O Level Chemistry: Energy Changes (Endothermic & Exothermic Reactions)

15 Oct

Posted by: sean in: Chemistry Notes & Tips

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Let’s check out this question on Energy Changes:

Question:
When solid potassium chloride is added to water, the temperature of the mixture falls. What conclusion can be made from this observation?
A. The process is endothermic.
B. The process is exothermic.
C. All solids dissolve with a fall in temperature.
D. Very little potassium chloride dissolves in water.

Anyone would like to share your answers for above Energy Changes questions?

PS: Leave your suggested answers and explanations in the “Comment” section below.

Rgds
Sean

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14 Responses

illuminate-x
16|Oct|2007 1

It should be A: the process if endothermic. When the temperature of the mixture falls, it means that the atoms making up potassium chloride take in heat from the surrounding liquid (in this case). The temperature of the mixture will then fall. In any case, the dissolving of hydrated salts in water is an endothermic reaction…

I hope I’m right! xD

Reply
LinwooD
16|Oct|2007 2

i think it’s A

the process is endothermic

Reply
Anonymous
16|Oct|2007 3

the asn shld be B rite??

Reply
lijun(:
17|Oct|2007 4

A.

when the reaction is endothermic, it takes in heat energy, thus the temperature of the surrounding air will fall.

Reply
pf
18|Oct|2007 5

i thought it will be b?
cos now it’s the temperature of the mixture that falls and not the surrounding temp

Reply
ruth
18|Oct|2007 6

I kinda get why some of you think it’s B, but it’s A. Cause even though their energy level has gone up, it doesn’t mean that their temperature goes up. Know what I mean? Energy doesn’t exist only just as heat energy. That’s why it’s A. Does that make sense to you?

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Anonymous
18|Oct|2007 7

It is exothermic because the reaction gives out energy. Thus the fall in temperature of the products. You might have confused it with the fact that heat is given out to the surrounding thus the temperature of the surrounding rises.

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Spirit-Breaker
18|Oct|2007 8

A. Endothermic. Heat energy is taken in to decrease the temperature.

Reply
Sean Chua
20|Oct|2007 9

An endothermic reaction is any chemical reaction that absorbs heat from its environment.

Mixing of Potassium Chloride with water is indeed experimentally proven to a an endothermic reaction. Potassium Chloride takes in heat energy from the solution and thus the temperature of the solution (which is in this case the environment) falls.

Here’s a list of examples of endothermic reactions:
- Reaction of barium hydroxide octahydrate crystals with dry ammonium chloride
- Dissolving ammonium chloride in water
- Reaction of thionyl chloride (SOCl2) with cobalt(II) sulfate heptahydrate
- Mixing water and ammonium nitrate
- Reacting ethanoic acid with sodium carbonate
- Photosynthesis (chlorophyll is used to react carbon dioxide plus water plus energy to make glucose and oxygen)

Hope this helps.

Sean
SimpleChemConcepts
“Learning Experience with A Difference”

Reply
Anonymous
23|Oct|2007 10

the reaction is endo….because it absorb heat to break bond so the temperature will fall

Reply
Nitratez
08|Apr|2008 11

Isn’t it B ???

Reply
sean
08|Apr|2008 12

Hi. Answer is A. Check out my earlier comments to understand the reason.
Always be clear what is the subject (body) and what is the environment. When
they ask about temperature change, ask yourself this question – are they asking
about the subject or the environment.

To Your Academic Success in Chemistry,

Sean
Master Trainer
Author

Reply
small boy
01|Feb|2009 13

i think is a

Reply
sean Reply:
February 9th, 2009 at 4:20 pm
Hi “small boy”,

Care to share the reason behind your answer being A??

Look forward to your response.

Cheers
Sean

Reply