2016 H2 Chemistry JC1 & 2 students post your questions here

last SG · 2016-02-03T02:17:14+00:00

Hello, would like to ask a questionAs to why c=c is stronger than c-c bond, is it because of the additional pi bond, or because the sigma bond in c=c is shor...

UltimaOnline SG

Ultima, for A levels purposes can I simple assume that polar will always be able to be soluble in polar reagents and non polar solvents wil mix with non polar reagents ? Ionic salts will always dissolve because they can form ionic like bonds with polar water molecules. Anything that can form hydrogen bonds iwith water can dissolve in water. as long as the bonds holdingh the solid together is weaker than the bonds that will form with the solvent, solid will surely dissolve. please correct any misconceptions ! im struggling with the whole solubility thing ( not the energetics kind but the bonding of solvent solution )

Not so simple. At O levels, everything is simplistically 'always'. At Uni levels, nothing is simplistically 'always'. A levels is between O levels and Uni levels, expect anomalies and having to examine case-by-case.

'Like dissolves like' is usually applicable for most cases, but the question can add in more variables to get you to discuss relative solubilities of a polar solute in 2 or 3 different solvents, all of which are polar (for example).

Not all ionic salts are soluble, hence you have Qsp and Ksp. But species like phenylammonium ions and phenoxide ions, are indeed more soluble than their molecular conjugate acid/base forms, due to ion - permanent dipole interactions which are stronger than hydrogen bonds (provided the formal charge is on a small period 2 atom).

Even if a molecule can form hydrogen bonds with water, it may not be very soluble, because of the non-polar parts of the molecule (eg. benzene ring) thermodynamically preferring to form van der Waals forces with the non-polar parts (eg. benzene ring) of the other solute molecules, while the snobbish water solvent molecules thermodynamically prefer to form hydrogen bonds with other water solvent molecules. Hence, aqueous solubility of alcohols decrease with increasing length of its non-polar hydrocarbon chain.

So once you see it as a beautiful complex spectrum of solubilities, rather than an oversimplistic black-and-white binary system of just 'soluble' or 'insoluble', you can let go of your frustrations and it will finally make logical sense for you.

UltimaOnline SG

Also e.g say when an amine is added into HCl, the solution surely dissolves and forms a salt rite? but isnt it reacting with the acid which permanently changes its chemical formula and not dissolving ? compared to maybe amine being dissolved in water which only forms hydrogen bonds with water and does not react.

Also Hormones must be made up of 2-amino carboxylic acids like proteins or any kind of amino acid?

Yes, it is reacting with the acid to be protonated, and it's formula changes from its conjugate base form to its conjugate acid form, and its conjugate acid form is more soluble than its conjugate base form, because ion - permanent dipole interactions are stronger than hydrogen bonding, as long as the formal charge is on a small period 2 atom.

Hormones may or may not be protein in nature (H2 Bio students have an advantage over H2 Physics students for this topic). Only proteins are polymers of alpha-amino acids.

gohby SG

Hello UltimaOnline,

I have returned with some questions to clarify and would appreciate your help :)

Q1 VJC/Prelim 09/P1/38

Why is 3 not an answer, given that ethanol could be formed from the hydrolysis of ester, and the potassium dichromate (VI) would oxidise the ethanol to ethanoic acid, thereby changing its colour from orange to green?

Q2 A sample of apple juice has a pH of 3.50. A sample of HCl also has a pH of 3.50. If 1dm³ of each of apple juice and this hydrochloric acid were treated separately with the same excess amount of zinc powder, which would yield a larger volume of hydrogen gas at room temperature? Explain. (From previous parts it was found out that the concentration of the apple juice is 0.110M; Ka of apple juice ~9.08x10^-7M)

- I think both would yield the same volume of hydrogen gas - even though the concentration of the apple juice greater than that of hydrochloric acid, it is the number of moles of H+ that will react with the excess zinc power that will affect the volume of hydrogen gas produced. Even though apply juice has a higher concentration than HCl, the number of moles of H+ produced from the apple juice and that of HCl are the same given they have the same pH (same H+ concentration, same volume = same no of moles).

- However the suggested answer states “the apple juice would yield a larger volume of H2 as [HA] (0.110) > [HCl] 3.16x10^-4)

DHS/Prelim 09/P1/25

Remarks: The answer is B. However why would the O-H2C be formed on the left hand side instead of HOH2C (as suggested by A)?

UltimaOnline SG

Hello UltimaOnline,

I have returned with some questions to clarify and would appreciate your help :)

Q1 VJC/Prelim 09/P1/38

- However the suggested answer states “the apple juice would yield a larger volume of H2 as [HA] (0.110) > [HCl] 3.16x10^-4)

DHS/Prelim 09/P1/25

Remarks: The answer is B. However why would the O-H2C be formed on the left hand side instead of HOH2C (as suggested by A)?

Yo Gohby, good to have you back.

VJC/Prelim 09/P1/38, all 3 options are correct, including the ester, which as you pointed out will be hydrolysed to a carboxylic acid and a primary alcohol, which will be oxidized by the K2Cr2O7.

Apple juice qn : the given answer is correct, because as predicted by Le Chatelier's principle, additional H+ from the weak acid will dissociate to oxidize the Zn to Zn2+, and be reduced to additional H2(g).

DHS qn : you're right, the answer should be A, not B.

