O Level Chemistry: Energy Changes – Exothermic & Endothermic Reactions

This is a question sent by a local student that is reading this Chemistry Blogsite very closely.

Hi there i would like to ask a qn regarding the chapter on energy changes and hope you will reply. (:

Q) When steam is passed through white-hot coke, a reaction occurs and the temperature of the coke falls.What does this indicate?

Answer: The reaction is endothermic(this is a mcq qn)

My question to you is: “Why is the answer endothermic?Since the temperature of the coke falls, isnt it giving out energy?By giving out energy, thus the temperature falls.And the definition of exothermic is heat exits while for endothermic it is heat taken in.So, why isnt the answer exothermic then?

P.S i find this chapter the terms exothermic and endothermic quickly deceiving and confusing.

This chapter is in fact a very short chapter that does not require much memorization or calculation to score. But you do need to have a good foundation of the concepts behind it.

Below will be my suggested answer. Important concept to grasp for this topic on Energy Changes is to understand & differentiate SYSTEM (i.e. reactants , products) and its Surrounding (i.e. container, surrounding air, solution, etc). Click on image to have a clearer view)

PS: Check “comments” section below for suggested answers

Muhammad Ashfaq

April 10, 2008 | 3:58 pm

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sean Reply:
February 9th, 2009 at 4:56 pm

Hi Muhammad,

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Thank you.

To Your Academic Success in Chemistry,
Sean
Master Trainer
Author

cheezy

February 1, 2009 | 5:30 pm

i want to know how to know if the qn is endo or exo like 2C4H10 + 13O2 – 8CO2 + 10H2O

sean Reply:
February 9th, 2009 at 5:01 pm

Hi “Cheezy”,

Where are you from?

Whether a reaction is endothermic or exothermic, you would need to have the Enthalphy Change information given to you in the question.

If not given, which is commonly the case in GCE O Level Chemistry PAPER 1 (MCQ), you would need to know the common chemical reactions..

Based on the equation you have given above, it shows that a hydrocarbon (fuel) has been combusted (burned) in the presence of oxygen gas.

In combustion, the energy changes is EXOTHERMIC, since more heat energy is given out! to the environment.

Hope you find this clear for you.

cheezy Reply:
February 10th, 2009 at 6:47 pm

THkS

cheezy Reply:
February 10th, 2009 at 6:48 pm

O Level Chemistry: Energy Changes – Exothermic & Endothermic Reactions

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