VCE Chemistry Question Thread

[Crowley99]

Polar solvent: so the more polar molecule will move faster (elute first)

And peak area is proportional to concentration, so the larger the peak area, the more compound, and vice versa.

Does that help?

i feel so dumb, makes sense, thanks.

[kalopsia]

Hi,

When writing equations for organic pathways, eg. H₂C=CH₂ + Cl₂ →  CH₂ClCH₂Cl   or 
 CH₃CH₂CH₂OH + CH₃CH₂COOH → CH₃CH₂COOCH₂CH₂CH₃ + H₂O what states should they be?

And what about food chem? eg. a reaction between fatty acids and glycerol?

[gemmaruffin]

Do we have to know about gravimetric analysis equations and calculations? Or have I just missed a massive part of the course...

[Pinkydoo]

Do we have to know about gravimetric analysis equations and calculations? Or have I just missed a massive part of the course...

Pretty sure it's out of the new study design

[gemmaruffin]

Pretty sure it's out of the new study design

so i don't have to learn it? Good
Also acidity constants and whatever K_a means....

[-273.15]

HELLO
does a time given in a question count for sig figs?
So if you have 60 minutes in the question, does that automaticallly mean your answer has to be 2 sig figs too?
For some reason ive always ignored the time for sig figs and taken the lowest from another value that has been measured but in a companys answers to the sample exam they took 60 minutes as 2 sig figs???
Please help

thank you

[Bri MT]

HELLO
does a time given in a question count for sig figs?
So if you have 60 minutes in the question, does that automaticcaly mean your answer has to be 2 sig figs too?
For some reason ive always ignored the time for sig figs and taken the lowest from another value that has been measured but in a companys answers to the sample exam they took 60 minutes as 2 sig figs???
Please help

thank you

If time influences the results, then you have to take this into consideration when calculating sig figs

[zxcvbnm18]

Hi is mcq question 20 on the northern hemisphere exam still relavant to the new studydesign?

[LPadlan]

If a cell is being recharged, why does the anode still send electrons to the cathode? Shouldn't the anode be recharging it's electric charge?
I know the anode is always oxidation and cathode always reduction, but this kinda confuses me.

[Syndicate]

If a cell is being recharged, why does the anode still send electrons to the cathode? Shouldn't the anode be recharging it's electric charge?
I know the anode is always oxidation and cathode always reduction, but this kinda confuses me.

When a cell is recharging, the opposite reaction is happening to when it was discharging. So oxidation occurs at the positive electrode, whilst reduction occurs at the negative one. As you said, oxidation always occurs at the anode and vice versa... we think of the positive electrode as the anode in this case (because oxidation is occurring here), and the negative one as the cathode (reduction is occurring here).

[keltingmeith]

Hi,

When writing equations for organic pathways, eg. H₂C=CH₂ + Cl₂ →  CH₂ClCH₂Cl   or 
 CH₃CH₂CH₂OH + CH₃CH₂COOH → CH₃CH₂COOCH₂CH₂CH₃ + H₂O what states should they be?

And what about food chem? eg. a reaction between fatty acids and glycerol?

You may need to refer to your data book, you might find some of these mentioned there. When in doubt, just say aq.

Although, never put aq for anything fatty. They should be l or g. You might even get told by the question which they should be - eg, "the student adds 5 drops of glycerol", clearly a liquid.

so i don't have to learn it? Good
Also acidity constants and whatever K_a means....

Ka is exactly the same as K. It's not explicitly mentioned in the study design, but you should still be able to do any question in regards to it.

Hi is mcq question 20 on the northern hemisphere exam still relavant to the new studydesign?

I mean, it's a pretty standard concentration question, don't see why it wouldn't be.

[usernameincorrect]

Can someone please explain this to me?

N2(g) + 3H2(g) ⇌ 2NH3(g) enthalpy change is -92kj/mol
Question 11
If an inert gas is added to the equilibrium system at a constant temperature and a constant volume, the concentration of H2 in the mixture will
A. increase.
B. decrease.
C. not change.
D. decrease then increase.
 
Answer is C, but I thought that adding an inert gas will increase pressure in the container, and so drive Eq to the right with fewer moles? Feel like I'm missing something simple... Cheers

[lzxnl]

If a cell is being recharged, why does the anode still send electrons to the cathode? Shouldn't the anode be recharging it's electric charge?
I know the anode is always oxidation and cathode always reduction, but this kinda confuses me.

I wrote this four years ago; might be of some use to you now.
https://atarnotes.com/forum/index.php?topic=148802.0

Can someone please explain this to me?

N2(g) + 3H2(g) ⇌ 2NH3(g) enthalpy change is -92kj/mol
Question 11
If an inert gas is added to the equilibrium system at a constant temperature and a constant volume, the concentration of H2 in the mixture will
A. increase.
B. decrease.
C. not change.
D. decrease then increase.
 
Answer is C, but I thought that adding an inert gas will increase pressure in the container, and so drive Eq to the right with fewer moles? Feel like I'm missing something simple... Cheers

Pressure itself doesn't really matter here. What we care about is the partial pressure of the individual reactants/products, i.e. the pressure that the nitrogen, hydrogen or ammonia molecules exert. From the perspective of kinetic theory, the presence of an inert gas will simply cause the nitrogen, hydrogen and ammonia molecules to make more collisions with inert gas molecules, but this acts as 'randomising' or mixing up their paths even more. As we assume that particles move randomly anyway to begin with, further randomising their velocities and positions with inert (energy-conserving) collisions won't affect the chemical reaction.

However, introducing more hydrogen gas, for instance, will increase the rate of nitrogen-hydrogen collisions, which DOES affect the reaction. Collisions between nitrogen and, say, helium, doesn't do anything.

[usernameincorrect]

Pressure itself doesn't really matter here. What we care about is the partial pressure of the individual reactants/products, i.e. the pressure that the nitrogen, hydrogen or ammonia molecules exert. From the perspective of kinetic theory, the presence of an inert gas will simply cause the nitrogen, hydrogen and ammonia molecules to make more collisions with inert gas molecules, but this acts as 'randomising' or mixing up their paths even more. As we assume that particles move randomly anyway to begin with, further randomising their velocities and positions with inert (energy-conserving) collisions won't affect the chemical reaction.

However, introducing more hydrogen gas, for instance, will increase the rate of nitrogen-hydrogen collisions, which DOES affect the reaction. Collisions between nitrogen and, say, helium, doesn't do anything.

Ahh ok, thanks  but now I'm a bit confused about the case with adding an inert gas, but with constant pressure. How is this different? Will this cause the volume to increase and concentrations of all gases to drop (equally?) and what happens from here?

[lzxnl]

Ahh ok, thanks  but now I'm a bit confused about the case with adding an inert gas, but with constant pressure. How is this different? Will this cause the volume to increase and concentrations of all gases to drop (equally?) and what happens from here?

Constant what pressure? Total gas pressure? Think of gas pressure as proportional to the number of particles you have. If you want to keep this constant (assuming constant volume and temperature), you have to release other gas particles. There's a difference between the total gas pressure and the partial pressure of one of the gases in the mixture.