pm1

Mar 21, 2012
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The passage gives Reaction 1

2 HI --> H2 + I2 and states that the reaction is exothermic.

4. Assuming that Reaction 1 is reversible, increased temperature within the reaction system would most likely produce:
A. increased initial rate of the forward reaction and increased equilibrium concentration of HI.
B. increased initial rate of the forward reaction and increased equilibrium concentration of H2 and I2.
C. decreased initial rate of the reverse reaction and decreased equilibrium concentration of HI.
D. decreased initial rate of the reverse reaction and increased equilibrium concentration of H2 and I2.

Answer: A

The answer explanation says that "An increase in temperature ALWAYS results in an increase in the initial rate of the forward reaction" - which makes sense conceptually. However, I'm used to the idea of considering temperature as product in a exothermic rxn so I was looking for an answer that would favor the reverse reaction, but it seems to not be the case?

Thank you!
 

chiddler

5+ Year Member
Apr 6, 2010
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You are confusing rate of reaction with equilibrium.

For rate of reaction,



this is the same k in the rate of reaction equations like rate = k [A]

now lets look at equilibrium constant

ΔG = -RTln(Keq)
-ΔG / RT = ln(Keq)
Keq = e^ (-ΔG/RT)

This shows dependence of both these constants on temperature. BUT. They are distinct!
 

MedPR

Removed
Dec 1, 2011
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The passage gives Reaction 1

2 HI --> H2 + I2 and states that the reaction is exothermic.

4. Assuming that Reaction 1 is reversible, increased temperature within the reaction system would most likely produce:
A. increased initial rate of the forward reaction and increased equilibrium concentration of HI.
B. increased initial rate of the forward reaction and increased equilibrium concentration of H2 and I2.
C. decreased initial rate of the reverse reaction and decreased equilibrium concentration of HI.
D. decreased initial rate of the reverse reaction and increased equilibrium concentration of H2 and I2.

Answer: A

The answer explanation says that "An increase in temperature ALWAYS results in an increase in the initial rate of the forward reaction" - which makes sense conceptually. However, I'm used to the idea of considering temperature as product in a exothermic rxn so I was looking for an answer that would favor the reverse reaction, but it seems to not be the case?

Thank you!
As chiddler points out, you are confusing equilibrium and reaction rate. However, the answer you were looking for is hidden in the second part of answer A. "An increased equilibrium concentration of HI." So because the reaction is exothermic, as you add heat equilibrium shifts to the left. Your understanding of equilibrium is good, you just missed the point of the question.

So even if you didn't know that increasing temperature always results in an increase in the initial rate of the forward reaction (I'm not even sure I accept this statement, but whatever) you can pick the right answer because only one of the answers says that equilibrium shifts to the left.
 

DrRichand1

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May 26, 2008
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Only temp can alter the equilibrium of a reaction.
The temp is increasing so wouldn't the products side be favored since there is more moles of gas on product side. That would increase the concentration of products.

Does larger equilibrium concentration of HI mean larger P/R quotient, so more products would be made giving a larger deltaK constant for HI?
 

MedPR

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Dec 1, 2011
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Only temp can alter the equilibrium of a reaction.
The temp is increasing so wouldn't the products side be favored since there is more moles of gas on product side. That would increase the concentration of products.

Does larger equilibrium concentration of HI mean larger P/R quotient, so more products would be made giving a larger deltaK constant for HI?
Larger equilibrium concentration of HI means that equilibrium shifts to the left. You'd have a smaller P/R.

The question doesn't say anything about constant pressure or constant volume, so you can't make any assumptions about how temperature will shift the reaction based on moles of gas alone.

Also, if the volume was constant and temperature was increasing, that means pressure would be increasing and you would shift to the side with less moles of gas, not more moles.
 

DrRichand1

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May 26, 2008
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So how could the initial forward rxn be increasing HI--> H2 + I2, and the equ. constant for HI be shifting to the left, wouldn't it decrease going to the right?
 

MedPR

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Dec 1, 2011
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So how could the initial forward rxn be increasing HI--> H2 + I2, and the equ. constant for HI be shifting to the left, wouldn't it decrease going to the right?
Reaction rate and equilibrium concentration are different concepts. Things presumably dissociate when heated, thus promoting dissociation of 2HI into H2+I2. However, since the reaction is exothermic, the equilibrium will shift away from products when heated.
 

DrRichand1

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May 26, 2008
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Oh my bad totally did not read "this is exothermic" makes sense thanks.