Need urgent help! Chem paper tomorrow! Watch

Nikita Verma
Badges: 14
Rep:
?
#1
Report Thread starter 3 years ago
#1
Hi! There's a question in one of the chemistry past papers (9701_s14_qp_23 que 4 a) which says:
Explain the decreasing trend in the reactivities of the halogens with hydrogen in terms of bond energies.
The marking scheme says :
X-X bond strength decreases from chlorine to iodine
But decreasing strength of H-X down group more significant.

Can someone please explain why?? How does decreasing bond energy lead to a DECREASE in reactivity......
please help as soon as possible.
0
reply
me2*
  • Political Ambassador
Badges: 11
Rep:
?
#2
Report 3 years ago
#2
which unit is this ?
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#3
Report 3 years ago
#3
(Original post by Nikita Verma)
Hi! There's a question in one of the chemistry past papers (9701_s14_qp_23 que 4 a) which says:
Explain the decreasing trend in the reactivities of the halogens with hydrogen in terms of bond energies.
The marking scheme says :
X-X bond strength decreases from chlorine to iodine
But decreasing strength of H-X down group more significant.

Can someone please explain why?? How does decreasing bond energy lead to a DECREASE in reactivity......
please help as soon as possible.
Because of the atomic radius of the halogens, it gets bigger down the group so bond strength decreases as the atomic radius gets bigger. The bigger the atomic radius the easier it is to react because bonds are easier to break. Goes from not very reactive (small atomic radius halogen)-chlorine to very reactive (big atomic radius halogen)- iodine
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#4
Report 3 years ago
#4
(Original post by Nikita Verma)
Hi! There's a question in one of the chemistry past papers (9701_s14_qp_23 que 4 a) which says:
Explain the decreasing trend in the reactivities of the halogens with hydrogen in terms of bond energies.
The marking scheme says :
X-X bond strength decreases from chlorine to iodine
But decreasing strength of H-X down group more significant.

Can someone please explain why?? How does decreasing bond energy lead to a DECREASE in reactivity......
please help as soon as possible.
http://www.xtremepapers.com/revision...add/symhbr.php


look at this, this will help explain it better. There is a biit on this page about halides, just scroll down.
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#5
Report 3 years ago
#5
WAIT HOLD ON! Decrease is reactivity? no it's an increase in reactivity.
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#6
Report 3 years ago
#6
(Original post by Nikita Verma)
Hi! There's a question in one of the chemistry past papers (9701_s14_qp_23 que 4 a) which says:
Explain the decreasing trend in the reactivities of the halogens with hydrogen in terms of bond energies.
The marking scheme says :
X-X bond strength decreases from chlorine to iodine
But decreasing strength of H-X down group more significant.

Can someone please explain why?? How does decreasing bond energy lead to a DECREASE in reactivity......
please help as soon as possible.

Me again!

What they mean here by decreasing trend is just what decreases and that's why they mentioned bond energies because they want you to talk about the bond but not the reactivity pattern, not the decrease in reactivity because that doesn't happen. There is an increase.

sorry I hope that helps.
0
reply
Nikita Verma
Badges: 14
Rep:
?
#7
Report Thread starter 3 years ago
#7
(Original post by me2*)
which unit is this ?
I'm not sure...i guess it's the unit on group 7.
0
reply
Nikita Verma
Badges: 14
Rep:
?
#8
Report Thread starter 3 years ago
#8
(Original post by TheonlyMrsHolmes)
Me again!

What they mean here by decreasing trend is just what decreases and that's why they mentioned bond energies because they want you to talk about the bond but not the reactivity pattern, not the decrease in reactivity because that doesn't happen. There is an increase.

sorry I hope that helps.
Hey. I'm sorry. I probably should have posted the question properly. Here it is:
4. The halogens and their compounds have a wide variety of uses and the chemical and physical properties show regular patterns related to their positions in Group VII.
a) Chlorine,bromine and iodine all react with hydrogen.
i) state the trend in the reactivities of the halogens with hydrogen.
ii) Explain the trend in terms of bond energies.


