Chemistry 3/4 2013 Thread

[Mr Keshy]

Don't take these as correct yet, I'm not entirely sure but I'll fix any errors as soon as someone points it out. I'm actually kinda confused about this too.. In some diagrams they put the states in there, in some, they don't. Some of this I found in my A+ notes book for Chem too.

[brightsky]

Yeah ethanol is polar because of the hydroxyl functional group. But it is also non-polar. It serves a dual purpose.

Water is only polar. So in water you can only dissolve polar solutes. In ethanol, you can dissolve both.

Therefore, in the context of this question: when should ethanol be used instead of water? If the solute is non-polar.

And ethanol is not necessarily pure, for the same reasons as water. Yeah, it's pretty vague 

ahh i see what you mean now. i stand corrected.

[lzxnl]

I'd say it's good practice to put states in And yeah, I do believe you need to have then written down.

I'm not too confident in this myself so please correct me if I'm wrong! But from my understanding thus far

So to go from an Alkane to a Chloroalkane you'd need Cl₂(g) and the catalyst is light/heat. To go from an Alkene to a Chloroalkane you need HCl(g) and the catalyst is AlCl₃(s).

To go from a Chloroalkane to an Alkanol, you need NaOH(aq) and again, there's no specified catalyst for that. To go from an Akene to a Chloroalkane you need H₂O(g) where H₃PO₄(s) is the catalyst.

To go from an Alkanol to a Carboxylic acid you need either Cr₂O₇^2-(aq) or MnO₄^- and H⁺(aq) acts as the catalyst

Then to go from a Carboxylic acid to an ester, you need the Alcohol (ethanol/methanol.. etc) and your catalyst (H₂SO₄(l)

Strictly speaking, I wouldn't call heat a catalyst, as heat merely provides energy to overcome the activation energy barrier. A catalyst speeds up both the forwards and the backwards reaction equally and does not affect the equilibrium position of a reaction. Heat and light, as forms of energy, will affect equilibrium. Also, this heat and light are absorbed and are not emitted after the reaction. Therefore, they are consumed as part of the reaction; a catalyst isn't.

[Mr Keshy]

Strictly speaking, I wouldn't call heat a catalyst, as heat merely provides energy to overcome the activation energy barrier. A catalyst speeds up both the forwards and the backwards reaction equally and does not affect the equilibrium position of a reaction. Heat and light, as forms of energy, will affect equilibrium. Also, this heat and light are absorbed and are not emitted after the reaction. Therefore, they are consumed as part of the reaction; a catalyst isn't.

Oh okay. What is the heat/light classified as then? I thought light/heat sped the reaction, but what you said makes sense. So a catalyst merely speeds up a reaction, so it's not absolutely necessary right? It'll just take longer for the reaction to finish.

But for reactions like an Alkane to Chloroalkane where light/heat is required, you need the light/heat energy or else the reaction won't occur?

[brightsky]

Technically, yes, but at times if you do not provide a suitable catalyst, the reaction will literally take hundreds of years to reach 'completion', and for obvious reasons, we don't have that amount of time to spare.

Yes, in the case of free radical halogenation, UV radiation is required or else the reaction will not take place. This is because UV radiation is required for the first step of the reaction mechanism involved. Without UV light, the chlorine molecule will not undergo homolysis, which is indispensable to this particular kind of reaction.

[lzxnl]

Yes, catalysts are not necessary for the reaction to take place; they are only required to speed the reaction up. UV light here is an essential part of the reaction as it splits the chlorine molecule into chlorine radicals. However, sulfuric acid in the addition of water to ethene, for instance, is not absolutely necessary; it does, however, provide a reaction mechanism with a lower activation energy. A reaction IS possible without sulfuric acid, but I'm not sitting around waiting for the product.

[Daniel_Le]

Hello, I wanting to ask what sort of organic compounds we must know? i.e. we must know what kind of carboxyl derivatives? i.e. acyls, salt of carboxyls, nitriles, amides? or just esters? I'm revising over these, but there's a lot of info on their formation, reduction and oxidation, and I don't think I can remember all the processes and info of all these functional groups? My teacher saying they can put anything on there, but please if anyone has any info on this would be many thanks.

[bucklr]

Hello i have a question:

Why is there a bold line at the bottom of carbohydrates? And why does the disaccharide composed of two fructose molecules comprise of one fructose that has been flipped?

Thanks in advance.

[Scooby]

Hi guys,

Does anyone know what actually determines whether a substance is a strong acid or base? I mean, considering oxygen is more electronegative than chlorine, why is HCl a stronger acid than CH3COOH? A considering an OH bond is highly polarised, why are water and alcohols such weak acids?

