help chem mcqs....

[hassam]

doesn't the rate of reaction with hydrogen of halogens increase down the group

 

[abcde]

For 36, I think 1 is correct. Halogens are oxidising agents so 3 is incorrect. The alkali metals react explosively with hydrogen, not halogens so 2 is also incorrect. Halogens form diatomic bonds and since astatine is less reactive than chlorine, it's compound is relatively less stable and thus dissociates more readily than chlorine molecules.
For 37, I think the correct answer would be 2.
Kindly, correct me if I'm wrong!

 

[soumayya]

i fink the ans to 37 is 1, 2 n 3...all 3 seem logical...

 

[hassam]

i dont understand abt one.....
@abcde...i found later in book that flourine reacts explosively with hydrogen EVEN AT -200 deg celsius and reactivity decrease down the group thats why i was wrong at first

 

[abcde]

Oh.. Are my answers correct?

 

[hassam]

ya

 

[hassam]

but
37 all three are correct i dont undrstand y one is correct

 

[abcde]

I think that's because the reaction between nitrogen and oxygen to form oxides of nitrogen are combustion reactions. All combustion reactions are exothermic, so 1 is incorrect! Primarily, the two gases do not react at r.t.p. because they require a high activation energy, which is provided by the high temperature of the hot car engine. C suggests that a great deal of energy is required to break the diatomic nitrogen bond but since the reaction is exothermic, even more energy is required to FORM oxides of nitrogen. Do I make sense?

 

[hassam]

well....its an endothermic reaction at first.....due to high bond energy of Nitrogen..its reactions are usually endothermic

 

[abcde]

:| No, I think this is exothermic because ALL combustion reactions are!

 

[hassam]

check from book

 

[Zishi]

:| No, I think this is exothermic because ALL combustion reactions are!

abcde, not all reaction with oxygen is burning (combustion).

Take for instance the corrosion of metals in air. This is a slow process of oxidation, which certainly cannot be described as burning.

The reaction of nitrogen with oxygen could also included as simple oxidation.

Combustion requires that the reaction with oxygen be self-sustaining. very exothermic and rapid.

Materials that undergo combustion almost invariably produce a vapour which combines exothermically with oxygen via a free radical process.

 

[beacon_of_light]

The reaction b/w N2 and O2 to form nitrogen monoxide is endothermic.... the bond energy of N2 is really very high due to triple covalent bonds!!!
AND
here's the proof...

N2 + 02 ---------> 2NO

Bond energies :
N2 = 994Kj/mol
O2 = 496KJ/mol
NO = 607Kj/mol

Energy required to break bonds = 994 + 496 = + 1490 KJ/mol
Energy released when bonds are formed = 2x 607 = -1214KJ/mol

Enthalpy change = +1490-1214 = + 276 KJ/mol

So the reaction is endothermic and all the three options are correct in 37 !

 

[abcde]

Thank you for correcting me. I'm sorry for providing the wrong information for MCQ 37. How is it possible that it can have all three options correct?

 

[abcde]

Maybe that's because the reason why the two gases do not react at r.t.p. is 2. The other two statements i.e. 1 and 3 may be correct but they do not explain why oxygen and nitrogen do not react together at r.t.p.

 

[beacon_of_light]

I think all three should be correct because nitrogen has a high bond energy so it can not react with oxygen without the input of extra energy ... therefore, activation energy of reaction is high that also accounts for the reaction not occurring without the input of energy !

 

[usman]

N2 + O2 --> 2 NO delta H = 43.2 kcal.

It takes heat to make 2NO i.e. the reaction is endothermic, taking 43.2 kcal per mol