Chemistry: Post your doubts here!

[qwertypoiu]

How do we know if it is tetrahedral or square planar ?

Well this isn't a question about complexes firstly. So we should expect a tetrahedral structure.

Secondly, even if it were a question about complexes I'd suggest going with tetrahedral. Cisplatin is the only complex we need to know that is square planar, as far as I know.

 

[nowaisir10]

anybody giving variant 2 (S3) this may???

 

[nowaisir10]

View attachment 59711

in 2014 may/june paper 42 mark scheme the optical isomers are given like this.

View attachment 59713

In the syllabus, it's given that potical isomers should be drawn like this.

My question is in the mark scheme why is the OH as a dashed line and then in the second isomer as a wedge but in the syllabus it is not ?

are you giving variant 12 and 22 this may?

 

[qwertypoiu]

The answer is D. But the question asks what we can deduce from THESE observations. So how do we know that H2SO4 is a stronger oxidiser than iodine from these observations. And also, iodine is supposed to be a reducing agent and iodide is supposed to be an oxidising agent. It's the opposite here.View attachment 59691

• A strong oxidising agent is a specie that is really good at oxidising others.
• So it is really good at getting reduced.
• So it is really good at accepting electrons.

The strongest oxidising agent is the one that is the best at taking electrons and keeping it.
Let's look at the statements:

A - Phosphoric acid is a stronger oxidising agent than sulphuric acid?! No way! The experiments demonstrated that the SO4 2- ions were able to steal away the electrons from I- and keep it, but PO4 3- ions couldn't do that. So this statement is opposite. Sulphuric acid should be stronger oxidising agent.

B - Phosphoric acid better oxidising agent that iodine? Wait a sec. Where does I2 even come from? We can imagine this equilibrium existing:

I2 + 2e <----> I-​

I2 is an oxidising agent because it can take away electrons from others to become I-. One way to measure its oxidizing ability would be to see if it is able to keep these electrons and remain in the form of I-. If a specie comes along and oxidises (snatches away electrons from) I-, it means I2 as an oxidising agent was not strong enough to keep these electrons and remain I-. Hope this makes sense.

So was phosphoric acid strong enough to take away electrons from I- to form I2? Nope. So I2 is actually a stronger oxidising agent than H3PO4.

C - For the same reason as above, since H2SO4 was not able to oxidise Cl- to Cl2, Cl2 is the stronger oxidising agent.

D - In this case, sulphuric acid WAS able to oxidise I- to I2. So H2SO4 is the stronger oxidising agent (when compared to I2)

 

[qwertypoiu]

View attachment 59687
I think both A and D are possible answerstandards but it's D in the ms. :/

Hydrogen bonds between molecules is an intermolecular force of attraction. Intermolecular forces are best shown by looking at energy required to vaporize, since the process makes the molecules as far away from each other as possible (thus destroying all intermolecular forces)

Bond dissociation energy (which is same thing as bond energy I assume) will just show energy required to break H-X bond (where X is halogen like Cl)
These values are completely irrelevant, since these H-X bonds aren't even being broken. I hope you understand this. In a reaction, H-X bonds could be broken, but during vaporization, HX will remain in its molecular state. Only the molecules will move further apart from one another.

 

[qwertypoiu]

View attachment 59671

Can someone please explain the second part please ? Why can it only be CH3+ why cant it be H2 or CH2?

In general you should assume only C-C bonds are broken and that C-H bonds are not.

 

[qwertypoiu]

Ik the answer is A but can someone tell me how the oxidation number of silver does not change? View attachment 59689

Oxidation state of Ag in AgCl: +1.
Oxidation state of Ag in [Ag(S2O3)2]3- : +1.

Note that S2O3 is 2- charge.

 

[holoholo]

Starting pH is 11.9 and final pH is 1.8. Vertical line is at 15cm^3.
Strong acid with weak base titration.
How do we label the endpoint ? At the start of the vertical line or at the end or at the middle ?

 

[holoholo]

Can someone please draw this fully labelled ?

 

[qwertypoiu]

View attachment 59727

Starting pH is 11.9 and final pH is 1.8. Vertical line is at 15cm^3.
Strong acid with weak base titration.
How do we label the endpoint ? At the start of the vertical line or at the end or at the middle ?

Middle is best

 

[qwertypoiu]

Can someone please draw this fully labelled ?

View attachment 59728

I think something like this should be good

 

[holoholo]

Middle is best

Isn't that the equivalence point ?

 

[holoholo]

Can someone please give me notes on the biochemistry part ?

 

[qwertypoiu]

Isn't that the equivalence point ?

I could be mistaken but I think the endpoint and the equivalence point is the same thing.

 

[alevelpleasebegood]

Can someone explain question 3iv) for me? I looked for the mark scheme but I couldn't understand it. http://studyguide.pk/Past Papers/CIE/International A And AS Level/9701 - Chemistry/9701_w07_qp_4.pdf

 

[The Sarcastic Retard]

Can someone explain question 3iv) for me? I looked for the mark scheme but I couldn't understand it. http://studyguide.pk/Past Papers/CIE/International A And AS Level/9701 - Chemistry/9701_w07_qp_4.pdf

As potential value of Cr2O7(2-)/Cr(3+) is more than Br2/Br(-) So Cr(VI) would oxidise Br- to Br2 easily.

 

[Saad the Paki]

Thank you so much qwertypoiu

 

[holoholo]

For solution C, I wrote [Cu(NH3)4(H2O)2]SO4^2-

But in the mark scheme it is

Can you please explain ?

 

[qwertypoiu]

View attachment 59735

For solution C, I wrote [Cu(NH3)4(H2O)2]SO4^2-

But in the mark scheme it is View attachment 59736

Can you please explain ?

They specifically mentioned ANHYDROUS copper sulphate, dissolved in LIQUID ammonia at -33degrees.

There can be no justification for including H2O molecules in your answer; where can H2O possibly come from? There is no water involved here.
Since Cu2+ has coordination number of 6, the examiner expects the students to realise that 6 molecules of NH3 will form coordinate bonds with the central Cu2+ ion.

 

[holoholo]

Do we have to know what affects the enzymes like pH temperature etc ? What exactly do we have to know about enzymes because i can't seem to understand it from the syllabus ?