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/molAny 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.
Related Articles:
- O Level Chemistry: Energy Changes – Exothermic & Endothermic Reactions
- Chemistry Phenomenon: Energy Changes (Endo/Exo) in Cold Packs
- O Level Chemistry: Common Errors in Energy Changes (Endothermic / Exothermic)
- Chemistry Phenomenon: Endothermic / Exothermic Reactions in Everyday Life
- Chemistry Phenomenon: Thermit Process to weld steel railway lines together
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
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
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
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
reaction is exothermic bc its a bond making reaction and energy of bond formation 862kj is greater than bond breaking.
Reply
zulfiqar Reply:
March 6th, 2011 at 11:55 pm
i am pakistani u ? any website which tell about questions about electrolysis ,helping to prepare tests plz thanx
Reply
Pls I would like to know if an exothermic reaction can be reversed and then the same amount of heat regenerated over and over again? and if it is possible what comination of chemicals would one require for that? Looking forward to hearing from you. Thanks for your anticipated answer.
Reply