So the trend is definitely decreasing...
0
reply
littleangel9914
Badges: 15
Rep:
?
#9
Report 3 years ago
#9
(Original post by Nikita Verma)
Hey. I'm sorry. I probably should have posted the question properly. Here it is:
4. The halogens and their compounds have a wide variety of uses and the chemical and physical properties show regular patterns related to their positions in Group VII.
a) Chlorine,bromine and iodine all react with hydrogen.
i) state the trend in the reactivities of the halogens with hydrogen.
ii) Explain the trend in terms of bond energies.


So the trend is definitely decreasing...
Your right it is decreasing as fluorine is the most reactive halogen due to its electronegativity. What the answer is trying to say is that while halogen-halogen bond strength decreases meaning it's easier to split them and react the strength of the bonds of H-X is more important and fluorine can displace all other halogens as it forms the strongest bonds with hydrogen. Hence halogen reactivity decreases.


That what it sounds like to me but A-levels were a few years ago now.
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#10
Report 3 years ago
#10
(Original post by Nikita Verma)
Hey. I'm sorry. I probably should have posted the question properly. Here it is:
4. The halogens and their compounds have a wide variety of uses and the chemical and physical properties show regular patterns related to their positions in Group VII.
a) Chlorine,bromine and iodine all react with hydrogen.
i) state the trend in the reactivities of the halogens with hydrogen.
ii) Explain the trend in terms of bond energies.


So the trend is definitely decreasing...
(ii) Trend in terms of bond energies is a decrease in bond strength due to atomic radius increasing which makes bonds easier to break. (i) But reactivity is increasing.
0
reply
littleangel9914
Badges: 15
Rep:
?
#11
Report 3 years ago
#11
(Original post by TheonlyMrsHolmes)
(ii) Trend in terms of bond energies is a decrease in bond strength due to atomic radius increasing which makes bonds easier to break. (i) But reactivity is increasing.
I'm pretty sure that halogens (F,Cl, Br etc.) decrease in reactivity down the group which is what the question is referring to. I think your thinking about H-F, H-Cl etc. which are halides whose reactivity decreases as you go down.
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#12
Report 3 years ago
#12
(Original post by littleangel9914)
I'm pretty sure that halogens (F,Cl, Br etc.) decrease in reactivity down the group which is what the question is referring to. I think your thinking about H-F, H-Cl etc. which are halides whose reactivity decreases as you go down.
Yes I am talking about halides because that whats the mark scheme said, but iodine is definitely the fastest reaction so it increases as you go down. Fluorine is the slowest.

'But decreasing strength of H-X down group more significant'

it's a pretty unclear question tbh, it also mentions X-X bond in the mark scheme

'X-X bond strength decreases from chlorine to iodine '
0
reply
littleangel9914
Badges: 15
Rep:
?
#13
Report 3 years ago
#13
(Original post by TheonlyMrsHolmes)
Yes I am talking about halides because that whats the mark scheme said

'But decreasing strength of H-X down group more significant'

it's a pretty unclear question tbh, it also mentions X-X bond in the mark scheme

'X-X bond strength decreases from chlorine to iodine '
To form a halide two things happen:

The X-X bond is broken which takes energy so weaker bonds will require less energy. Less energy required more reactive to an extent.

The H-X bond is formed which releases energy. Stronger bonds release more energy. H-X bond strength decreases down the group meaning less reactive. As H-X is more important this is what determines reactivity of halogens and so reactivity of halogens decreases down the group.
I'm not sure about reaction speed but fluorine is the most reactive as its can displace H-Cl whereas Cl cannot displace H-F.
0
reply
Nikita Verma
Badges: 14
Rep:
?
#14
Report Thread starter 3 years ago
#14
(Original post by littleangel9914)
To form a halide two things happen:

The X-X bond is broken which takes energy so weaker bonds will require less energy. Less energy required more reactive to an extent.

