Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread

It's simply hydration enthalpy of Sr²⁺ + 2 x hydration enthalpy of F^-

see that's why I'm getting confused...

^ is the (b) part of the question... then here why do we use the provided lattice enthalpy to get this answer for (b) when we didn't use it for (a)? if we solve the same way for (b) as we did (a) we get a wrong answer.

How do you know 'when to do what?'?

(edited 7 years ago)

Reply 701

Neha121101

1

Original post by blueynuey

We know that thermodynamically stable means that the total entropy is less then 0
The entropy of system for this reaction is positive as it is going from a solid --> gas
Therefore stability must come from the entropy of surrounding being more negative then the system, which must mean that enthalpy is positive for surrounding to be negative (look at the formula).

activation energy affects kinetic stability not thermodynamic stability therefore answer can only be B.

thank you!

Reply 702

MacroniCheese

2

Original post by ayvaak

I dont understand why the conc is 0.05. Could someone please help?
Thanks in advance

I got confused there too, but the examiners report said it was a simple dilution.
I guess since you have the same concentrations in the mixture, you are basically putting the 0.1 moldm-3 solutions into twice the volume that it originally was in(25cm3 --> 50cm3). Therefore all you do is 0.1/2 = 0.05moldm-3.

Sorry if it's a bit confusing :/

Reply 703

Neha121101

1

Original post by ihaspotato

see that's why I'm getting confused...

^ is the (b) part of the question... then here why do we use the provided lattice enthalpy to get this answer for (b) when we didn't use it for (a)? if we solve the same way for (b) as we did (a) we get a wrong answer.

How do you know 'when to do what?'?

for part a) you're just working out the overall enthalpy of hydration for both ions, using the data they've given

for part b) you need to use the lattice enthalpy and the data for the enthalpy of hydration to work out the enthalpy of solution. the best thing to do for this type is to draw a hess diagram and use it to find the enthalpy of the solution

hope that helped?

Reply 704

RUNSran

2

Original post by Funky_Giraffe

Could someone explain this? I put D because I thought the less the molecule interacts with the eluent, the faster it travels through. However, the answer is B. Could someone please explain? :smile:

In one type of high-performance liquid chromatography, the stationary phase is non-polar and a polar solvent is used as the eluent. Which of the following would travel through the chromatography column most quickly?
a) tetrachloromethane
b) chloromethane
c) iodomethane
d) hexane

Because the stationary phase is non polar, all the non polar compounds will interact with the stationary phase so they will take much longer to travel through the column- Hexane and tetrachloromethane are both non polar. Now with chloromethane and iodomethane, iodomethane has the greater dispersion forces so interact more with the stationary phase than chloromethane so therefore chloromethane will travel through the column the quickest.

Reply 705

Funky_Giraffe

10

Original post by ihaspotato

see that's why I'm getting confused...

^ is the (b) part of the question... then here why do we use the provided lattice enthalpy to get this answer for (b) when we didn't use it for (a)? if we solve the same way for (b) as we did (a) we get a wrong answer.

How do you know 'when to do what?'?

The questions are asking you to do two different things so you cannot solve them in the same way. You are not supposed to use all of information provided in the table at the stem of the question to answer (a), because it is simply asking you to use hydration enthalpies to calculate the enthalpy change. Part (b) is asking you to work out the lattice enthalpy. Notice how they are two different questions.

Reply 706

Funky_Giraffe

10

Original post by RUNSran

Because the stationary phase is non polar, all the non polar compounds will interact with the stationary phase so they will take much longer to travel through the column- Hexane and tetrachloromethane are both non polar. Now with chloromethane and iodomethane, iodomethane has the greater dispersion forces so interact more with the stationary phase than chloromethane so therefore chloromethane will travel through the column the quickest.

