O Level Chemistry: Energy Changes (Exo/Endo) & Bond Energy

21 Jun

Posted by: sean in: Chemistry Notes & Tips

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During the Intensive Revision Bootcamp, the concepts surrounding the chapter on Energy Changes (Endothermic/Exothermic) are dissected and the correct concepts are shared with the class.This chapter requires little memorization and in fact, 95% of the chapter involved the understanding of the concepts and applying them accordingly in application questions, especially for those that involves Bond-Forming & Bond-Making.

First we need to define Exothermic and Endothermic properly.

Exothermic Reactions:
- Reactions that gives out heat energy to the surroundings
- T of reaction mixture rises. Container feels hotter

Endothermic Reactions:
- Reactions that takes in heat energy from the surroundings
- Temperature of reaction mixture falls. Container feels cold.

Energy changes in reactions are caused by the making & breaking of chemical bonds

• Bond Making –> Exothermic Process

• Bond Breaking –> Endothermic Process

Tell if a Reaction is Exothermic or Endothermic by:

• Exothermic Reaction: ?H bond breaking < ?H bond making

• Endothermic Reaction: ?H bond breaking > ?H bond making

Q) Is this reaction Exothermic or Endothermic then? What is the value of Enthalpy Change?

Given:
Cl-Cl bond energy = 242 kJ/mol
H-H bond energy = 436 kJ/mol
H-Cl bond energy = 431 kJ/mol

Any takers? Write down your suggested answers in the Comment section below, and we will discuss it accordingly.

PS: Once again –> Participation is Impt! No point me showing you all the answers and working, and u dont learn.

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8 Responses

Amanda
26|Jun|2008 1

Total energy used for breaking bond is 678 while energy for forming bonds is 862. So reaction is… exothermic?

Reply
sean Reply:
December 15th, 2008 at 11:21 am
Hi Amanda,

Yes. You are absolutely spot on.

It is indeed an Exothermic Rxns.

Might be better if you show the proper working step by step in Chemistry Exams.

Examiners are looking for the word “Overall Heat Change” in the chemical reaction is “-184 kj/mol”.

Hope this is clear
Sean
Master Trainer
Author

PS: Keep up the good work.

Reply
Gary
03|Jul|2008 2

Total Energy required to break H-H and Cl-Cl bond = (+436) + (+242)
= +678KJ

Total energy released during bond forming = 2 x (-431)
= -862

Overall heat change = (+678) + (-862)
= -184

The reaction is exothermic because the amount of energy released during bond forming is greater than the energy taken in during bond breaking.

Reply
sean Reply:
December 15th, 2008 at 11:23 am
Hi Gary,

Well done. Your answer is correct and i like the way you present it.

Examiners marking your exam papers will be equally happy to see it too.

PS: Just note that units “kj/mol” need to be reflected at your final answer of “-184 kj/mol”. If not marks will be taken away.

Hope this helps.

Cheers
Sean
Master Trainer
Author

Reply
Brinda
07|Dec|2008 3

Energy required to break H-H and Cl-Cl bond= (+436) + (+242) = +678
Energy required to form HCl bond=2 (-431) =-862

Overall heat change= (+678) + (-862)
=-184
The reaction is exothermic because the amont of energy released during bond forming is greater than energy taken in during bond breaking.

Reply
sean Reply:
December 15th, 2008 at 11:25 am
Hi Brinda,

Well done. Your answer is correct and i like the way you present it.

Examiners marking your exam papers will be equally happy to see it too.

PS: Just note that units “kj/mol” need to be reflected at your final answer of “-184 kj/mol”. If not marks will be taken away. It seems like you made the same error like Gary. Rmbr to reflect the units when showing your final answer/working.

Hope this helps.

Cheers
Sean
Master Trainer
Author

Reply
Saud
20|Feb|2010 4

from the given information

H-H bond’s energy=436 kj
Cl-Cl bond’s energy=242 kj

o total energy on reactants side= 436+242

=678 kj

similarly,

H-Cl bond’s energy=431 kj

so total enery on products side =2 x 431

=862 kj

so by the formula del H rteactants-del H products

678-862= -184 and “-” sign indicates an exothermic reaction as total energy of bond forming is greater than total energy of bond breaking………….. am i corect??????????

Reply
sadia
16|Apr|2010 5

reaction is exothermic bc its a bond making reaction and energy of bond formation 862kj is greater than bond breaking.

Reply