will the equilibrium constant for the reaction increase, decrease, or stay the same?

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equilibrium constants and changing conditions

A look at the relationship between equilibrium constants and Le Chatelier's Principle.

Explanation in terms of the constancy of the equilibrium constant

The equilibrium constant, Kc for this reaction looks like this:

If you have moved the position of the equilibrium to the right (and so increased the amount of C and D), why hasn't the equilibrium constant increased?

This is actually the wrong question to ask! We need to look at it the other way round.

Let's assume that the equilibrium constant mustn't change if you decrease the concentration of C - because equilibrium constants are constant at constant temperature. Why does the position of equilibrium move as it does?

If you decrease the concentration of C, the top of the Kc expression gets smaller. That would change the value of Kc. In order for that not to happen, the concentrations of C and D will have to increase again, and those of A and B must decrease. That happens until a new balance is reached when the value of the equilibrium constant expression reverts to what it was before.

The position of equilibrium moves - not because Le Chatelier says it must - but because of the need to keep a constant value for the equilibrium constant.

If you decrease the concentration of C:

Changing pressure

This only applies to systems involving at least one gas.

The facts aren't changed if you change the pressure of the system. The only thing that changes an equilibrium constant is a change of temperature. changed if you change the pressure. According to Le Chatelier's Principle, the position of equilibrium moves in such a way as to tend to undo the change that you have made.

That means that if you increase the pressure, the position of equilibrium will move in such a way as to decrease the pressure again - if that is possible. It can do this by favouring the reaction which produces the fewer molecules. If there are the same number of molecules on each side of the equation, then a change of pressure makes no difference to the position of equilibrium.

Explanation

Let's look at the same equilibrium we've used before. This one would be affected by pressure because there are 3 molecules on the left but only 2 on the right. An increase in pressure would move the position of equilibrium to the right.

Because this is an all-gas equilibriium, it is much easier to use Kp:

Once again, it is easy to suppose that, because the position of equilibrium will move to the right if you increase the pressure, Kp will increase as well. Not so!

To understand why, you need to modify the Kp expression.

Remember the relationship between partial pressure, mole fraction and total pressure?

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Source : www.chemguide.co.uk

The Effect of Changing Conditions

This page looks at the relationship between equilibrium constants and Le Châtelier's Principle. Students often get confused about how it is possible for the position of equilibrium to change as …

The Effect of Changing Conditions

Last updated Aug 15, 2020 The Contact Process The Haber Process picture_as_pdf Readability Cite this page Donate Jim Clark

Former Head of Chemistry and Head of Science at Truro School in Cornwall

This page looks at the relationship between equilibrium constants and Le Chatelier's Principle. Students often get confused about how it is possible for the position of equilibrium to change as you change the conditions of a reaction, although the equilibrium constant may remain the same.

Changing concentrations

Equilibrium constants are not changed if you change the concentrations of things present in the equilibrium. The only thing that changes an equilibrium constant is a change of temperature. The position of equilibrium is changed if you change the concentration of something present in the mixture. According to Le Chatelier's Principle, the position of equilibrium moves in such a way as to tend to undo the change that you have made.

Suppose you have an equilibrium established between four substances A, B, C and D.

A+2B⇌C+D (1) (1)A+2B⇌C+D

According to Le Chatelier's Principle, if you decrease the concentration of C, for example, the position of equilibrium will move to the right to increase the concentration again.

Explanation in terms of the constancy of the equilibrium constant

The equilibrium constant,

K c Kc

for this reaction looks like this:

K c = [C][D] [A][B ] 2 (2) (2)Kc=[C][D][A][B]2

If you have moved the position of the equilibrium to the right (and so increased the amount of

C C and D D

), why hasn't the equilibrium constant increased? This is actually the wrong question to ask! We need to look at it the other way round.

Let's assume that the equilibrium constant must not change if you decrease the concentration of

C C

- because equilibrium constants are constant at constant temperature. Why does the position of equilibrium move as it does?

If you decrease the concentration of

C C , the top of the K c Kc

expression gets smaller. That would change the value of

K c Kc

. In order for that not to happen, the concentrations of

C C and D D

will have to increase again, and those of

A A and B B

must decrease. That happens until a new balance is reached when the value of the equilibrium constant expression reverts to what it was before. The position of equilibrium moves - not because Le Chatelier says it must - but because of the need to keep a constant value for the equilibrium constant.

If you decrease the concentration of

C C :

Changing pressure

This only applies to systems involving at least one gas. Equilibrium constants are not changed if you change the pressure of the system. The only thing that changes an equilibrium constant is a change of temperature. The position of equilibrium may be changed if you change the pressure. According to Le Chatelier's Principle, the position of equilibrium moves in such a way as to tend to undo the change that you have made.

That means that if you increase the pressure, the position of equilibrium will move in such a way as to decrease the pressure again - if that is possible. It can do this by favoring the reaction which produces the fewer molecules. If there are the same number of molecules on each side of the equation, then a change of pressure makes no difference to the position of equilibrium.

Case 1: Differing Numbers of Gaseous Species on each side of the Equation

Let's look at the same equilibrium we've used before. This one would be affected by pressure because there are three molecules on the left, but only two on the right. An increase in pressure would move the position of equilibrium to the right.

A (g) +2 B (g) ⇌ C (g) + D (g) (3)

(3)A(g)+2B(g)⇌C(g)+D(g)

Because this is an all-gas equilibrium, it is much easier to use

K p Kp : K p = P c P D P A P 2 B (4) (4)Kp=PcPDPAPB2

Once again, it is easy to suppose that, because the position of equilibrium will move to the right if you increase the pressure,

K p Kp

will increase as well. Not so! To understand why, you need to modify the

K p Kp

expr ession. Remember the relationship between partial pressure, mole fraction and total pressure?

P A

=(mole fraction of A)(total pressure)

(5)

(5)PA=(mole fraction of A)(total pressure)

P A = χ A P tot (6) (6)PA=χAPtot

Replacing all the partial pressure terms in

4 4 by mole fractions χ A χA

and total pressure (

P tot Ptot ) g ives you this: K p ( χ C P tot

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Source : chem.libretexts.org

Chem 1102 Exam 3 Chapter 17 Flashcards

Memorize flashcards and build a practice test to quiz yourself before your exam. Start studying the Chem 1102 Exam 3 Chapter 17 flashcards containing study terms like Which of the following would occur if solid NH4Cl was added to an aqueous solution of NH3?, Which of the following conjugate acid-base pairs will function as a buffer?, The pH of the equivalence point of a weak acid-strong base titration is _________. and more.

Chem 1102 Exam 3 Chapter 17

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Which of the following would occur if solid NH4Cl was added to an aqueous solution of NH3?

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The pH decreases.

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Which of the following conjugate acid-base pairs will function as a buffer?

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HF/F-

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Terms in this set (13)

Which of the following would occur if solid NH4Cl was added to an aqueous solution of NH3?

The pH decreases.

Which of the following conjugate acid-base pairs will function as a buffer?

HF/F-

The pH of the equivalence point of a weak acid-strong base titration is _________.