Chemistry: Post your doubts here!

[Student12]

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w04_qp_1.pdf
Can anyone explain Q3,12,19,23,31 ?? In detail please.

 

[asexamskillme111]

Hey fellows need explanations for some paper 1 mcqs​

Here are the links and questions....​

Thanks.......​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_1.pdf​

Q 8 Why is the answer D not A?​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s07_qp_1.pdf​

Q 12 Shouldnt the answer be C not B?​

And please also explain Q 16,18,22,34,40 of this same paper.​

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w05_qp_1.pdf​

Q 14 Why is the answer not C?​

Q 33?​

For Q8,
The graph plotted for pressure against volume always looks something like this http://www.chemistryexplained.com/images/chfa_02_img0350.jpgPressure is inversely proportional to volume. The answer is D, which shows that volume is directly proportional to temperature (when p is constant, of course.)
For Q12 S07
The option C is incorrect because Aluminium chloride has full covalent character, not some. It is a covalent compound.Magnesium chloride however displays some covalent character.
For Q16
The answer is obviously 6. The bond energy for C-Cl is higher than that for C-I. More energy will be needed to break the C-Cl bond which is why the reaction's enthalpy is more negative.
For q18
When compounds containing the ammonium ion react with an alkali, NH3 is given off so the fertiliser has to be NH4NO3 which has an empirical formula of N2H4O3, so D.
For 22,
The answer is A. You have to know organic mechanisms of reactions. 'A' is an electrophillic addition where the bromine molecule develops a dipole because the double bond is a region of high electron density. In the intermediate the Br+ forms a bond with carbon so a positive ion on the compound is developed. The Br- ion then forms another bond forming the final compound.
For 34,
Look at the equation carefully. It says right hand side of the manometer so the volume in R has to increase when temperature is increased. (remember the gases are placed in flasks of fixed volume). So with the first equation (endothermic reactions favoured by increase in temperature) the forward reaction is favoured which has a 1:2 ratio so volume increases hence pushes the liquid to move up right hand side. This doesn't apply to the rest. Answer is D.
For 40,
you need an acid or base to break down the ester. Only the first option is an acid. Answer is D.
For W05 Q14
Phosphorous has a higher ionization energy than sulphur because the 3p subshell is singly filled. Sulphur has 2 electrons in one of the orbitals in the 3p subshell, which repel so IE decreases. Phosphorous has a lower melting point than sulphur because it forms P4 molecules while sulphur forms S8 molecules (more electrons so stronger Van Der Waal's forces) Answer is obviously D.
Q33
It says in the question that the compound has a graphite structure so option 3 is definitely correct. The atoms of boron and nitride are arranged alternately so ratio has to be 1:1. So far options 1 and 3 are correct so the only possible choice is A (all options). That's how I came up with it.
Hope I helped.

 

[hmlahori]

Q8: You've to think about pV=nRT. At constant T, p is inversely proportional to V, and A doesn't show a graph of inversely proportional. At constant p, V and T are directly proportional, which D shows correctly.
Q12: The electro negativity difference between Al and Cl isn't that great as between Mg and Cl, so AlCl3 is mainly covalent.
As for rest of the questions, could you tell why are you not able to do them and where are you facing the problem?

Thanks for your explanations! the rest of the questions have been explained by asexamskillme111. So its ok. but thanks anyways.

 

[hmlahori]

For Q8,
The graph plotted for pressure against volume always looks something like this http://www.chemistryexplained.com/images/chfa_02_img0350.jpgPressure is inversely proportional to volume. The answer is D, which shows that volume is directly proportional to temperature (when p is constant, of course.)
For Q12 S07
The option C is incorrect because Aluminium chloride has full covalent character, not some. It is a covalent compound.Magnesium chloride however displays some covalent character.
For Q16
The answer is obviously 6. The bond energy for C-Cl is higher than that for C-I. More energy will be needed to break the C-Cl bond which is why the reaction's enthalpy is more negative.
For q18
When compounds containing the ammonium ion react with an alkali, NH3 is given off so the fertiliser has to be NH4NO3 which has an empirical formula of N2H4O3, so D.
For 22,
The answer is A. You have to know organic mechanisms of reactions. 'A' is an electrophillic addition where the bromine molecule develops a dipole because the double bond is a region of high electron density. In the intermediate the Br+ forms a bond with carbon so a positive ion on the compound is developed. The Br- ion then forms another bond forming the final compound.
For 34,
Look at the equation carefully. It says right hand side of the manometer so the volume in R has to increase when temperature is increased. (remember the gases are placed in flasks of fixed volume). So with the first equation (endothermic reactions favoured by increase in temperature) the forward reaction is favoured which has a 1:2 ratio so volume increases hence pushes the liquid to move up right hand side. This doesn't apply to the rest. Answer is D.
For 40,
you need an acid or base to break down the ester. Only the first option is an acid. Answer is D.
For W05 Q14
Phosphorous has a higher ionization energy than sulphur because the 3p subshell is singly filled. Sulphur has 2 electrons in one of the orbitals in the 3p subshell, which repel so IE decreases. Phosphorous has a lower melting point than sulphur because it forms P4 molecules while sulphur forms S8 molecules (more electrons so stronger Van Der Waal's forces) Answer is obviously D.
Q33
It says in the question that the compound has a graphite structure so option 3 is definitely correct. The atoms of boron and nitride are arranged alternately so ratio has to be 1:1. So far options 1 and 3 are correct so the only possible choice is A (all options). That's how I came up with it.
Hope I helped.

