VCE Chemistry Question Thread

[knightrider]

Acid: increases, but to different extents if its a weak or strong acid. If strong, the solution consists mainly of products, so H+ decreses and pH increases. But if its a weak acid, water is a reactant so the equilibrium will shift to the right so to partially oppose the change. In this way, the concentration of H+ does decrease upon dilution, but it decreases less than a strong acid would in solution.

Does this make sense? Same logic applies to bases.

Thanks warya ! Made sense.

[knightrider]

How would you do this question attached?

Can anyone help with this?

[sunshine98]

How would you do this question attached?

water is always neutral meaning [H3O+]=[OH-] at all times , so the first two statements are correct.
The last statement is incorrect for more obvious reasons.

[knightrider]

water is always neutral meaning [H3O+]=[OH-] at all times , so the first two statements are correct.
The last statement is incorrect for more obvious reasons.

Thanks sunshine98

Why is the last statement incorrect and how is the 2nd statement correct?

[qwerty123456]

DO we need to know about ibuprofen

[warya]

DO we need to know about ibuprofen

no, just aspirin

[Fullmethyl Alchemist]

How would you do this question attached?

The second statement pertains the Kw- so the ionisation constant of water. That changes with temperature- so the amount that water self-ionises so the pH can be less than 7. BUT because the reaction is still stoichiometric (2H2O -> H3O+ and OH-) so it's still neutral.

The last statement Is false due to aforementioned change in Kw with temperature. Self ionisation is endothermic btw, so when the temperature decreases [H3O+] may drop below 10^-7 but because it's pure water it's still neutral ie not alkaline.

[knightrider]

The second statement pertains the Kw- so the ionisation constant of water. That changes with temperature- so the amount that water self-ionises so the pH can be less than 7. BUT because the reaction is still stoichiometric (2H2O -> H3O+ and OH-) so it's still neutral.

The last statement Is false due to aforementioned change in Kw with temperature. Self ionisation is endothermic btw, so when the temperature decreases [H3O+] may drop below 10^-7 but because it's pure water it's still neutral ie not alkaline.

Thanks Fullmethyl Alchemist 

but whats Kw ?

[zsteve]

Kw is the ionic product of water, or the eqm constant for water's self ionisation
we omit H2O(l) from the denominator because it is heterogeneous reactant.

Does anyone know if pH changes can alter the secondary structure of a protein? I have an inkling that it doesn't.

[warya]

Kw is the ionic product of water, or the eqm constant for water's self ionisation
we omit H2O(l) from the denominator because it is heterogeneous reactant.

Does anyone know if pH changes can alter the secondary structure of a protein? I have an inkling that it doesn't.

Hmm, how would hydrogen bonds be altered by pH changes? I think its just the tertiary structure, so amino acids losing H's off carboxyl groups or gaining H's on amino groups, and therefore becoming cat/anions, that is affected by pH alterations.

[knightrider]

Kw is the ionic product of water, or the eqm constant for water's self ionisation
we omit H2O(l) from the denominator because it is heterogeneous reactant.

Thanks zsteve 

[zsteve]

VCAA 2014 Section A Q6: the titration curve has a shape similar to weak base + strong acid, but the final pH at about 2 is very confusing - I took that to be a sign of a weak acid?
Could someone explain how this one worked? Usually strong acid would have been about pH < 1?

Also, when drawing reaction profiles, do you need to write the molecule formulas on either side? Or just the curve

Carboxyl groups - is it correct to refer the C=O as the 'carbonyl group' within the carboxyl group? I.e. a peak at ~1750 cm^-1 indicates that the compound contains a carbonyl group, and the O-H (acids) peak at ~3000 cm^-1 further indicates that the compound contains a carboxyl group

[warya]

VCAA 2014 Section A Q6: the titration curve has a shape similar to weak base + strong acid, but the final pH at about 2 is very confusing - I took that to be a sign of a weak acid?
Could someone explain how this one worked? Usually strong acid would have been about pH < 1?

Also, when drawing reaction profiles, do you need to write the molecule formulas on either side? Or just the curve

Carboxyl groups - is it correct to refer the C=O as the 'carbonyl group' within the carboxyl group? I.e. a peak at ~1750 cm^-1 indicates that the compound contains a carbonyl group, and the O-H (acids) peak at ~3000 cm^-1 further indicates that the compound contains a carboxyl group

\

2: I learnt the hard way just to label everything, reactants, products, activation energy and delta H
3: Yes thats fine, a Carbonyl group is a carbonyl group no matter the context. How else would you differentiate IR spectra of esters and carboxylic acids?

[zsteve]

And another relevant question - what about fuel cells that often go beyond the course in terms of reaction mechanisms and states.
The 2014 exam sprang a question about a 'gel' and correct states kind of needed a bit of an assumption that stuff could be (aq) while in the gel, although there were indications of that
Further, in Q10, the question talks about a redox flow battery that runs on 'quinones/hydroxyquinones'. At the beginning of section B, there is a note that "states should be included" - I've taken this as a rule to include states ALWAYS EXCEPT where specified 'states are not required'.
However, the examiner's report doesn't include states.
When is it safe to drop the states? I felt like that when I was doing the question, but such was my faith in the exam that I didn't

[jyce]

VCAA 2014 Section A Q6: the titration curve has a shape similar to weak base + strong acid, but the final pH at about 2 is very confusing - I took that to be a sign of a weak acid?
Could someone explain how this one worked? Usually strong acid would have been about pH < 1?

Hi zsteve,
Strong acids can definitely have a pH > 1; they are just not particularly concentrated. For example, a 0.01 M HCl solution has a pH of 2, since -log₁₀(0.01) = 2.