Equilibrium

IB Textbook Chapter 15

Reversible reactions

Some reactions will take place in both directions (reactants → products and reactants

← products)

The reactions occur simultaneously in both directions

Chemical equilibrium

Equilibrium: the forward reaction and reverse reaction take place at the same rate Aka: The Boring Point Written like this: A + B C The word “rate” is important because

There is not the same AMOUNT of product & reactant

You can speed up and slow down the reaction, but it’s still going to go at the same RATE forward and backwards

Keq Equilibrium is a predictable thing Scientists have studied equilibrium

systems and determined a constant for EACH system

The call it… .Keq K = [products]power of the # of moles

[reactants]power of the # of moles

It’s often shown like this:

aA + bB cC + dDKeq = [C]c[D]d

[A]a[B]b

Keq Examples (more? See p. 548 #13)

Try to write K for this equilibrium:2NO2 N2O4

Keq = [N2O4]

[NO2]2

Note: K actually comes in several forms:

Kc Ksp Ka Kb Kw or Kp but, you don’t need all that mess yet…

Keq Examples (more? See p. 548 #13)

And this one… But, K can be a little tricky because you never include liquids (l) or solids (s):

CaCl2(s) Ca2+ (aq) + 2Cl- (aq)

Keq = [Ca2+][Cl-]2

Finally…Ca(s) + 2HOH(l) Ca(OH)2(aq) + H2(g)

Keq = [Ca(OH)2][H2]

Interpreting K

So… What does it mean if

K < 1

K > 1

K = 1

Bottom of the ratio is bigger; There are more reactants than products

Top of the ratio is bigger; There are more products than reactants

The top and bottom are the same; The amounts of products and reactants are equal

Page 548 #19

Le Châtelier’s Principle If an equilibrium system is stressed, it will

adjust/shift to relieve the stress There are 3 ways it can be stressed

Concentration If the con. of reactants or products is increased, the

equilibrium will shift away from the increase If the con. Is decreased, it shifts toward the decrease

Pressure If the pressure on a system is increased, equilibrium

will shift toward the side with least moles of gas Temperature

Heat can be added or removed, and should be treated as a reactant (endothermic) or product (exothermic)

Try it…NH4OH(g) + heat NH3(g) + HOH(g)

What will happen if… Heat is added? The system is cooled? More NH4OH is added? HOH is added? Pressure is decreased? Volume is decreased?

Le Châtelier’s Principle and K?

K can only change when the temperature changes the pressure changes

Changes in concentration still balance out to the same K (power point #2!)

Try it…with KNH4OH(g) + heat NH3(g) + HOH(g)

What will happen if… Heat is added? The system is cooled? More NH4OH is added? HOH is added? Pressure is decreased? Volume is decreased?

K

K

K sameK

sameK

sameK

same