Ah right - so two non-polar molecules interact? I didn't know that :colondollar:

Reply 707

dj3k

9

The pH of 3 solutions with concentration 1 moldm-3 was measured.
Solution 1 = NH3
Solution 2 = CH3COONa
Solution 3 = NH4Cl
Which of the following shows the 3 solutions in order of increasing pH?
A = 1, 2, 3
B = 3, 2, 1
C= 3, 1, 2
D = 2, 3, 1
Why is the answer B? (jan 16 ial)

Reply 708

maria17xo

I am really stuck on this one marker question, i have tried but can't seem to get the right answer. can someone please explain it. it is question 3d on multiple choice. Thanks

http://qualifications.pearson.com/content/dam/pdf/A%20Level/Chemistry/2013/Exam%20materials/6CH04_01R_que_20130612.pdf

Reply 709

Supermanxxxxxx

5

any help on this guys

Reply 710

ihaspotato

2

Original post by Funky_Giraffe

The questions are asking you to do two different things so you cannot solve them in the same way. You are not supposed to use all of information provided in the table at the stem of the question to answer (a), because it is simply asking you to use hydration enthalpies to calculate the enthalpy change. Part (b) is asking you to work out the lattice enthalpy. Notice how they are two different questions.

So finding 'standard enthalpy change' by default is to find the lattice enthalpy?

help d:

Original post by ihaspotato

So finding 'standard enthalpy change' by default is to find the lattice enthalpy?

No - you need to look at the state symbols and how they change in the equations to work out what data to use.

Reply 713

n2697

11

and this one! Thanks in advance

Reply 714

MacroniCheese

2

Original post by Supermanxxxxxx

any help on this guys

You first calculate the moles of HCl - 4x24/1000= 0.096. Since you know that in the all the ammonia in the 25 cm3 must have reacted with HCl to produce that titre the ratio is 1:1. Therefore the number of moles of ammonia in the aqueous layer must be 0.096.
You can then calculate the concentration by doing 0.096x1000/25 which is 3.84moldm-3.

Then just initial concentration minus 3.84 will give you the concentration in the trichloromethane layer

Reply 715

ihaspotato

2

Original post by dj3k

The pH of 3 solutions with concentration 1 moldm-3 was measured.
Solution 1 = NH3
Solution 2 = CH3COONa
Solution 3 = NH4Cl
Which of the following shows the 3 solutions in order of increasing pH?
A = 1, 2, 3
B = 3, 2, 1
C= 3, 1, 2
D = 2, 3, 1
Why is the answer B? (jan 16 ial)

Sol. 1 has hydrogen bonding therefore hard to dissociate. (strongest)
Sol. 2 All carboxylic acid types (COOX) are weak acids
Sol. 3 ??????

Reply 716

tootfruity

11

Original post by ihaspotato

Sol. 1 has hydrogen bonding therefore hard to dissociate. (strongest)
Sol. 2 All carboxylic acid types (COOX) are weak acids
Sol. 3 ??????

3 can dissociate into NH3 and HCl, and then the HCl can dissociate further to produce H+ ions and Cl- ions which are strongly acidic?

Reply 717

ihaspotato

2

Original post by n2697

and this one! Thanks in advance

The answer is C?

1) cm3 > dm3

10/1000 = X

2) X/(990/1000) = Y

3) -log10(Y) = Ans

If C is the right answer....

Reply 718

Don Pedro K.

9

Original post by blueynuey

We know that thermodynamically stable means that the total entropy is less then 0
The entropy of system for this reaction is positive as it is going from a solid --> gas
Therefore stability must come from the entropy of surrounding being more negative then the system, which must mean that enthalpy is positive for surrounding to be negative (look at the formula).

activation energy affects kinetic stability not thermodynamic stability therefore answer can only be B.

Dang, that's a world class explanation right there :smile:

!

Reply 719

MacroniCheese

2

Original post by dj3k

The pH of 3 solutions with concentration 1 moldm-3 was measured.
Solution 1 = NH3
Solution 2 = CH3COONa
Solution 3 = NH4Cl
Which of the following shows the 3 solutions in order of increasing pH?
A = 1, 2, 3
B = 3, 2, 1
C= 3, 1, 2
D = 2, 3, 1
Why is the answer B? (jan 16 ial)

The salt of a weak acid and strong base is slightly basic. This is because ethanoate ions react with water CH3COO- + H2O --> CH3COOH + OH-.

The salt of a strong acid and weak base is slightly acidic. Again because the salt is hydrolysed
NH4+ + H2O --> NH3 + H3O+.

We know ammonia is a weak base so it has to have the highest pH out of the 3. Hence the answer is B

Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread

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