VCE Chemistry Question Thread

[turtlebanana]

You have to draw a line back from the max temp to when the current was turned on to account for heat loss. This is the absolute maximum temperature change.
This diagram explains it

What do you mean from max temp? Isn't the line extrapolated back from the last temperature recorded? (so in this case from approximately the dot where it is at 450 seconds as shown in diagram)

[sk2000]

What do you mean from max temp? Isn't the line extrapolated back from the last temperature recorded? (so in this case from approximately the dot where it is at 450 seconds as shown in diagram)

Using the extrapolated line is an overestimate because it is based on the final rate of heat loss and it assumes a constant rate of heat loss during heating. However the initial rate of heat loss was zero, and the rate of heat loss would increase as the temperature rises. So the change in temp would be less than the 4.4 degrees from the diagram.

[KiNSKi01]

I have been given inconsistent info on zwitterions

Using aspartic acid as an example

Is it when the amino acid has an overall charge of zero

OR

 is it when the amine groups and carboxyl groups have accepted / donated hydrogen atoms (thus giving an overall charge of -1)

[sweetiepi]

I have been given inconsistent info on zwitterions

Using aspartic acid as an example

Is it when the amino acid has an overall charge of zero

OR

 is it when the amine groups and carboxyl groups have accepted / donated hydrogen atoms (thus giving an overall charge of -1)

From my understanding, a zwitterion has an overall charge of zero- as the \(\ce{COO-}\) group and the \(\ce{NH3+}\) are oppositely charged, and the charges cancel out. Although there are two charged parts to a zwitterion, there is an overall net zero charge. The amino acid becomes zwitterionic once an internal transfer (the H of the carboxyl is donated and the amino group receives it) is completed.

I hope this clarifies

[KiNSKi01]

Yeah that's what I thought 

Came across a TSSM q asking what the overall charge of the zwitterion of glutamine is and the answer was +1 

[turtlebanana]

Question about significant figures!

For this VCAA question (attached below), you round your final answer to the number with least significant figures in the question right? In this case, it is 3 significant figures, so why does VCAA round it down to 2? (i.e. answer is 6.9 kg instead of 6.92 kg)

Also, i initially used PV = nRT, but when i calculated the mols (n), i used H instead of H₂. Do you just assume that its H₂ because in the question it says hydrogen gas at the beginning?

 

[KiNSKi01]

When discussing why electrodes may be porous what are the most important factors

Is it to increase rate of reaction by maxamising surface area/catalyst or should we also mention that it allows for gaseous reactants to interact with the electrode (or I have got this wrong- I thought the only way gaseous reactants can take part in galvanic cells is if the electrodes are porous)

[KiNSKi01]

Question about significant figures!

For this VCAA question (attached below), you round your final answer to the number with least significant figures in the question right? In this case, it is 3 significant figures, so why does VCAA round it down to 2? (i.e. answer is 6.9 kg instead of 6.92 kg)

Also, i initially used PV = nRT, but when i calculated the mols (n), i used H instead of H₂. Do you just assume that its H₂ because in the question it says hydrogen gas at the beginning?

 

It appears they didn't take marks off for sig figs anyway

[Matthew_Whelan]

Question about significant figures!

For this VCAA question (attached below), you round your final answer to the number with least significant figures in the question right? In this case, it is 3 significant figures, so why does VCAA round it down to 2? (i.e. answer is 6.9 kg instead of 6.92 kg)

Also, i initially used PV = nRT, but when i calculated the mols (n), i used H instead of H₂. Do you just assume that its H₂ because in the question it says hydrogen gas at the beginning?

 

In chem you usually answer to the smallest decimal place. The question had each value to 1decimal place so the answer was to one decimal place. Physics goes by sig figs though, I’m not really sure why they’re different.

[turtlebanana]

In chem you usually answer to the smallest decimal place. The question had each value to 1decimal place so the answer was to one decimal place. Physics goes by sig figs though, I’m not really sure why they’re different.

Are you sure it's to the smallest decimal place? I've usually always rounded it to the least significant figures.

They say this in their website:
Calculations in chemistry often involve numbers having different numbers of significant figures. In mathematical operations involving:

addition and subtraction, the student should retain as many digits to the right of the decimal as in the number with the fewest significant digits to the right of the decimal, for example: 386.38 + 793.354 - 0.000397 = 1179.73
multiplication and division, the student should retain as many significant digits as in the number with the fewest significant digits, for example: 326.95 x 10.2 ÷ 20.322 = 164.

If you're talking about addition/subtraction, then you round it to the number with the least decimal places.

However, in my question before, i multiplied so shouldn't it be to to the least significant figures?

[Matthew_Whelan]

I don’t think it’s too important, 6.92 and 6.9 are both correct so the examiner would have to be pretty callous to not give the mark. I could be wrong though...

[turtlebanana]

Thanks,

Question about titration! (asking this again cos didn't get an answer last time  )

Do we have to know what is the ''most suitable indicator'' for different reactions?

For example:
The most suitable indicator for a titration of NaOH against benzoic acid, C₆H₅COOH, is:
A. bromophenol blue
B. methyl orange
C. thymol blue
D. phenol red

I don't recall learning how to do this in class. If this is still in the study design, how do we know which indicator to use? How do we know if a certain compound is a ''weak'' or ''strong'' acid/base?

[Bri MT]

Thanks,

Question about titration! (asking this again cos didn't get an answer last time  )

Do we have to know what is the ''most suitable indicator'' for different reactions?

For example:
The most suitable indicator for a titration of NaOH against benzoic acid, C₆H₅COOH, is:
A. bromophenol blue
B. methyl orange
C. thymol blue
D. phenol red

I don't recall learning how to do this in class. If this is still in the study design, how do we know which indicator to use? How do we know if a certain compound is a ''weak'' or ''strong'' acid/base?

In previous study designs it was expected for students to know common strong & weak acids and bases but since pH isn't part of units 3&4 I don't see how they could ask you that now.

Yeah that's what I thought 

Came across a TSSM q asking what the overall charge of the zwitterion of glutamine is and the answer was +1 

The reason they would have done this is because of the R group

[KiNSKi01]

The reason they would have done this is because of the R group

So zwitterions don't necessarily always have an overall charge of zero? Cos I've been taught if we are to draw out the zwitterion of an amino acid (one that has either a basic or acidic group) we ensure that it still has an overall charge of zero

[Flyfishy]

can someone real quick explain the bromine test and how it can be used to distinguish alkenes and alkanes

ty