Equilibrio Químico

17-1

Kinetics applies to the speed of a reaction, the concentration of product that appears (or of reactant that disappears) per unit time.
Equilibrium applies to the extent of a reaction, the concentration of product that has appeared after an unlimited time, or once no further change occurs.
At equilibrium:

rateforward = ratereverse

A system at equilibrium is dynamic on the molecular level; no further net change is observed because changes in one direction are balanced by changes in the other.

17-2

Reaching equilibrium on the macroscopic and molecular levels.
N2O4(g)

17-3

2NO2(g)

Equilibrium ≠ Equal
• the rates of the forward and reverse reactions are equal at equilibrium
• …ver más…

SOLUTION:

[NO2]4[O2]

(a) 2 N2O5(g) ⇌ 4 NO2(g) + O2(g)

Qc =

[N2O5]2
[CO2]3[H2O]

(b)

17-8

C3H8(g) +

5O2(g)

⇌

3CO2(g) +

4H2O(g)

Qc =

[C3H8][O2]5

4

Q - The Reaction Quotient
At any time, t, the system can be sampled to determine the amounts of reactants and products present.
Q is calculated in the same manner as K c [D]d

[A]

Q=

[C]

a

[B]b

We use the molar concentrations of the substances in the reaction. This is symbolised by using square brackets - [ ].
For a general reaction aA + bB ⇌ cC + dD where a, b, c, and d are the numerical coefficients in the balanced equation, Q
(and K) can be calculated as
17-9

Calculating Variations on Q and K

aA + bB ⇌ cC + dD

[C] c [D]d
Qc =
[A]a [B]b

cC + dD ⇌ aA + bB

Q’ =

1
Qc

n

aA + bB ⇌ cC + dD

Qc’ = (Qc)n

For a sequence of equilibria, Qoverall = Q1 x Q2 x Q3 x …

17-10

Sample Problem 17.2

Writing the Reaction