VCE Chemistry Question Thread

[persistent_insomniac]

For qs 2a from last year's exam (attatched) is the answer suggesting 2 reasons for why increasing temperature will increase the rate, so:
1) increasing temperature increases the energy of all particles which results in more collisions and more successful collisions in a given time
2) increasing temperature increases the kinetic energy of all particles which means more particles are able to overcome the Ea barrier, increasing no. of collisions and no. of successful collisions in a given time?

[xxxjss]

For qs 2a from last year's exam (attatched) is the answer suggesting 2 reasons for why increasing temperature will increase the rate, so:
1) increasing temperature increases the energy of all particles which results in more collisions and more successful collisions in a given time
2) increasing temperature increases the kinetic energy of all particles which means more particles are able to overcome the Ea barrier, increasing no. of collisions and no. of successful collisions in a given time?

Yeah in essence temperature affects both the frequency and proportion of successful collisions; proportion just is recognised as having a more significant impact
For 2) I would explicitly mention a higher proportion of particles with enough energy to overcome Ea

[persistent_insomniac]

Yeah in essence temperature affects both the frequency and proportion of successful collisions; proportion just is recognised as having a more significant impact
For 2) I would explicitly mention a higher proportion of particles with enough energy to overcome Ea

Ok thankyou!

[rgversaGRa]

"Hey guys,

when turning ethene into ethane, what is the catalyst used in what state?
Also, to what extent do we need to know about monomers and polymers?"
 
Most sutaible catalysts for the reaction ethene to ethane are mainly metal catalysts for e.g platinum (pt) and nickel (Ni)

[Bri MT]

For qs 2a from last year's exam (attatched) is the answer suggesting 2 reasons for why increasing temperature will increase the rate, so:
1) increasing temperature increases the energy of all particles which results in more collisions and more successful collisions in a given time
2) increasing temperature increases the kinetic energy of all particles which means more particles are able to overcome the Ea barrier, increasing no. of collisions and no. of successful collisions in a given time?

The phrasing of your answer is slightly wrong here which could cost you marks. I have edited accordingly:
1) increasing temperature increases the energy of all particles which results in more collisions and thus/therefore/consequently more successful collisions in a given time period
2) increasing temperature increases the kinetic energy of all particles which means more particles are able to overcome the Ea barrier, increasing no. of collisions and the proportion of collision which are successful and thus the no. of successful collisions in a given time period

[rgversaGRa]

Hey guys
Just wondering  if  i can use a mechanical pencil throughout the chemistry exam,usually  fix my sig figs and calcs after i have left over time.

[KiNSKi01]

Came across an MCQ on benzene..
Does it have double bonds or not?

Question was like which statement is correct about C6H6:
Correct answer was "The compound can undergo substitution reactions because there are no carbon-carbon double bonds in the molecule'

I picked the option 'The compound can undergo addition reactions because three carbon-carbon double bonds are present in the molecule'

[Bri MT]

Hey guys
Just wondering  if  i can use a mechanical pencil throughout the chemistry exam,usually  fix my sig figs and calcs after i have left over time.

I love mechanical pencils but I personally wouldn't risk the chance of it not being picked up by the scanner. Cross out working and replace it if you need to.

Came across an MCQ on benzene..
Does it have double bonds or not?

Question was like which statement is correct about C6H6:
Correct answer was "The compound can undergo substitution reactions because there are no carbon-carbon double bonds in the molecule'

I picked the option 'The compound can undergo addition reactions because three carbon-carbon double bonds are present in the molecule'

That's a bit of a yucky question.

You certainly can add hydrogens to it (e.g. to make cyclohexane) but I also wouldn't feel confident saying that benzene has 3 double bonds either since it's more like there's 1.5 bonds between each of the carbons due to the delocalised electrons being shared around the whole ring. The high stability of the benzene ring means that it can preferentially undergo substitution reactions rather than addition reactions and in the end I'd go with their answer over yours.

[KiNSKi01]

That's a bit of a yucky question.

You certainly can add hydrogens to it (e.g. to make cyclohexane) but I also wouldn't feel confident saying that benzene has 3 double bonds either since it's more like there's 1.5 bonds between each of the carbons due to the delocalised electrons being shared around the whole ring. The high stability of the benzene ring means that it can preferentially undergo substitution reactions rather than addition reactions and in the end I'd go with their answer over yours.

Ok thank you for explaining! 

[3086]

If anyone wants to figure out quickly how many double bonds a compound has (maybe for a fatty acid or to distinguish between isomers perhaps) the following is a great reference:
2(Num of carbon atoms in single bonds+1)-(Num of hydrogen atoms in the whole molecule) /2

So for example: C17H33COOH (we know it has one double bond)
So according to the equation: 2(17+1)-34/2 = 1 (That's what we wanted!)

[jnlfs2010]

If anyone wants to figure out quickly how many double bonds a compound has (maybe for a fatty acid or to distinguish between isomers perhaps) the following is a great reference:
2(Num of carbon atoms in single bonds+1)-(Num of hydrogen atoms in the whole molecule) /2

So for example: C17H33COOH (we know it has one double bond)
So according to the equation: 2(17+1)-34/2 = 1 (That's what we wanted!)

Thanks for the tip!

Question, do we need to know extraneous variables in analysis of experimental design and validity?

[Matthew_Whelan]

Thanks for the tip!

Question, do we need to know extraneous variables in analysis of experimental design and validity?

The study design does not list it as key knowledge although it doesn’t hurt to know it anyway.

[jnlfs2010]

The study design does not list it as key knowledge although it doesn’t hurt to know it anyway.

Thanks for the help

[Erutepa]

Thanks for the tip!

Question, do we need to know extraneous variables in analysis of experimental design and validity?

Yes you do need to know about this.
While it is not listed under the AOS's for Unit 3/4 chemistry - it is listed under the key science skills which apply to all of units 3 and 4 (these are a list of skills found at the start of the study design).
I have included the relevant section which states that students will be expected to know how to:

[jnlfs2010]

Yes you do need to know about this.
While it is not listed under the AOS's for Unit 3/4 chemistry - it is listed under the key science skills which apply to all of units 3 and 4 (these are a list of skills found at the start of the study design).
I have included the relevant section which states that students will be expected to know how to:
(Image removed from quote.)

Alright thanks got it