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Calculating the Equilibrium Constant from measured equilibri

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Anmol Arora

on 14 February 2015

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Transcript of Calculating the Equilibrium Constant from measured equilibri

- from measured equilibrium concentrations
Calculating the Equilibrium Constant
~ ICE tables ~
Calculating the Equilibrium Constant
aA+ bB --> cC +dD
<--
K << 1: reverse reaction is favored

K = 1: neither direction favored

K >> 1: forward reaction is favored
I
1.0M 0.0M

C

-0.3 +0.5

E

0.7
0.5
Kc = _[Z]_
[X]
Solution:
Initial:
The concentration that is given for the beginning reaction

C
hange:
The change that occurred in the concentrations to reach equilibrium

E
quilibrium:
The final concentrations after the reaction reached equilibrium

I-C-E

An Introduction
I-C-E

An example:
Solution:
Now determine the equilibrium constant:
The equilibrium constant (K)
What does K mean?
By: Anmol Arora
University of New Mexico
CAPS Supplemental Instruction Program
~ a way to quantify the concentrations of reactants and products at equilibrium
K = _[C]_[D]__
[A] [B]
c

d
a b

3X --> 5Z
<--
Find equilibrium constant, Kc, given initial 1.0 M X, 0.0 M Z, and equilibrium 0.5 M of Z.

3
X -->
5
Z
<--
0.5
mol Z *
3
mol X =
0.3mol
X being used
/
5
mol Z
-> Since
0.5
moles of Z are produced, we use this number with Stoichiometric Coefficients for the ratio from balanced equation to determine how much X is used to produce
0.5
moles of Z when the ratio is
3
X:
5
Z
-> To determine how much X is left over, we simply subtract 0.3 from initial 1.0 M to get 0.7 equilibrium concentration
What to do:
-Make an ICE table
-Determine what is given
-Use stoichiometry to determine how much of the reactant is being used or product is being produced
-Find equilibrium concentrations
-Determine Kc
5
3
-> Use equilibrium concentrations from the ICE table (last row) to determine Kc
Kc = _
[0.5]
_
[0.7]
5
3
==> _
0.03125
_ ==>
Kc = 0.09
0.343
-> Can be concluded that reverse reaction is favored (K<<1)
I-C-E

Practice yourself:
1) 3.7 M H2 (g) reacts with 2.90 M I2 (g) to produce some amount of product, 2HI (g). At equilibrium, 0.09 M of I2 (g) remain. Determine the equilibrium constant for this reaction.


2) 0.1 M I2 (g) and 0.1 M Cl2 (g) react together to produce some amount of product, 2 ICl (g). The equilibrium constant, Kc, for this reaction was determined to be 81.9. Find equilibrium concentrations of I2, Cl2, and ICl.
I-C-E

Another example:
Find equilibrium concentrations of reactants and product, given initial 1.0 M CO (g), 2.0 M NH3 (g) that react to form HCONH2 (g). The equilibrium constant for this reaction is 0.700.
CO (g) + NH3 (g) --> HCONH2 (g)
<--
CO (g) + NH3 (g) --> HCONH2 (g)
<--
I
1.0M 2.0M 0.0M

C

-x -x +x

E

(1-x) (2-x) x
Kc = _[HCONH2]_
[CO]*[NH3]
0.7 = ____[X]____ => 0.7(1-x)(2-x) = x
[1-x]*[2-x]
0.7(2-x-2x+x) = x => 1.4-2.1x+0.7x = x
2
2
->Use Quadratic to determine, x, the change
in: 0.7x-3.1x+1.4 = 0

x = 0.5

Therefore, equilibrium concentrations:
CO = 1-0.5= 0.5 M
NH3 = 2-2(0.5) = 1.0 M
HCONH2 = 0.5 M
-> manipulate equation to write in Quadratic form
-x
-x
2
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