Thanks for all the explanations. Really appreciate it but i still have some doubts.

For question 8(M/J 08) is the pressure against volume at constant temperature graph even for an ideal gas a curve because i thought it was a straight line and a curve for a real gas thats why my answer was A.

And for the question 16(M/J 07) Why isnt the answer D because that also makes sense?

 

[Pals_1010]

12) You need to know the Boltzmann curve for this question. There's no other way of doing this question. When you increase the temperature the peak moves to the right indicating the average energy of the molecules has increased. Answer is B.

23) Most of us will require some sort of 'structure-drawing' in this question. Here are the all the possible substitutions:
1-bromoethane; 1,1-dibromoethane; 1,2-dibromoethane; 1,1,1-tribromoethane; 1,1,2-tribromoethane; 1,2,2-tribromoethane; 1,1,1,2-tetrabromoethane; 1,1,1,2,2-pentabromoethane and finally 1,1,1,2,2,2-hexabromoethane. (It's not as complicated as it sounds. Try drawing all of them)
Ans: D

24) We haven't learnt electrophilic substitution yet, so C and D can be ruled out. (It usually only happens in aromatic compounds). Carbonyls react with CN- ions by nucleophilic substitution so B is the answer.

25) Saturated compounds = alkanes
A,B and C can be ruled out since these reactions require a double bond. D is the correct answer.
(oops sorry, I accidentally did this question; I see it's not on your list)

36) Thermal stability decreases down the group of Hydrogen Halides. (HI decomposes with just a lit match) So 1 is ruled out. This renders C as the answer. However, let's look at the other two statements anyway. 2 - Bond length increases down the group of HXs. This is because the radii of halogens increase down the group so their nuclei move further and further away from the bonding electrons with hydrogen. 3 - Ease of oxidation increases down the group of HXs. Recall that H2SO4 can not oxidize HCl but oxidizes HI to I2.

37) This question is pretty confusing. I need to confirm it from my teacher, first. Thank you so much for bringing this to our attention. :-]
The marking scheme says the answer is A.

38) 1 - this has more Cl atoms so obviously the amount of NaCl formed will be greater. 2 - the bond length of C-Br is more that that of C-Cl and hence it's broken more easily, so obviously NaBr should also be greater. 3 - the bond length of C-I is still longer. So answer is A.

Thank You So Much

 

[hendrixgirl]

hey everyone, i need desperate help on this.
may june 2007 chemistry paper 5 .. question number 2) part e)
How would i find the formula from the graph?
please pleeeeeeeeeeeease help me out!

 

[Amy Bloom]

Can somebody explain this to me:
Predict the change in Ecell of the Daniell cell if the concentration of
(i) CuSO4 is increased?
(ii) ZnSO4 is increased?
I found that in both cases the Ecell increases, but i'm told dat for Zn it decreases. I'm totally !!! Can somebody out there clarify this??

 

[geek101]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_1.pdf

Q8!! i dunno...both A an D seem right. any help?? thnx in advance

 

[Amy Bloom]

Can somebody explain this:
H-F----------------------- ▲Hf = -271 Kjmol-1
H-Cl---------------------- ▲Hf = -92 Kjmol-1
H-Br---------------------- ▲Hf= -36 Kjmol-1
H-I------------------------ ▲Hf = +26.5 Kjmol-1
Do i need to speak about the decreasing stability of the hydrides???

Explain Why's the trend in ▲H formation becoming more endothermic down Group VII????

