Rod's Chemistry 3/4 Questions Thread

[brightsky]

ethanol is oxidised and the dichromate ion is reduced. not sure why/how i know that, but it's just something that you need to know.

and yeah, much of chem is about memorisation, unfortunately. 

[Rod]

ethanol is oxidised and the dichromate ion is reduced. not sure why/how i know that, but it's just something that you need to know.

and yeah, much of chem is about memorisation, unfortunately.

All good, I'll try and find out a way to understand these equations though. I hate memorising.

Would I be asked that in a VCAA exam? The breath test question? It provides absolutely no information on what could get reduced or oxidised.

[jgoudie]

Yeah, a questions like that in Heinemann can often require knowledge that is in the textbook pages before hand (page 48 to be precise)  General chemistry knowledge should have you remember that dichromate is something that is usually reduced to Cr3+.  You have probably balanced that half equation like 50 times by now!!

VCAA should give you a little more background info, at least products if not and unbalanced equation.

ethanol is oxidised and the dichromate ion is reduced. not sure why/how i know that, but it's just something that you need to know.

and yeah, much of chem is about memorisation, unfortunately.

[Rod]

Yeah, a questions like that in Heinemann can often require knowledge that is in the textbook pages before hand (page 48 to be precise)  General chemistry knowledge should have you remember that dichromate is something that is usually reduced to Cr3+.  You have probably balanced that half equation like 50 times by now!!

VCAA should give you a little more background info, at least products if not and unbalanced equation.

Thank you!!!

[Rod]

Hey guys what's the difference between amide and amino? Is it that NH is amine and NH2 is amino?

And when do we use 'amino' and 'amine'

Thanks!

[lzxnl]

Hey guys what's the difference between amide and amino? Is it that NH is amine and NH2 is amino?

And when do we use 'amino' and 'amine'

Thanks!

First, you mention amide and amino. Then, you switch from "amide" to "amine". There is a difference.

For starters, amide has two meanings. In VCE chemistry, amide means a group where there is a carbon both double-bonded to oxygen and single-bonded to a nitrogen atom. Its other meaning is the NH2- anion, the conjugate base of ammonia (yes that exists; sodium amide, for instance, is a remarkably powerful base as you can imagine). Forget the latter

Now, "amine" and "amino" are equivalent when referring to the functional group. They are used differently when naming though; "amino" is used at the front, like "aminoethanoic acid), while "amine" is used at the back, like "butan-1-amine".

[Rod]

Didn't understand sorry

So amide = NH
     amino = NH2

When naming hydrocarbons, for amides we use;

1. Amino (at the start, so primary)
2. -amine (at the end, secondary)

We use primary ???when??? we use the secondary ??when??

[scribble]

no, amide is C=ONH, so think of the bond you get when you join two amino acids in vce chemistry

if something has an amine group, and no other functional groups, then it will be in the form of _______-amine.
if the compound has multiple functional groups, theres sort of like, a hierarchy we use when it comes to naming. carboxylic acids are above amines in this hierarchy, so when we name something with an amine group and also a carboxylic acid group, we use "ic acid" at the end, and amino at the front. eg. 3-aminopropanoic acid.

[Rod]

Hey guys,

I've just been having trouble with states.

When writing organic reactions, I seem to always get the states wrong. Eg, what states do I put in a reaction where an alcohol + carboxylic acid makes an ester? Or when alkene and water makes an alcohol? or when an alkane and bromine make a haloalkane??

All I know is that methane, ethane, are always gases. In alkanes carbons 5-16 are liquids, and above 17 are soft solids.

How do I understand states??

Also , are the reagents really important to in organic reactiosn??

Thanks!!

[lzxnl]

Hey guys,

I've just been having trouble with states.

When writing organic reactions, I seem to always get the states wrong. Eg, what states do I put in a reaction where an alcohol + carboxylic acid makes an ester? Or when alkene and water makes an alcohol? or when an alkane and bromine make a haloalkane??

All I know is that methane, ethane, are always gases. In alkanes carbons 5-16 are liquids, and above 17 are soft solids.

How do I understand states??

Also , are the reagents really important to in organic reactiosn??

Thanks!!

Esters are generally liquid; they're not very soluble. Smaller alcohols are aqueous, just like with acids.
When alkenes and water react to form an alcohol...hmm...depends on the reaction conditions. Nowadays the hydration of ethene requires phosphoric acid at 250 degrees as a catalyst, so in that case water and alkene would both be dissolved in phosphoric acid. Alkenes don't react readily with water and need some catalyst.
For alkanes reacting with bromine, bromine is generally a liquid, while the alkane is either liquid or gaseous depending on size. Chlorine is always a gas, while iodine is generally solid.

States are related to the intermolecular forces that hold the molecules together. With increasing molecular mass, there are more electrons available that contribute to dispersion forces (more electrons, greater instantaneous dipoles, greater attractions), so the molecules are held together more strongly and boil/melt at lower temperatures.