LavXuan SG

Is it possible to remove 2 carbon atoms at one go from the oxidation of an alkene? How

LavXuan SG

Does elimination reaction still occur for alcohol to form alkene when alcohol is REFLUXED with conc H2SO4 instead of carrying out the reaction at 170 degree celsius?

UltimaOnline SG

Is it possible to remove 2 carbon atoms at one go from the oxidation of an alkene? How

Yes of course, if using KMnO4 (but not K2Cr2O7).

The =CH2CH2= group will be oxidized to ethandial, then ethandioic acid, then 2 mol of carbonic(IV) acid, which decomposes into 2 mol of CO2(g) + H2O(l), with thermodynamically favorable positive entropy change (since CO2(g) is gaseous), and as predicted by Le Chatelier's principle, since CO2(g) leaves the reaction solution, pulling the position of equilibrium over to the RHS.

UltimaOnline SG

Does elimination reaction still occur for alcohol to form alkene when alcohol is REFLUXED with conc H2SO4 instead of carrying out the reaction at 170 degree celsius?

It's not "heat" versus "reflux". You're supposed to heat under reflux at 170 deg C.

Christabellw SG

" Comparing HF and NH3 F is more electronegative than N, thus H-F bond is mote polar than N-H bond" What does it mean when a bond is more polar than the other?

UltimaOnline SG

" Comparing HF and NH3 F is more electronegative than N, thus H-F bond is mote polar than N-H bond" What does it mean when a bond is more polar than the other?

It means the dipole moment (in Debye units) has a larger magnitude. The larger the magnitude of electronegativity difference between the 2 atoms of different elements, the more polar the bond, the larger its dipole moment.

Eg. the dipole moment in H-F is 1.91 D, which is larger than the dipole moment in H-Cl which is 1.08 D, which is larger than the dipole moment in H-Br which is 0.80 D.

Shinyphua SG

h2 chemistry A levels 2007 Paper 2

Qn 4, C (ii)

suggest the size of the cl-c-cl bond, my answer is 115deg but the answer from tys is 120deg

Qn 4, d (i)

for stage ll, my answer is decomposition but tys answer is elimination

UltimaOnline SG

h2 chemistry A levels 2007 Paper 2

Qn 4, C (ii)

suggest the size of the cl-c-cl bond, my answer is 115deg but the answer from tys is 120deg

Qn 4, d (i)

for stage ll, my answer is decomposition but tys answer is elimination

It's obviously trigonal planar, 120 degrees. Think of it as a ketone, with the 2 alkyl groups replaced by Cl atoms.

Best answer is to combine both, ie. decomposition via elimination. When in doubt as to which answer Cambridge wants, write both answers. As long as your answers don't contradict, Cambridge will credit the required answer, and ignore the other answer. BFJC ftw.

Fxwhy SG

https://twitter.com/doublehalfarrow/status/746629255522443264 How does A undergo intramolecular nuclophilic substitution between its amine and alkyl halide to form D?

Christabellw SG

Explain why gaseous ethanoic acid, Ch3COOH has an apparent molecular mass of 120 instead of 60

Ans: this is because the the gas particles consist of hydrogen bonded dimers

Is it because organic acid in gaseous state have molecular mass twice of what it is expected??? If yes, why so??? So what if it consist of hydrogen bonded dimers?

nicolemantou SG

https://twitter.com/doublehalfarrow/status/7466…
Why is [OH-] divided by 2 in the answer?

UltimaOnline SG

https://twitter.com/doublehalfarrow/status/746629255522443264 How does A undergo intramolecular nuclophilic substitution between its amine and alkyl halide to form D?

What do you mean, "how does it undergo intramolecular nuclophilic substitution?". It undergoes because of the mechanism, that's how (by definition)!

If you mean "how do you know" then it's because the no. of C atoms in A and D are the same, and the degree of unsaturation increases by 1 (due to formation of cyclic ring), that's how you know.

UltimaOnline SG

Explain why gaseous ethanoic acid, Ch3COOH has an apparent molecular mass of 120 instead of 60

Ans: this is because the the gas particles consist of hydrogen bonded dimers

Is it because organic acid in gaseous state have molecular mass twice of what it is expected??? If yes, why so??? So what if it consist of hydrogen bonded dimers?

Not for all organic acids, but especially for small carboxylic acids. And not just gaseous state, but in the liquid state as well, and in non-polar solvents as well.

For such questions, you're required to draw (and label!) the following hydrogen-bonded dimer :

http://www.chemguide.co.uk/organicprops/acids/background.html

UltimaOnline SG

https://twitter.com/doublehalfarrow/status/7466…
Why is [OH-] divided by 2 in the answer?

Because adding equal volumes of both reactant solutions, doubles the volumes, hence halves the molarities of the aqueous ions.

ACA-Milkshake SG

For the bond angle of cl-c-cl bond of cocl2, even with the presence of the double bond with O, the bond angle won't be affected and remain at 120 degrees?

ACA-Milkshake SG

h2 chemistry A Level TYS

2007 Paper 3 qns 2(e)ii: My answer for the reaction I is base hydrolysis but the ans is neutralisation. Why is it neutralisation?

2007 Paper 3 qns 3(d)i: How do the product CH3CHO form from the reaction?

2007 Paper 3 qns 3(d)ii: Can CH2O decolourise purple KMnO4 and liberate CO2 gas? Cos i thought that ketones can't do so...

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