Transcript Chemical Equilibrium

Chemical
Equilibrium
Intro:
 Many
chemical reactions go to
completion (where all reactants are
consumed to form product)
 This unit deals with reactions that do not
completely convert all reactants to
products.
Chemical Equilibrium
A
state where the rate of the forward chemical
reaction is equal to the rate of the reverse chemical
reaction
 Double arrows show that a system is at equilibrium
Characteristics of a system at
equilibrium
1.
2.
3.
4.
Reversible reaction must be possible
There is a dynamic state of balance
between both the forward and
backward reaction
There is no change in concentration
of reactants or products once
chemical equilibrium is reached
There is no bulk change in properties
of the system (ex: no colour or
pressure change)
Characteristics cont.
5.
6.
7.
8.
It is a closed system (no heat or matter
in or out)
The temperature of the system remains
constant
It can be reached from either direction
Any change to the system at equilibrium
can be reversed if restored back to
original equilibrium conditions.
Examples at Equilibrium
 Water
evaporating and condensing in a
jar with lid.
 Any solution at the saturation point.
 A bottle of unopened pop, carbon
dioxide is in equilibrium in solution and air
above pop.
 Walking the wrong way on an escalator.
Equilibrium System Simulation
 With
a partner, take two graduated cylinders
and fill them with a random amount of water
(not full).
 Record the volume of water in each cylinder.
 Take straws and place one into each of the
cylinders, wet your finger and place it on the
top of the straw.
 Transfer the water trapped in the straw to the
other cylinder (do not spill).
 Record new volume in each cylinder.
 Repeat until the volumes are unchanged for
three trials.
 Plot a graph of volume vs trials
Analysis
How were you able
to tell when your
system established
equilibrium?
Would the system be
described as an
open or closed
system?
How does this
simulation model a
dynamic equilibrium
system?
Homework
 Read
Section 7.1
 Page 428 #1,2
 Changing
the temperature affects the
rates of forward and reverse reactions
differently because they have different
activation energies
 Kc = expressed in terms of molar
concentration (mol/L)
Calculating Kc
N2 + 3Cl2 <--> 2NCl3
Temp remains constant in 5 L flask.
Equilbrium was reached and the following was
found: 0.007 mol N2 , 0.0022 mol Cl2 and
0.95 mol of NCl3. Calculate the Kc for this
reaction.


Find molar concentration of everything
(mol/L)
Use equilibrium expression based on
chemical equation
Answer
1.
2.
[N2] = 0.007 mol / 5 L = 0.0014 M
[Cl2] = 0.0022 mol / 5 L = .00044 M
[NCl3] = 0.95 mol / 5 L = 0.19 M
Kc = [NCl3]2 = [0.19]2
[N2][Cl2]3 [0.0014][0.00044]3
= 3.0 x 10 11
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (g)
1. Write an expression for Keq
2. Calculate K at a given
temperature if [CH4] = 0.020 M,
[O2] = 0.042 M, [CO2] = 0.012 M,
and [H2O] = 0.030 M at
equilibrium. (include units)
K eq = 0.306
 Time
to practice…
Practice:
 Read
section 7.2
 Pg 431 #1-3
 Pg 436 #2,3
The value of K
 The
value or magnitude of K tells us the
extent to which reactants have been
converted into products.
 Remember in the ratio for K, the
concentration of products divided by the
concentration of reactants.
A
small value for K means that very little of
the reactants were converted into
products before equilibrium was reached.
 This is stated as “reactants are favoured”.
 A large value of K means that most of the
reactants were converted into products
before equilibrium was reached.
 This is stated as “products are favoured”.
The Magnitude of Equilibrium
Constants
If K >1, then products dominate at equilibrium
and equilibrium lies to the right.
If K <1, then reactants dominate at equilibrium
and the equilibrium lies to the left.
 If
K = 1 neither reactants nor products are
favoured. The value of
 K does not indicate how long it takes for
equilibrium to be reached. The value of K
varies with temperature and that’s why its
usually mentioned with K
2NO2 (g) <--> N2O4 (g)
 Four
experiments
were performed.
 The initial
concentrations of the
two chemicals were
different in each
experiment and the
concentration of
each gas was
measured once the
system reached
equilibrium.
At Equ ilibrium
Exp #
[NO2]
[N2O4]
1
0.104
1.19
2
0.048
0.254
3
0.136
2.04
4
0.202
4.48
Calculate
 Using
the data on the previous page,
calculate the K for each experiment.
 What
did you find?
Homework