VCE Chemistry Question Thread

[Shadowxo]

Thanks. Do you know how to do this question? When 1.0 g of ethene undergoes complete combustion, what mass of water vapour is produced? I keep on getting 1.3 g, but the answer says 3.6 g. What am I doing wrong?

Yep, you're right
The equation is \(\ce{C2H4 + 3O2 -> 2H2O + 2CO2}\) (with states)
n(C2H4) = 1.0/28.0 = 0.036 mol
n(H2O)= 2*n(C2H4) = 0.071 mol
m(H2O)=n*M= 0.071*18.0 = 1.3g

[zhen]

Ok, thanks Syndicate and Shadowxo 

[deStudent]

Often in multiple choice they try to "trick" you, especially like this.
C just says number of collisions, and overall there are the same number of collisions in both scenarios - same number of reactions overall, which is why it's "wrong" even though it implies more collisions in a given period of time.
D could have been worded better - the "than" shouldn't be there. But they want you to notice the concentration increases the rate of reaction due to the increased frequency of collisions. It should have been worded better though

Yeah thanks haha.

--
http://m.imgur.com/a/swGM7
Part c) I talked about the temperature here, apparently this wasn't what the answer wanted.
Anyway, when the answer says that a decreased air pressure means water boils at a lower temperature. Is this just a general fact we should know because I can't find it in the collision theory section?

[Shadowxo]

Yeah thanks haha.

--
http://m.imgur.com/a/swGM7
Part c) I talked about the temperature here, apparently this wasn't what the answer wanted.
Anyway, when the answer says that a decreased air pressure means water boils at a lower temperature. Is this just a general fact we should know because I can't find it in the collision theory section?

I think it's just general knowledge, I don't think you'd have to know that for the end of year exam but I'd check with your teacher just to make sure

[deStudent]

Thanks Shadowxo
--
http://m.imgur.com/a/kUR7G For Q7b) just to double check, would my working be correct? The answer didn't show working,

[Shadowxo]

Thanks Shadowxo
--
http://m.imgur.com/a/kUR7G For Q7b) just to double check, would my working be correct? The answer didn't show working,

Yep your working is fine

[monashvivek]

Am doing unit 1 and 2 chemistry, and i badly want advice to improve my chemistry skills

[Quantum44]

I literally can't get my head around this. Apparently when you form a chemical bond it releases energy? Last year in bio we learnt that chemical energy was stored in bonds, which makes a lot of sense given how ATP works but now that's wrong apparently. Can someone please help explain this.

[sweetiepi]

Am doing unit 1 and 2 chemistry, and i badly want advice to improve my chemistry skills

Hey! I have a few tips in order to improve your chemistry skills:
1. Summarise your notes (trust me, this helps)
2. Try out some practice questions from 1/2 exams, ask your teacher for some.
3. Try out checkpoints, they are really good for following the study design with regards to exam questions.
4. Ask your teacher for clarification/help when you need it.
5. You can always ask your questions here!

I literally can't get my head around this. Apparently when you form a chemical bond it releases energy? Last year in bio we learnt that chemical energy was stored in bonds, which makes a lot of sense given how ATP works but now that's wrong apparently. Can someone please help explain this.

Hey!
Excuse me if I'm a bit rusty but here goes nothing:
In order for a chemical reaction to occur, you must have a minimum amount of energy (also known as the activation energy, or E_A), so the reactants react.

In endothermic reactions, a small bit of energy is released, as it's more than enough energy for the reaction to happen, although most of the energy created in the reaction gets absorbed, as it is endothermic. (Endothermic reactions absorb energy into its system)(Its \(\Delta H\) is positive energy is sucked in)

In exothermic reactions, the activation energy gives enough energy for the reaction to commence, however instead of absorbing the energy, it releases the energy, so it gives off heat (generally), therefore the \(\Delta H\) is negative- hence a release of energy. An example of this is the combustion of a fuel.

(Let me know if I got a concept wrong)

[deStudent]

For this question http://m.imgur.com/xnjP6wE
What's the correct method? I can't use mole ratios because I can't form a balanced chemical equations, I had: C + O2 -> CO2 + H2O. I can't use n = V/V_m because I don't know the volume which also means PV = nRT can't work.

Thanks

[Shadowxo]

For this question http://m.imgur.com/xnjP6wE
What's the correct method? I can't use mole ratios because I can't form a balanced chemical equations, I had: C + O2 -> CO2 + H2O. I can't use n = V/V_m because I don't know the volume which also means PV = nRT can't work.

Thanks

Hint:
C(s) + O2(g) -> CO2(g) is the equation
It gives you the mass of coal, and says carbon is 25% of it. So you know mass of carbon therefore you know the number of moles of carbon and CO2

[gameboy99]

Don't get how to do Q.13 a)  https://imgur.com/gallery/mEu3X
Can someone help?
Thanks

[Shadowxo]

Don't get how to do Q.13 a)  https://imgur.com/gallery/mEu3X
Can someone help?
Thanks

You know the molecule is K₂Sn(OH)₆, K is 1+ (K+), so Sn(OH)₆ is 2-, OH is 1- (OH-), so you know Sn is 4+ (4-6= -2)

[gameboy99]

You know the molecule is K₂Sn(OH)₆, K is 1+ (K+), so Sn(OH)₆ is 2-, OH is 1- (OH-), so you know Sn is 4+ (4-6= -2)

The answer for 13 a) , however is +4 charge for the tin ion.

[Shadowxo]

The answer for 13 a) , however is +4 charge for the tin ion.

Like I said, the Sn (tin) is 4+, aka +4