The H-X bond is formed which releases energy. Stronger bonds release more energy. H-X bond strength decreases down the group meaning less reactive. As H-X is more important this is what determines reactivity of halogens and so reactivity of halogens decreases down the group.
I'm not sure about reaction speed but fluorine is the most reactive as its can displace H-Cl whereas Cl cannot displace H-F.
Yes this makes perfect sense. Thank you so much for your help!
0
reply
Nikita Verma
Badges: 14
Rep:
?
#15
Report Thread starter 3 years ago
#15
[05/05 19:22] Ma'am: This is due to the increased atomic size which increases the energy levels this lessens the attraction for valence electron s of other atoms decreasing reactivity. This decrease also happens due to decrease in electronegativty down group
[05/05 19:28] Nikita Verma: Ohh okay. But ma'am how is this related to the bond strength of H-X?
[05/05 19:40] Nikita Verma: Ma'am, does this make sense: iodine is not that electronegative so it cannot pull the electron from hydrogen completely, so the bond is not strong as it would not take much energy to pull the electron back to hydrogen and separate the atoms. So bond energy decreases and so does reactivity.... This can be related to the inductive effect.

^^ this is what I gathered....thought it might help others with similar doubts
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#16
Report 3 years ago
#16
(Original post by littleangel9914)
To form a halide two things happen:

The X-X bond is broken which takes energy so weaker bonds will require less energy. Less energy required more reactive to an extent.

The H-X bond is formed which releases energy. Stronger bonds release more energy. H-X bond strength decreases down the group meaning less reactive. As H-X is more important this is what determines reactivity of halogens and so reactivity of halogens decreases down the group.
I'm not sure about reaction speed but fluorine is the most reactive as its can displace H-Cl whereas Cl cannot displace H-F.
That's a misunderstanding on my part, I was talking about the reaction speed of the halides. Of course they increase as there is a decrease in bond energies. I wasn't referring to the halogens themselves, my bad!

sorry for that.
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#17
Report 3 years ago
#17
(Original post by Nikita Verma)
[05/05 19:22] Ma'am: This is due to the increased atomic size which increases the energy levels this lessens the attraction for valence electron s of other atoms decreasing reactivity. This decrease also happens due to decrease in electronegativty down group
[05/05 19:28] Nikita Verma: Ohh okay. But ma'am how is this related to the bond strength of H-X?
[05/05 19:40] Nikita Verma: Ma'am, does this make sense: iodine is not that electronegative so it cannot pull the electron from hydrogen completely, so the bond is not strong as it would not take much energy to pull the electron back to hydrogen and separate the atoms. So bond energy decreases and so does reactivity.... This can be related to the inductive effect.

^^ this is what I gathered....thought it might help others with similar doubts
can I just ask where you asked this question? another forum?
0
reply
Nikita Verma
Badges: 14
Rep:
?
#18
Report Thread starter 3 years ago
#18
(Original post by TheonlyMrsHolmes)
can I just ask where you asked this question? another forum?
If you're referring to answer above then no, it wasn't a forum, just a discussion with my chem teacher and best friend :P
Also thanks for responding to my original question. It lead to quite an enriching discussion
0
reply
TheonlyMrsHolmes
Badges: 17
Rep:
?
#19
Report 3 years ago
#19
(Original post by Nikita Verma)
If you're referring to answer above then no, it wasn't a forum, just a discussion with my chem teacher and best friend :P
Also thanks for responding to my original question. It lead to quite an enriching discussion
No problem, I also learnt something too what exam do you have tomorrow? good luck for it
1
reply
Nikita Verma
Badges: 14
Rep:
?
#20
Report Thread starter 3 years ago
#20
(Original post by TheonlyMrsHolmes)
No problem, I also learnt something too what exam do you have tomorrow? good luck for it
Thank you! Sorry for my late reply. My chemistry theory, biology MCQ and Psychology papers are over! Phew!
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Edge Hill University
    Undergraduate and Postgraduate - Campus Tour Undergraduate
    Mon, 18 Feb '19
  • University of the Arts London
    MA Innovation Management Open Day Postgraduate
    Mon, 18 Feb '19
  • University of Roehampton
    Department of Media, Culture and Language; School of Education; Business School Undergraduate
    Tue, 19 Feb '19

Do you give blood?

Yes (46)
9.26%
I used to but I don't now (14)
2.82%
No, but I want to start (177)
35.61%
No, I am unable to (116)
23.34%
No, I chose not to (144)
28.97%

Watched Threads

View All