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HOW ARE EQUILIBRIUM EXPRESSIONS WRITTEN?. Q VS . K. “Systems”: two reactions that differ only in direction. Any reversible reaction. H 2 + I 2 ↔ 2HI. noted by the double arrow; ↔. Reversible Reactions. H 2 + I 2 ↔ 2HI the products may react back to original reactants.

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systems two reactions that differ only in direction
“Systems”: two reactions that differ only in direction
  • Any reversible reaction

H2 + I2↔ 2HI

noted by the double arrow; ↔

reversible reactions
Reversible Reactions

H2 + I2 ↔ 2HI

  • the products may react back to original reactants.
  • “closed system”:

ONLY if all reactant are present

  • If one piece is completely gone it has ”gone to competition” and no longer reversible
equilibrium
Equilibrium
  • The state in which a chemical reaction and its reverse reaction occur at the same rate.
properties of an equilibrium
Properties of an Equilibrium

Equilibrium systems are

  • DYNAMIC (in constant motion)
  • REVERSIBLE
  • can be approached from either direction

E + Co(H2O)6Cl2nCo(H2O)4Cl2 + 2 H2O

le chatelier s principle
Le Chatelier’s Principle

Can we explain WHY?

  • If a system at equilibrium is stressed, it will react to undo the stress.
the reacton quotient q
The Reacton Quotient, Q

at equilibrium, Q = K

In general, all reacting chemical systems are characterized by their REACTION QUOTIENT

reaction quotient
Reaction Quotient

At anypointin the reaction

H2 + I2 --->2 HI

the equilibrium constant
THE EQUILIBRIUM CONSTANT

For any type of chemical equilibrium of the type

aA + bB--->cC + dD

the following is a CONSTANT (at a given T)

If K is known, then we can predict concs. of products or reactants.

writing and manipulating k expressions
Writing and Manipulating K Expressions

Solids NEVER appear in equilibrium expressions.

S(s) + O2(g) ---> SO2(g)

writing and manipulating k expressions1
Writing and Manipulating K Expressions

Liquids NEVER appear in equilibrium expressions.

NH3(aq) + H2O(liq) ---> NH4+(aq) + OH-(aq)

product or reactant favored
Product- or Reactant Favored

Product-favored

Reactant-favored

slide18
For: N2(g) + 3 H2(g) ---> 2 NH3(g)

Using K: Is the reaction product-favored or reactant-favored?

When K is much greater than 1

the reaction is strongly

product-favored.

slide19
For

AgCl(s) g Ag+(aq) + Cl-(aq)

Kc = [Ag+] [Cl-] = 1.8 x 10-5

If K is much less than 1

The reaction is strongly reactant-favored.

Ag+(aq) + Cl-(aq) gAgCl(s)

is product-favored.

using k
Using K:

Can determine if the reaction is at equilibrium.

slide22
If [iso] = 0.35 M and [n] = 0.15 M, are you at equilibrium?

If not, which way does the reaction “shift” to approach equilibrium?

slide23
REACTION QUOTIENT, Q

Characterize all chemical systems

If Q = K, then system is at equilibrium.

Q (2.33) < K (2.5)

Reaction is NOT at equilibrium,

[iso] must ________ and [n] must ____________.

experimental determination of equilibrium constant k
Experimental Determination of Equilibrium Constant, K

2 NOCl(g) --->2 NO(g) + Cl2(g)

Place 2.00 mol of NOCl is a 1.00 L flask. At equilibrium you find 0.66 mol/L of NO.

Calculate K.

Set of an “ICE” table of concentrations

[NOCl] [NO] [Cl2]

Initial 2.00 0 0

Change

Equilibrium 0.66

determining k
Determining K

2 NOCl(g) ---> 2 NO(g) + Cl2(g)

[NOCl] [NO] [Cl2]

Initial 2.00 0 0

Change -0.66 +0.66 +0.33

Equilibrium 1.34 0.66 0.33

slide26
2 NOCl(g) ---> 2 NO(g) + Cl2(g)

[NOCl] [NO] [Cl2]

Initial 2.00 0 0

Change -0.66 +0.66 +0.33

Equilibrium 1.34 0.66 0.33