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Will decreasing heat on a exothermic reaction increase the value of equilibrium...

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tv2295 | Student, Grade 11 | Honors

Posted October 30, 2012 at 3:22 AM via web

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Will decreasing heat on a exothermic reaction increase the value of equilibrium constant?

I am doing my chemistry homework.

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jerichorayel | College Teacher | (Level 1) Senior Educator

Posted October 30, 2012 at 5:47 AM (Answer #1)

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If we will consider this following equations:

ΔG = -RTlnK                 eq1

ΔG = ΔH - TΔS             eq2

we can rewrite eq1 to solve for K

K = e^(-ΔG/RT)            eq3

In this form, we can clearly see the relationship of ΔG and K.

An exothermic reaction has -ΔH thus the value of ΔG would be negative (assume that it is enthalpy driven).

Let me show you a hypothetical example.

if ΔH = -4550kJ and ΔS = .10kJ/K at 298 K

ΔG = -4550 - (298 x 0.1)

ΔG = -4579.8 kJ

K = e^(-ΔG/RT)

K = e^(-(-4579.8 kJ/8.31447*298))

K = 6.34

 

if  ΔH value is changed to -4000 kJ (assume  ΔS would be the same) 298 K.

ΔG = -4000 - (298 x 0.1)

ΔG = -4029.8 kJ

K = e^(-ΔG/RT)

K = e^(-(-4029.8/8.31447*298))

K = 5.08

 

we can summarize it as (enthalpy driven):

ΔH      -4550    ---->  -4000

ΔG      -4579.8 ---->  -4029.8

K          6.34    ---->   5.08

 

Conclusion:

 As the heat is decreased in an exothermic reaction, the value of the delta H becomes more positive (-4550 to -4000), the value of K decrease. 

 

 

 

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