VCE Chemistry Question Thread

[swagsxcboi]

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Help with b ii) please

I understand that removal of products would have caused the back reaction to drop but why would the forward reaction rate decrease? Wouldn't it increase as the system shifts right to increase [products]?

I think there is an increase of products?
So, according to Le Chatelier's Principle, it shifts to the left (increasing the rate of back reaction and decreasing rate of forward reaction) as it partially opposes the effect of the change (increase of products) to decrease the amount of products.

[Robert123]

Yeah I am aware. I thought the 286kJ/mol was for the thermochemical equation 2H2 + O2 -> 2H2O instead of the equation H2 + 1/2O2 -> H2O

The thermochemical equations given in the data book are for the combustion of 1 mole of substance. So if you combust 1 mole of H2 you would release  286kj of energy, if you combust 1 mole of methane, you would release 889 kj of energy etc.

[Yacoubb]

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Help with b ii) please

I understand that removal of products would have caused the back reaction to drop but why would the forward reaction rate decrease? Wouldn't it increase as the system shifts right to increase [products]?

I was going to say a removal of reactants:
In order to partially oppose this decrease in reactant concentration, a net back reaction that results in the formation and subsequent increase in the concentration of the reactant will be favoured. This explains the increase in the back reaction and decrease in forward reaction until they eventually become equal (when equilibrium is established).

[psyxwar]

I was going to say a removal of reactants:
In order to partially oppose this decrease in reactant concentration, a net back reaction that results in the formation and subsequent increase in the concentration of the reactant will be favoured. This explains the increase in the back reaction and decrease in forward reaction until they eventually become equal (when equilibrium is established).

this doesn't explain the initial decrease in the back reaction

[ikiwi]

(Image removed from quote.)

Help with b ii) please

I understand that removal of products would have caused the back reaction to drop but why would the forward reaction rate decrease? Wouldn't it increase as the system shifts right to increase [products]?

-The removal of products causes less product particles to be available to react, therefore the rate of the back reaction decreases.
-As the forward reaction is now higher than the back reaction, there is a net forward reaction.
-According to Le Chatelier's principle, the rate of the back reaction increases after the initial decrease to partially oppose the decrease in reaction rate.
-As the system is re-establishing equilibrium, the forward reaction is always higher than the back reaction so there is a net forward reaction.
-As the change can only be partially opposed, the rate of the forward reaction decreases until the forward and back reactions occur at equal rate at equilibrium.

[Yacoubb]

this doesn't explain the initial decrease in the back reaction

Ah okay I see! Thanks for that!

[0KxBaNa2]

this doesn't explain the initial decrease in the back reaction

well according to the solutions: the reverse reaction rate decreases due to a drop in [products] then increases as the system returns to equilibrium.

...so no explanation as to why the forward reaction decreases :S but then again that wasn't really required in the answer - I'm just asking cos I'd like a better understanding of rates

[0KxBaNa2]

-The removal of products causes less product particles to be available to react, therefore the rate of the back reaction decreases.
-As the forward reaction is now higher than the back reaction, there is a net forward reaction.
-According to Le Chatelier's principle, the rate of the back reaction increases after the initial decrease to partially oppose the decrease in reaction rate.
-As the system is re-establishing equilibrium, the forward reaction is always higher than the back reaction so there is a net forward reaction.
-As the change can only be partially opposed, the rate of the forward reaction decreases until the forward and back reactions occur at equal rate at equilibrium.

Thanks ikiwi! That was the bit I didn't quite get

[Observer22]

Can anyone tell me the essential skills for units 3/4. You see I didn't do unit's 1/2, and I'm picking chem up for year 12. Most likely not the smart decision but I'm sticking to it.
I have started studying units 1/2. I really just need to know the specifics of what carries over? The obvious stuff like the mole and the related equations, i have already learnt.

What do i need to know, specifically?
Thanks!!!

[knightrider]

How would you do this question?

A solution of sodium hydroxide (NaOH) has a pH of 10.  10mL of this solution is mixed with 990ml of water. The pH of the diluted solution is closest to:
A) 8
B) 9
C) 11
D) 12
pls include working

[Mieow]

Is it advisable to answer wordy "describe" or "explain" questions in dot points, or are full sentences required?

[lzxnl]

How would you do this question?

A solution of sodium hydroxide (NaOH) has a pH of 10.  10mL of this solution is mixed with 990ml of water. The pH of the diluted solution is closest to:
A) 8
B) 9
C) 11
D) 12
pls include working

Final solution = 1000 mL
100 fold dilution => [OH-] drops by factor 100 => [H+] increases by factor 100 => pH drops by 2 => final pH = 8

Is it advisable to answer wordy "describe" or "explain" questions in dot points, or are full sentences required?

I always used full sentences

Can anyone tell me the essential skills for units 3/4. You see I didn't do unit's 1/2, and I'm picking chem up for year 12. Most likely not the smart decision but I'm sticking to it.
I have started studying units 1/2. I really just need to know the specifics of what carries over? The obvious stuff like the mole and the related equations, i have already learnt.

What do i need to know, specifically?
Thanks!!!

You'd need to know the organic chemistry section (like, ALL of it from 1/2), acid-base chemistry, pH, redox chemistry, oxidation numbers, electrochemical series stuff, balancing half-equations, stoichiometry, concentrations, concept of the mole, ideal gas law, intermolecular forces (e.g. hydrogen bond, dispersion, dipole-dipole etc), intramolecular forces (ionic, covalent bonds)...anything I've left out guys?

[AngelWings]

Is it advisable to answer wordy "describe" or "explain" questions in dot points, or are full sentences required?

I've heard from my teacher and a lecturer that they don't actually mind if you use dot points, against what lzxnl has said.  (The latter said he prefers it, but that's his opinion.)
I mean, it's direct, it's easy and it's faster.
Then again, sentences have their own benefits too...

[Yacoubb]

I've heard from my teacher and a lecturer that they don't actually mind if you use dot points, against what lzxnl has said.  (The latter said he prefers it, but that's his opinion.)
I mean, it's direct, it's easy and it's faster.
Then again, sentences have their own benefits too...

I think it would depend on the situation. I personally prefer dot points too because I think it makes putting you answer down in accordance to the mark allocation/space available much better, but sometimes both are good. For instance, you would really use dot points to highlight the difference between A and B.

[LiquidPaperz]

Just wondering with chem how you guys think the way to succeed is?

im doing methods, eng, spesh etc this year and been getting 95+s, however for chem its my worst subject usually around low 80s - mid 90s, but mostly about 87s etc. i really want to pick this standard up before next year, since i really like chem but i find most of the time im sitting their using the same formalised systematic approach of using formulas without knowing what it means, like moles etc i just use the formula , gas laws PV =nRT , again i just use formulas and if somethings missing, use stoich to get it. Unlike maths where you actually will probably need to know the stuff, and the info is just given to you without any strings attached. for example chem, this is soluble except with x and y, it has oxidisation number of -1 except with x and y etc, this stuff i hate. How can i improve?