Chemistry Questions Thread

[Becky2012]

Food essences are usually dissolved in ethanol. These essences contain an ester as their active ingredient. Suggest why:
a) water is not a sutiable solvent
b) hexane is not a suitable solvent

[kenhung123]

Esters are would react with water to form its alcohol and carboxylic acid derivatives
R-OH + R'-COOH -> R-COOR' +H2O
The reverse of the above equation forms the reactants when excess water is present

Hexane is non polar, esters are polar (like dissolves like substances)
When you have substances that are non polar, they form stronger bonds when non polar solvents are present. (Thermodynamically stable)

[Becky2012]

thanks! could you explain part a further? I don't think I quite understand..

[Becky2012]

Also:
Write flow diagrams showing all organic and inorganic reagants required to prepare:
a) propanoic acid from propane
b) ethyl ethanoate from ethene

How would I go about this?

[kenhung123]

propane+Cl2 -> (uv) 1-chloropropane
1-chloropropane + NaOH -> 1-propanol
1-propanol -> (K2Cr2O7) propanoic acid

Similar for b) but ethene + H2O -> ethanol
ethanol -> (K2Cr2O7) ethanoic acid

react the acid and alcohol to form ethyl ethanoate

[Becky2012]

Okay, so I've been doing the organics reactions part of Unit 3, and I've been getting rather confused with which states to use in each equation, particularly in whether to use (aq) or (l). Could someone explain this to me please?

[Hamdog17]

(l) usually. Most of the questions on papers are about the industrial production of esters, alcohols, etc. in this context I don't think the reactants are aqueous. In regards to condensation reactions forming H2O with esters for example I'd just go for (l) products, technically esterification is exothermic so any water formed may be evaporated. Same goes for catalysts (eg. VCAA in the 2011 U3 only gave marks for writing H2SO4 (l) as the catalyst NOT H2SO4 (aq) because a concentrate is needed)

[Becky2012]

thanks, that helped a fair bit!
okay, next question haha.

Calculate the percentage yield if 5.0g of ethanol is oxidised to produce 4.8g of ethanoic acid.

[Phy124]

thanks, that helped a fair bit!
okay, next question haha.

Calculate the percentage yield if 5.0g of ethanol is oxidised to produce 4.8g of ethanoic acid.

Never really enjoyed chem so neglected it a bit, but I'll give it a shot.

%yield = (actual/theoretical) x 100

Ethanol being oxidised to ethanoic acid is shown by the equation:

CH₃CH₂OH + H₂O -> CH₃COOH + 4H⁺ + 4e^-

The ratio of CH₃CH₂OH : CH₃COOH is 1:1

n(CH₃CH₂OH) = m/M = 5.0/46 = 0.109 mol

n(CH₃COOH) = n(CH₃CH₂OH) = 0.109 mol

m(CH₃COOH) = n x M = 0.109 x 60 = 6.54 g

%yield = (4.8/6.54) x 100 =  73 % (2 sig figs?)

*note: it may be necessary to go to more decimal places etc. (also, this could be totally wrong :/)

edit: put 4.7 not 4.8

[Becky2012]

Yep, the answer in the book was 72%, so thank you!

[Phy124]

Yep, the answer in the book was 72%, so thank you!

No worries, glad to help. Feel free to ask any more questions you may have.

[thushan]

Food essences are usually dissolved in ethanol. These essences contain an ester as their active ingredient. Suggest why:
a) water is not a sutiable solvent
b) hexane is not a suitable solvent

Actually, I disagree with kenhung here. Unless they're really small esters (like ethyl methanoate or sth) esters are actually insoluble in water. Whilst they do have a polar carbonyl group, the size of the ester means that dispersion forces between ester molecules are favoured over any form of dipole-dipole between the C=O and water. So water is not a suitable solvent because it's insoluble.

With hexane (this is under the assumption that food essences are consumed) it is actually quite toxic (hexane would dissolve most esters actually, as it's non-polar) and should not be consumed. [Ethanol, whilst somewhat toxic, is often taken by people in small doses anyway (or larger doses if you went to a party ;P) and can be metabolised by liver.] Think of drinking hexane as...drinking petrol

[kenhung123]

I agree, only for small esters...

[paulsterio]

Would esters dissolve in hexane?

[thushan]

Yeah, I'm pretty sure they dissolve in hexane.