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TODAY A new chapter: Equilibrium Writing equilibrium expressions

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TODAY A new chapter: Equilibrium Writing equilibrium expressions observing LeChatelier’s Principle Calculating equilibrium constants, K. TODAY A new wrinkle on Equilibrium: Q. Q, from the Q continuum, Star Trek ,The Next Generation No, not that Q . TODAY

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Presentation Transcript
slide1
TODAY
    • A new chapter: Equilibrium
    • Writing equilibrium expressions
    • observing LeChatelier’s Principle
    • Calculating equilibrium constants, K
  • TODAY
    • A new wrinkle on Equilibrium:
    • Q
  • Q, from the Q continuum,
  • Star Trek ,The Next Generation
  • No, not that Q ..
slide2
TODAY
    • A new chapter: Equilibrium
    • Writing equilibrium expressions
    • observing LeChatelier’s Principle
    • Calculating equilibrium constants, K
  • TODAY
  • A quick recap of Monday’s concepts
  • Magnitudes of Keq
  • Q: the reaction quotient
  • Q: how to use it
slide3
A quick recap on equilibrium expressions:

[products]

Keq

[reagents]

For this reaction:

[Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]++ H2O

But if reaction goes BOTH to right and to left,

which are reagents and which are products?

By convention”

reagents are species to the left of arrow

products are species to the right of arrow

slide4
A quick recap on equilibrium behavior:

You observed this chemical system:

[Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]++ H2O

When you added excess Cl- :

[Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]++ H2O

When you added excess H2O:

[Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]++ H2O

slide5
The series of reactions observed:

1. CuCl2(H2O)2+ 3H2O

[CuCl(H2O)5]+ + Cl-

2. [CuCl(H2O)5]+ + H2O

[Cu(H2O)6]2++ Cl-

3. [Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]2++ H2O

[CuCl(H2O)52+]

To calculate Keq

[Cu(H2O)62+][Cl]

0.80 M

Need Keq

concentrations

0.28,

NO UNITS!

0.40M x7.1 M

Pure liquids and solids don’t appear in Keq expression:

slide6
Combining reactions equals multiplying K’s

(a) CuCl2(H2O)2+ 3H2O

[CuCl(H2O)5]+ + Cl-

K1eq

(b) [CuCl(H2O)5]+ + H2O

[Cu(H2O)6]2++ Cl-

K2eq

Kneteq

Net CuCl2(H2O)2+ 4H2O

[Cu(H2O)6]2++ 2 Cl-

[CuCl(H2O)52+ [Cl]

[Cu(H2O)62+][Cl]

K1eq x K2eq

[CuCl(H2O)52+]

CuCl2(H2O)2

[Cu(H2O)62+][Cl]2

K1eq x K2eq

Kneteq

CuCl2(H2O)2

slide7
Are Your Eyes Misleading You?

What is in the graduated cylinder?

Visible electronic

spectra

mixture can

appear GREEN

Abs

[CuCl(H2O)5+]

[Cu(H2O)62+]

700 nm

400 nm

wavelength

slide8
Are Your Eyes Misleading You?

What is in the graduated cylinder?

Recall equilibrium concentrations:

[CuCl(H2O)5+]

0.8 M

Keq

0.28

0.4 M x7.1M

[Cu(H2O)62+][Cl]

Visible electronic

spectra

If Keq ~ 1, product concentrations are

similar to reagent concentrations

Abs

mixture can

appear GREEN

[CuCl(H2O)5+]

[Cu(H2O)62+]

700 nm

400 nm

wavelength

slide9
How large must Keq be for reaction to be “complete”?

Consider this reaction:

[Ni(H2O)6]2++ 6 NH3

[Ni(NH3)6]2++ 6H2O

Keq

2.0 x 108

[Ni(NH3)6]2++ 3 “en”

[Ni(en)3]2++ 6 NH3

Keq

7.3 x 109

slide10
What is Keq for this reaction: ?

[Ni(H2O)6]2++ 3 “en”

[Ni(en)3]2++ 6 H2O

[Ni(H2O)6]2++ 6 NH3

[Ni(NH3)6]2++ 6H2O

K1 = 2.0 x 108

[Ni(NH3)6]2++ 3 “en”

[Ni(en)3]2++ 6 NH3

K2 = 7.3 x 109

K = K1x K2

[Ni(H2O)6]2++ 3 “en”

[Ni(en)3]2++ 6 H2O

K = K1x K2 = (7.3 x 109)(2.0 x 108)

= 1.5 x 1018

slide11
What happens if the concentrations are equal:

[CuCl(H2O)5+]= 0.8 M, [Cu(H2O)62+]= 0.8 M, [Cl] = 0.8M

for this reaction:

[Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]++ H2O

You investigate by calculation:

[CuCl(H2O)5+]

0.8 M

1.25

[Cu(H2O)62+][Cl]

0.8 M x0.8 M

But you know Keq = 0.28 ≠ 1.25: what does this mean?

It’s not at equilibrium!!

slide12
So under these concentration conditions:

[CuCl(H2O)5+]= 0.8 M, [Cu(H2O)62+]= 0.8 M, [Cl] = 0.8M

[CuCl(H2O)5+]

1.25 > 0. 28 = Keq

[Cu(H2O)62+][Cl]

This tells you one definite thing:

there’s too much in numerator, or,

there’s too much product

How will reaction system species behave?

[Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]++ H2O

[CuCl(H2O)5+] decreases, [Cu(H2O)62+] increases,

[Cl] increases

slide13
This is Q!!! Ratio of Concentrations under

Non-Equilibrium conditions

[products]

Q

[reagents]

aA + bB

cC + dD

[C]c[D]d

Q

[A]a[B]b

Q: the Reaction Quotient

slide14
The reaction quotient Q can be determined for

any set of concentrations

Possible outcomes

[C]c[D]d

1. Q

Keq

[A]a[B]b

[C]c[D]d

2. Q

> Keq

[A]a[B]b

[C]c[D]d

3. Q

< Keq

[A]a[B]b

slide15
Example problems to be used with reaction:

[Cu(H2O)6]2++ Cl-

[CuCl(H2O)5]++ H2O

Keq = 0.28

A. [CuCl(H2O)5+]= 0 M, [Cu(H2O)62+]= 0.4 M, [Cl] = 0.4 M

B. [CuCl(H2O)5+]= 1 M, [Cu(H2O)62+]= 1 M, [Cl] = 0.5 M

C. [CuCl(H2O)5+]= 0.01 M, [Cu(H2O)62+]= 0.01 M, [Cl] = 0.01 M

Compare this result with earlier equimolar at 0.8M !!

slide16
Calculations, calculations, calculations,

Many types:

1. Calculating K from equil. concentrations

2. Calc’gKeq from initial and changed concentrations

3. Calc’g final concentrations from initial, change and Keq

4. Calculating a new K from adding 2 reactions

slide17
The ICEbox method

For these types:

2. Calc’gKeq from initial and changed concentrations

3. Calc’g final concentrations from initial, change and Keq

slide18
Lots of K’s

Keq – a general equilibrium constant

Kc - equilibrium constant in concentrations

Ksp - equilibrium constant for solubility

Kb - equilibrium constant for OH- formation

Kp - equilibrium constant in partial pressures

Ka - equilibrium constant for H+ dissociation

Kf - formation constant for metal complexes

Not to be confused with:k – rate constants

slide19
The K “Zoo”

Keq

Kf

Ksp

Kp

Ka

Kc

Kb

little k

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