed.

On this note, is it sufficient to state that 'activation energy is the energy required to break bonds in the reactants' (pretty sure that is ok, but just checking). No need to mention 'transition complex' or stuff outside the course? (reason I'm asking is because I'm not sure if formation of some intermediate complex might require even MORE energy than simply breaking bonds - I don't know)

[jyce]

On this note, is it sufficient to state that 'activation energy is the energy required to break bonds in the reactants' (pretty sure that is ok, but just checking). No need to mention 'transition complex' or stuff outside the course? (reason I'm asking is because I'm not sure if formation of some intermediate complex might require even MORE energy)

A good definition of the activation energy would be "the MINIMUM amount of energy required to break bonds within the reactants so that the products may form". It is not simply "the energy required to break bonds in the reactants", as different bonds require different amounts of energy to break and the activation energy is the minimum amount required to break all bonds in the reactants so that the reaction may proceed. But no, you do not need to mention the transition complex at all.

EDIT: I forgot 'minimum'. This is important in a definition of the activation energy: if you don't write 'minimum', you might be suggesting that going over the activation energy will not result in reaction, which is not the case.

[sunshine98]

I'm fairly certain an energy profile represents only how chemical energy changes over the course of a chemical reaction.
- The horizontal line at the beginning represents the stable chemical energy of the reactants.
- The rise up to the transition complex is where energy is being absorbed from the environment by the reactants to break bonds, so here chemical energy is being gained
- The peak is where chemical energy is greatest, it is where there are no bonds at all, and it is the least stable point
- The drop down represents energy being released to the environment as bonds form within the products, so here chemical energy is being lost
- The horizontal line at the end is the stable chemical energy of the products, once all the new bonds have formed.

Thank you 
My bad, I should've said in relation to collision theory. This what the video said:
There is an initial conversion of kinetic energy into chemical energy when  breaking of bonds and then later a subsequent conversion of chemical energy into kinetic energy in the formation of new bonds.
Also , my teacher said at the peak of the graph(transition complex)  , some of the bonds of the reactants are broken and some have formed (for the products) . Is this wrong?
Also also ,  a HUGE THANK YOU to jyce , your explanations are seriously amazing !!!!! 

[jyce]

Thank you 
My bad, I should've said in relation to collision theory. This what the video said:
There is an initial conversion of kinetic energy into chemical energy when  breaking of bonds and then later a subsequent conversion of chemical energy into kinetic energy in the formation of new bonds.
Also , my teacher said at the peak of the graph(transition complex)  , some of the bonds of the reactants are broken and some have formed (for the products) . Is this wrong?
Also also ,  a HUGE THANK YOU to jyce , your explanations are seriously amazing !!!!!

Yes, the video sounds right. As for whether your teacher is right or not, I'm not completely sure to be honest. I've always understood that there's no bonds at the transition complex. I tried quickly Googling the answer just then, but to no success. But I wouldn't worry since I've never seen a VCAA question asking about this. All you need to know regarding this complex is that in order to reach it you need to achieve the activation energy.

And you are very welcome!!

EDIT: You know what? I actually think you're teacher is right. I was thinking about substitution reactions: in these reactions, you really only need to break one or a few bonds, so it wouldn't make sense that all the bonds break.