VCE Chemistry Question Thread

[josh92012]

Can someone explain to me how to draw molecules with the valence bond structure.

All I know is that 1 bond = linear, 2 bonds = 'V' shaped, 3 bonds = pyramid and 4 bonds = tetrahedron. I'm not sure if this is correct but CO2 doesn't follow this when it has 2 double bonds. Are there any rules I'm missing?

The way i was taught is that you just have to rope learn which molecules are what shape of molecule, however i could be completely wrong and there could be a pattern and CO2 has 2 double bonds and its linear.

The 2 oxygen atoms want to get "as far away from" each other as possible and carbon is using all of its electrons in the covalent bond, therefore the oxygen atom will be opposite each other, making them linear.

[josh92012]

How can you explain why aluminium is lustrous by referring to the structure of the metals and the bonding that occurs within metal aluminium?

[Maz]

How can you explain why aluminium is lustrous by referring to the structure of the metals and the bonding that occurs within metal aluminium?

All metals, including aluminium are made with a metallic bond/net. The structure of this net is a lot of atomic nuclei netted together with electrons passing freely between them which causes light to bounce between them evenly and shine. This is what gives it the lustre.

[Maz]

Hey
i have a titration assessed practical coming up soon
the assessment is the we have to find the percentage of household ammonia in a cleaning product by tritrating it against HCl... with the solution diluted to 100mL and an aliquot of 10mL
I've done the calculation part to one...but we are also given density...and i don't quite know where the density fits into it?
the HCl has a concentration of 0.1082
Average volume of HCl titrate used
     = 4.85 cm3
No. of moles of HCl reacted = 0.1082 x (4.85 x10-3)
          = 5.248 x 10-4 mol

From equation, mole ratio of HCl ≡ NH3   is 1:1,
no. of moles of NH3 reacted = 5.248 x 10-4 mol
No. of moles of NH3 in volumetric flask (diluted) = (100 ÷ 10) × 5.248 x 10-4 mol
        = 5.248 x 10-3 mol
No. of mole of NH3 in 10mL (undiluted)   = 5.248 x 10-3 mol
Concentration of NH3   = (5.248 x 10-3) ÷ (10 x 10-3) = 5.248 x 10-1 moldm-3

Molar mass of NH3 = 14.01 + 3 (1.0079)
= 17.0337 g mol-1

Therefore, weight-volume percentage of NH3 in window cleaner 
= ((17.0337 x 5.248 x 10-1)/ 103) × 100%
= 0.894%

Percentage of ammonia in the window cleaner is: 0.894%
found the percentage of ammonia...but didnt use the density given...could you please tell me where i went wrong?
i'd really appreciate the help
Thankyou 

[vox nihili]

Hey
i have a titration assessed practical coming up soon
the assessment is the we have to find the percentage of household ammonia in a cleaning product by tritrating it against HCl... with the solution diluted to 100mL and an aliquot of 10mL
I've done the calculation part to one...but we are also given density...and i don't quite know where the density fits into it?
the HCl has a concentration of 0.1082
Average volume of HCl titrate used
     = 4.85 cm3
No. of moles of HCl reacted = 0.1082 x (4.85 x10-3)
          = 5.248 x 10-4 mol

From equation, mole ratio of HCl ≡ NH3   is 1:1,
no. of moles of NH3 reacted = 5.248 x 10-4 mol
No. of moles of NH3 in volumetric flask (diluted) = (100 ÷ 10) × 5.248 x 10-4 mol
        = 5.248 x 10-3 mol
No. of mole of NH3 in 10mL (undiluted)   = 5.248 x 10-3 mol
Concentration of NH3   = (5.248 x 10-3) ÷ (10 x 10-3) = 5.248 x 10-1 moldm-3

Molar mass of NH3 = 14.01 + 3 (1.0079)
= 17.0337 g mol-1

Therefore, weight-volume percentage of NH3 in window cleaner 
= ((17.0337 x 5.248 x 10-1)/ 103) × 100%
= 0.894%

Percentage of ammonia in the window cleaner is: 0.894%
found the percentage of ammonia...but didnt use the density given...could you please tell me where i went wrong?
i'd really appreciate the help
Thankyou 

It's there to trip you up, don't need it

[@#035;3]

Can someone explain to me the roles of H2SO4 as a catalyst and a dehydrating agent in Fischer Esterification reactions?
Can someone also confirm if these websites are correct.. 
(Talks about Mechanism for Esterification reaction,website bellow)               
 http://www.chemguide.co.uk/physical/catalysis/esterify.html   
and...      (Website bellow talks about H2SO4 as a dehydrating agent increases yield of ester)                                               https://answers.yahoo.com/question/index?qid=20080424082051AAkDDsA 

[Swagadaktal]

Can someone explain to me the roles of H2SO4 as a catalyst and a dehydrating agent in Fischer Esterification reactions?
Can someone also confirm if these websites are correct.. 
(Talks about Mechanism for Esterification reaction,website bellow)               
 http://www.chemguide.co.uk/physical/catalysis/esterify.html   
and...      (Website bellow talks about H2SO4 as a dehydrating agent increases yield of ester)                                               https://answers.yahoo.com/question/index?qid=20080424082051AAkDDsA

Is this related to unit 4? Because you don't need to know the process of the H2SO4 catalyst in an esterification reaction, just that it's there as a catalyst...

[@#035;3]

Nope unit3 esters sac ..... What would I say if I was asked the question: Explain how sulfuric acid increases the rate of this reaction (fischer esterification)?

[Swagadaktal]

Nope unit3 esters sac ..... What would I say if I was asked the question: Explain how sulfuric acid increases the rate of this reaction (fischer esterification)?

brah I finished that topic and no where did fischer ever come up who the hell is fischer.
I highly doubt they'll ask you HOW sulfiric acid increases the rate of reaction. I think they might ask you to demonstrate whether H2SO4 is there but im fairly certain that the mechanics of h2so4 in a reaction is not part of the study design.

If i'm wrong can someone please correct me?

[jyce]

Nope unit3 esters sac ..... What would I say if I was asked the question: Explain how sulfuric acid increases the rate of this reaction (fischer esterification)?

Hi,

In VCE Chemistry, you are expected to know how catalysts in general function to increase reaction rates; you are not expected to know how specific catalysts work for specific reactions (e.g. sulfuric acid in esterification reactions). The concept of how catalysts increase reaction rates is actually introduced in Unit 4 Area of Study 1. Anyways, the sulfuric acid provides an alternate reaction pathway that has a lower activation energy. This increases the proportion of reactant particles with sufficient energy to react.

[jyce]

brah I finished that topic and no where did fischer ever come up who the hell is fischer.
I highly doubt they'll ask you HOW sulfiric acid increases the rate of reaction. I think they might ask you to demonstrate whether H2SO4 is there but im fairly certain that the mechanics of h2so4 in a reaction is not part of the study design.

If i'm wrong can someone please correct me?

So yeah, how catalysts function is a part of the study design. But I'm guessing you weren't aware of this because this concept is covered in Unit 4, not Unit 3.

[Elizawei]

Which one? 

[jyce]

(Image removed from quote.)

Which one? 

My guess is B. HPLC can be used for the qualitative analysis of organic compounds, and it's highly sensitive. 

[Elizawei]

Thanks! _{And is your DP caffeine?} 

[jyce]

Thanks! _{And is your DP caffeine?} 

You're welcome, and yes