Thanks 

[brightsky]

acid dissociation constant (Ka), obtained via experimentation

[Alwin]

Hello, I wanting to ask what sort of organic compounds we must know? i.e. we must know what kind of carboxyl derivatives? i.e. acyls, salt of carboxyls, nitriles, amides? or just esters?
My teacher saying they can put anything on there, but please if anyone has any info on this would be many thanks.

Check the study design. Personally, I just committed everything to memory

Why is there a bold line at the bottom of carbohydrates? And why does the disaccharide composed of two fructose molecules comprise of one fructose that has been flipped?

It's to show the 3D shape of the molecule, the solid black line comes 'out' of the page. So they carbon ring is on a perpendicular plane to the the hydrogen and other functional groups attached to the carbon back bone. eg the glucose molecule has hydrogen and -OH going up and down from the carbon ring, not hydrogens going toward the centre. Hope it makes sense. 3D modelling and angle of bonds is no longer in the course I think.

Hmmm, I've never heard of a double fructose molecule, only know sucrose is glucose and fructose molecule. Someone else may know about your disaccharide composed of two fructose molecules

Does anyone know what actually determines whether a substance is a strong acid or base? I mean, considering oxygen is more electronegative than chlorine, why is HCl a stronger acid than CH3COOH? A considering an OH bond is highly polarised, why are water and alcohols such weak acids?

Unit 4 chem. You'll get there when you get there But if you still want a quick run down of it, I can give it a shot.
Expanding on what Brightsky has said, essentially (said simply) Ka tells you how likely the acid is to ionise. The table of values can be found in your data book, last page: entited 12. Acidity constants, Ka, of some weak acids at 25°C

EDIT: beaten to the last part. Added more detail.

[Scooby]

Unit 4 chem. You'll get there when you get there But if you still want a quick run down of it, I can give it a shot

Yeah, that'd be good, if that's alright

[Alwin]

Yeah, that'd be good, if that's alright

Okay, so the dissociation (ionisation) of an acid is an equilibrium reaction, ie it can go forwards and backwards.

where A represents the Acid.

For strong acids, such as HCl, the reaction goes forward a lot (for lack of a better word) so almost all of the acid dissociates.
For weak acids, such as CH3COOH, the reaction does not go forward "as much" so only some of the acid dissociates.

How do we measure this? With K_a
Skipping just a bit of equilibrium theory, I'll just jump straight to the equation


Using experimental data at 25°C (this is important), you can work out the concentration of H₃O⁺, A^- and HA in the resulting solution. You can see, that for strong acids they have high K_a since there is a very low concentration of HA (un-ionised) acid in the final solution. For weak acids, the majority is left not ionised, so there is a high concentration of HA hence low K

Hope that clears things up a bit!

[Patches]

Hello, I wanting to ask what sort of organic compounds we must know? i.e. we must know what kind of carboxyl derivatives? i.e. acyls, salt of carboxyls, nitriles, amides? or just esters? I'm revising over these, but there's a lot of info on their formation, reduction and oxidation, and I don't think I can remember all the processes and info of all these functional groups? My teacher saying they can put anything on there, but please if anyone has any info on this would be many thanks.

From the study design:

• structure including molecular, structural and semi-structural formulae, and International Union
of Pure and Applied Chemistry (IUPAC) nomenclature of alkanes, alkenes, amines, haloalkanes,
alkanols (Cn
H2n+1OH), alkanoic acids (Cn
H2n+1COOH) and esters up to C10
• common reactions of organic compounds including equations: addition reactions of alkenes (addition
of hydrogen halides and water limited to symmetrical alkenes), substitution reactions of alkanes
and primary haloalkanes, oxidation of primary alkanols, and esterification
• chemical bonding:
– primary, secondary and tertiary structures of proteins
– the role of the tertiary structure of proteins in enzyme action
– denaturing of proteins: effect of changes in pH and temperature on bonding
– primary and secondary structure of DNA
• organic reaction pathways including appropriate equations and reagents:
– production of esters from alkenes
– condensation reactions that produce lipids (limited to triglycerides)
– condensation and polymerisation reactions that produce large biomolecules including
carbohydrates, proteins and DNA
– production of biochemical fuels including the fermentation of sugars to produce ethanol
– function of organic molecules in the design and synthesis of medicines including the production
of aspirin from salicylic acid.

So alkanes, alkenes, alkanols, carboxylic acids, haloalkanes, amines, amides and esters. Shouldn't need to know anything else I don't think - though benzene and cycloalkanes seem to come up even if they're not mentioned here. Plus carbohydrates, proteins and DNA stuff too I guess.

[psyxwar]

Are we required to be able to balance redox reactions in basic conditions?