 

[Student12]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s08_qp_1.pdf

Q8!! i dunno...both A an D seem right. any help?? thnx in advance

As mentioned by asexamskillme111 above ---> The graph plotted for pressure against volume always looks something like this

Pressure is inversely proportional to volume. The answer is D, which shows that volume is directly proportional to temperature (when p is constant, of course.)

 

[Student12]

Can somebody explain this:
H-F----------------------- ▲Hf = -271 Kjmol-1
H-Cl---------------------- ▲Hf = -92 Kjmol-1
H-Br---------------------- ▲Hf= -36 Kjmol-1
H-I------------------------ ▲Hf = +26.5 Kjmol-1
Do i need to speak about the decreasing stability of the hydrides???

what exactly is the question ? Mcq ?

 

[hmlahori]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s08_qp_1.pdf

Q8!! i dunno...both A an D seem right. any help?? thnx in advance

This question has been explained to me by asexamskillme111 up on this same page. You can understand it from there.

 

[geek101]

As mentioned by asexamskillme111 above ---> The graph plotted for pressure against volume always looks something like this http://www.chemistryexplained.com/images/chfa_02_img0350.jpgPressure is inversely proportional to volume. The answer is D, which shows that volume is directly proportional to temperature (when p is constant, of course.)

oops ddnt see but thnx but for ideal gases the graph of volume and 1/pressure is directly proportional, hence a straight line....so if you want to make that a volume and pressure graph its going to be a straight line but with a negative gradient. so A seems right too :s

 

[Amy Bloom]

what exactly is the question ? Mcq ?

Explain the trend in ▲Hformation down group VII

 

[Student12]

Explain the trend in ▲Hformation down group VII

H-F----------------------- ▲Hf = -271 Kjmol-1
H-Cl---------------------- ▲Hf = -92 Kjmol-1
H-Br---------------------- ▲Hf= -36 Kjmol-1
H-I------------------------ ▲Hf = +26.5 Kjmol-1

So the bond enthalpy for HCL is +431 HBr is +366 and HI is +299 which is a given data.
Hydrogen halides formed are all simple molecular compounds which are gaseous at room temp. As the size oh the Hydrogen atom increases down the Gp, the H-X bond length also increase.Consequently, the H-X bond enthalpy decreases down the Gp.As you notice that the decrease in bond enthalpies means that the stabilities of the Hydrogen halides decrease down the group. HCl is stable at 1500 degrees, HBR decomposes at 800 decrease and HI decomposes at 500 thus HI is least stable of these three Hydrogen halides.

I've given a try

 

[hmlahori]

Can anyone please explain question 16 of this paper? why isnt the answer D? since its the enthalpy change of formation the bond energy of the reactants should matter most not of the products.
http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s07_qp_1.pdf

 

[geek101]

Can anyone please explain question 16 of this paper? why isnt the answer D? since its the enthalpy change of formation the bond energy of the reactants should matter most not of the products.
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s07_qp_1.pdf

inorder to form a H-I bond lesser energy is required since the bond energy is low and so the enthalpy of formation is high, and the H-Cl bond requires a higher value of energy since it has a greater bond energy value so the enthalpy of formation is low. so it basically depends on the bond energy values, which will show the overall energy of formation, which is more or less indirectly proportional to the bond energy values. does it sound confusing?

look at it this way....if you find out the enthalpy change of formation it is reactants - products. If the product lets suppose is H-I so the product has low energy. so the reactants-products gives a positive value. If the product is H-Cl it has a high energy so reactants-products gives you a low value. like in the question, so we know that it is defined using bod energies.

 

[hmlahori]

inorder to form a H-I bond lesser energy is required since the bond energy is low and so the enthalpy of formation is high, and the H-Cl bond requires a higher value of energy since it has a greater bond energy value so the enthalpy of formation is low. so it basically depends on the bond energy values, which will show the overall energy of formation, which is more or less indirectly proportional to the bond energy values. does it sound confusing?

look at it this way....if you find out the enthalpy change of formation it is reactants - products. If the product lets suppose is H-I so the product has low energy. so the reactants-products gives a positive value. If the product is H-Cl it has a high energy so reactants-products gives you a low value. like in the question, so we know that it is defined using bod energies.

I guess it makes sense now thanks.

 

[Student12]

Someone Describe the use of lime in agriculture ?

 

[Student12]

Someone Describe the use of lime in agriculture ?

is it to neutrilise acid and to lower the scidity of the soil?