VCE Chemistry Question Thread

[Snorlax]

Begun to do Chem practice exam.
I know it's probably been asked a million times a year, but which papers are the best (for chem)?
Relative to the VCAA exams in terms of difficulty etc...

Thanks!

[DJA]

With reaction pathways for Chloroalkanes to Alcohols what are the required conditions and reagents?

All I know is that it requires:
-H2O
-A Catalyst

I would like to know if 'heating' is also a required condition.

Cheers!

[swagsxcboi]

With reaction pathways for Chloroalkanes to Alcohols what are the required conditions and reagents?

All I know is that it requires:
-H2O
-A Catalyst

I would like to know if 'heating' is also a required condition.

Cheers!

nah, heating isn't required.
alternatively you could have either NaOH or KOH and no catalyst is required

[DJA]

In Chromatography, What are TLC, HPLC and GC all best used for in terms of analysing different kinds of substances?

TLC:

HPLC: Extremely sensitive analysis

GC: Volatile compounds

Could you guys help me complete this? Any examples of different substances under each type would be really helpful!

Thanks

[lzxnl]

With reaction pathways for Chloroalkanes to Alcohols what are the required conditions and reagents?

All I know is that it requires:
-H2O
-A Catalyst

I would like to know if 'heating' is also a required condition.

Cheers!

Your reaction will be quite slow; water isn't the best reactant to bring out this reaction. The oxygen doesn't have a high enough negative charge to quickly react with a chloroalkane. With hydroxide, it has a larger negative charge to attack the more positive C-Cl carbon. Best to stick with KOH or NaOH as mentioned.

In Chromatography, What are TLC, HPLC and GC all best used for in terms of analysing different kinds of substances?

TLC:

HPLC: Extremely sensitive analysis

GC: Volatile compounds

Could you guys help me complete this? Any examples of different substances under each type would be really helpful!

Thanks

TLC just costs less and is very fast
GC can't be used for compounds that undergo thermal decomposition at higher temperatures (like larger organic molecules for instance). HPLC is ideal for separating similar compounds like toluene and benzene.

Your textbook will have some for these; I've forgotten these to be honest.

[swagsxcboi]

primary standards should have a high molar mass (>110g/mol?) to minimise weight errors.
What does this mean exactly? and is specifying >110g/mol necessary?

[jgoudie]

Stoic calculations are made using moles.  Consider the following examples:

HCl  - low molar mass (36.45)
Real answer: 12grams, you got 13grams

12/36.5 = 0.32
13/36.5 = 0.35
difference = 0.03mol

Na₂CO₃  - higher molar mass (106)
Real answer: 12grams, you got 13grams

12/106 = 0.11
13/106 = 0.12
difference = 0.01

You can see how even though you have the same mass difference in each case, your number of mol difference is much less for a higher molar mass.  This reduces the amount of error during stoic calculations.

No real need to specify what molar mass is needed, unless your teacher has said to do so.

primary standards should have a high molar mass (>110g/mol?) to minimise weight errors.
What does this mean exactly? and is specifying >110g/mol necessary?

[soNasty]

Why is it that an increase in temperature for an exothermic reaction causes K (eq constant) to decrease?
Conversely, why is does an increase in temperature for an endothermic reaction cause K to increase?

[lzxnl]

primary standards should have a high molar mass (>110g/mol?) to minimise weight errors.
What does this mean exactly? and is specifying >110g/mol necessary?

Let's pretend you used lithium fluoride as a primary standard. Its molar mass is 6.9 + 19 = 25.9 g/mol
Now let's say you had 259 mg of LiF, but your scale uncertainty was 5 mg. The uncertainty in the number of moles is then 5 mg/molar mass.
However, if your primary standard was KCl, for instance, with a molar mass of 39 + 35.5 = 74.5 g/mol, the numerical uncertainty in the number of moles would decrease given the same mass used.

Why is it that an increase in temperature for an exothermic reaction causes K (eq constant) to decrease?
Conversely, why is does an increase in temperature for an endothermic reaction cause K to increase?

There are a few ways to explain this.
1. Using the explicit dependence of the equilibrium constant on temperature. This is out of the course
2. Le Chatelier's principle (this IS in the course)

OK, so let's say you have A => B and this is exothermic. If we add heat, the system will attempt to partially negate this disturbance by reducing the system temperature. The backwards reaction does this, so the equilibrium shifts backwards, forming more reactant and less product and thus decreases K. Try use this logic for your endothermic reaction.

3. Considering the effects on the reaction rate. Heating a reaction flask actually increases the rate of the forward AND backward reaction. It can be proved that for an exothermic reaction, heating the reaction flask increases the forward reaction rate less than that of the backward reaction rate, and vice-versa for cooling. This is again out of the course.

[Blondie21]

What does the study design mean by

addiction reactions of alkenes (addition of hydrogen halides and water limited to symmetrical alkenes)

[OutstandingInDivination]

What does the study design mean by

addiction reactions of alkenes (addition of hydrogen halides and water limited to symmetrical alkenes)

addition reactions involving water with an alkene

addition reactions involving a hydrogen halide (eg HCl) with an alkene

[Yacoubb]

What does the study design mean by

addiction reactions of alkenes (addition of hydrogen halides and water limited to symmetrical alkenes)

Hydrogen halides:
When you react an alkene with H---X (where X is a halogen) to produce an alkyl halide.

Hydration:
Reacting an alkene with water, in the presence of phosphoric acid at 300 degrees Celcius, to produce an alcohol.

[lzxnl]

Asymmetric alkene => addition reaction of water to propene can yield either isomer of propanol (reaction conditions will determine which isomer; there are many ways of effecting this reaction). VCAA avoids this complication by saying 'symmetric' alkenes only.

[swagsxcboi]

For the colorimetric determination of phosphate in fertiliser, why is it that the % phosphorous is greater than that of the manufacture's specified %?  So far all I can think of is ions (other than phosphate) absorbing the light.

[lzxnl]

Just say you're reacting methanol and salicylic acid and form methyl salicylate and water yeah, and say that the yield of methyl salicylate is 80%.. What does the other 20% exist as?

Let's talk about molar quantities.
So you've formed 80% of the maximum possible amount of methyl salicylate. The 20% can be thought of as reacting back to form the original reactants. As the reaction here is 1 methanol + 1 salicylic acid => 1 water + 1 methyl salicylate, the number of moles of unreacted methanol is 1/4 (20%/80%) that of the methyl salicylate. Similar idea with the salicylic acid. Note that this calculation only gives the additional number of moles of methanol or methyl salicylate in conjunction with any excess reactant you may have had.