Reagents are what cause the reactions to occur. They are just as important as the organic compound that you're trying to change.

[lzxnl]

^^ Thanks Lxnl

Here's a question from the prac we did yesterday. How would using solid alcohols and liquid carboxylic acids when making esters affect an experiment? I see that using a solid, more yield will be given off the the alcohol, and less to the carboxylic acid, but how do these 'gross errors' affect the experiment?

Huh? More yield given off the alcohol? I have no idea what you're trying to say there.
If you're trying to react a solid with a liquid, your rate of reaction is going to be lower because not all of the solid can actually react. If the alcohol was dissolved in the acid, more of the alcohol molecules would be available for reaction.

Also, you need to ensure that your liquid carboxylic acid is dissociated enough as the esterification reaction using an alcohol and ester requires an acid catalyst (there are better ways of preparing esters )

[Rod]

Huh? More yield given off the alcohol? I have no idea what you're trying to say there.
If you're trying to react a solid with a liquid, your rate of reaction is going to be lower because not all of the solid can actually react. If the alcohol was dissolved in the acid, more of the alcohol molecules would be available for reaction.

Also, you need to ensure that your liquid carboxylic acid is dissociated enough as the esterification reaction using an alcohol and ester requires an acid catalyst (there are better ways of preparing esters )

Thanks man!

[Rod]

Hey guys just need some help on states for organic reactions, do you guys have any rules or anything that could help me?

This is what I know;

Most alcohols are liquid
Combustion reactions only have gases
Meth,eth,prop usually gases
Anything reaction with aq would usually produce aq
Anything reacting with water usually produces aqueous, btu sometimes liquids
anything non-polar is a gas

[lzxnl]

Hey guys just need some help on states for organic reactions, do you guys have any rules or anything that could help me?

This is what I know;

Most alcohols are liquid
Combustion reactions only have gases
Meth,eth,prop usually gases
Anything reaction with aq would usually produce aq
Anything reacting with water usually produces aqueous, btu sometimes liquids
anything non-polar is a gas

Combustion reactions only have gases? I disagree. When burning fuel, which consists of hydrocarbons, the hydrocarbons are in liquid form when burnt. Similarly, sulfur dioxide is the product of burning sulfur, which is in solid form (I know this isn't organic). You just have to know what the states of the individual molecules are.

Meth, eth prop usually gases? Those alcohols are all liquid, propanamine is a liquid and none of the acids are gases (they're liquids). Propyl chloride is also a liquid. If it's non-polar, then the first three members of that homologous series are usually gases.

Aqueous reactions can form anything. Ethanoate ion reacting with chloroethane, all in water, can yield insoluble ethyl ethanoate; a similar product occurs with ethanoic acid and ethanol in acidifed water. Aqueous barium hydroxide reacts with sulfuric acid to form a precipitate of barium sulfate. Also, reacting sodium hydroxide with ammonium chloride, in water, can form gaseous ammonia if the reaction vessel is heated, while dehydrating ethanol in sulfuric acid in water can form gaseous ethene. Only assume that the product is aqueous if the product actually dissolves in water.

With your next one, if water is the solvent, you're generally correct on that.

Not all non-polar things are gases. Hydrocarbons aren't all gases, for instance. Stuff like decane is liquid normally. Similarly, molecular bromine is a liquid, while molecular iodine is a solid. Esters are quite non-polar (don't dissolve in water) but they're liquid.


As for rules, I don't have any hard and fast rules, but I think of them this way.
Most carboxylic acids are liquid (at least, the common ones)
Same with alcohols
Alkyl halides, if smaller, are gaseous, but at around three carbons they become liquid.
Butane is a gas, but pentane and higher are liquids (excluding isomeric effects on boiling points)

[Rod]

Hi everyone,

Just been stuck on a question for a while ;

2.5 mol of an unsaturated fatty acid was converted to a saturated fatty acid by reaction with 15g of h2 gas in the presence of a catalyst. Calculate the number of carbon to carbon double bonds present in the unsaturated fatty acid.

My thoughts;

So the double bonded, unsaturated fat is obviously going through a hydrogenation reaction to become a single bonded fatty acid.

n(h) used was 15/1 = 15 mol
n(unsaturated fatty acid) = 2.5 mol
n(saturated fatty acid) = we don't know

hmm. maybe divide the total mole of hydrogen with the unsaturated fatty acid --- 15/2.5 = 6...

So maybe 6 hydrogens added in during the hydrogenation reaction, so the fatty acid had 3 double bonds since each broken double bond equates to two hydrogens so 6/2 = 4

Can someone please help me? Even if I am right, can someone please explain to me how I did it lol! The last two sentences was just me randomly mucking around so I don't really know what I did.

Thank you!!

PS - No answers were supplied ;'(