5.4 Flashcards
Reactions Not at Equilibrium
•For the reaction conditions shown;
Concentration (M)
equilibrium is not reached until NO
concentration
reaches 0.0125 mol/L
0.08
0.06
0.04
0.02
line
Time
(a)
Chemical
Equilibrium
N
o
NO.
2NO
P2 (3)
0.08
0.06
0.04
The Reaction Quotient
Q
Keg
0.02
eq
0
Q-lT[D
[A|[B
Time
(bl
Chemical
•Reaction quotient (Q): a numerical value
determined by using the same formula as the
equilibrium constant (using data for a reversible
reaction) that may or may not be at equilibrium.
For: aA + bB cC + dD:
3-4/ 20
NO
Determining the Direction of a Reaction
NO
•The symbols forQ and K are placed on a number
•The reaction will move in the direction from Q to K.
eg
Determining the Direction of a Reaction
Q>K:
Determining the Direction ofa Rea( 67 /20
ratio of products
to reactants is
too large
reaction will
proceed LEFT to
reach
equilibrium.
Q=
[products]
[reactants]
Reaction proceeds
toward right
forming products
Example 1:
eqilitriur is
Q> K
net reaction to left
Reactions Not at Equl lilbrium
ui
The Reaction Quotlent
C
•Reaxtion ouotiernt (Q) a numerical value
using the sane torry
Q=
uil NO
:6centretion
tormuls zs the
raactlcnl that may or may nat be at equi lari am.
erminlng the Dlrectlon of a Reaction
K
Q=K:
Qand K
are olkced ur a uribe
raler wll mnes in the cirertiar fon Qto K
the system is at
equilibrium.
K= Q=
[products]
[reactants]
Q= K
Reaction proceeds
toward left
forming reactants
no net reaction
Q<K:
Determining the Direction of a Reaction
ratio of products
Calculate Q to determine the direction of reaction when
the concentrations are: [CH
]=0.100 M
M
[H
AAnilibciun constant for the reaction below is 5.67.
to reactants is to0
small
reaction
will proceed
RIGHT to reach
equilibrium.
Q=
[products]
[reactants]
Q<K
net reaction to right
CHạ (e) + H
O) Coig + 3H2()
Example 2:
Determining the direction of a Reaction
In the Haber process for manufacturing ammonia
nitrogen and hydrogen combine in the presence of a
catalyst:
Nzte) + 3H (a) 2NHa (e)
At 300 °C
the value of K._ for this reaction is 7.3.
eq
The following concentrations of gases are present in
a container at 300°C: [N
] = 0.10 mol/L
mol/L
and [NH
gases at equilibrium? If not
in which direction will
the reaction go to reach equilibrium?
Example 3: Calculating Equilibrium
Concentrations from Initial Concentration
Carbon monoxide reacts with water vapour to
produce carbon dioxide and hydrogen. At 900 °d
K = 4.200. Calculate the concentrations of all
entities at equilibrium if 4.000 mol of each entity
are initially placed in a 1.000 L closed container.
eo
Step 1: Calculate concentrations given c=nv
Step 2: Calculate the value of Q
Example 3 Continued
Step 3: Set up an ICE chart
Determining the Direction of a Reactinn
ERample
Initial conc. (mol/L)
Determining the Direction of a
pTcHad LEFT
ceullbdurh.
|Change in conc. (mol/4)
ratio of aradu.cti tha systam Is at rato of praeucts
snall reacrior
BISHT OrE8th
ing the Direction af a neaction
M. EH
O| 0.200 M and J]=0200 M
onc.
co(g)
4
= 4.000 mol/1.000 L
= 4.000 M
blve for X
Q = [C0
][H
= (4.000)(4.000)/(4.000)(4.
= 1.000.. NOT at equilibril
+ H
ole)
4
co
(e)
4
H
(e)
4
Step 1. Write the balanced equation for the reaction.
Step 2. Under the balanced equation
make a table that lists for each substance
involved in the reaction:
(a) The initial concentration
the
(b) The change in concentration on going to equilibrium
(c) The equilibrium concentration
In constructing the table
define x as the concentration (mol/L) of one of
he substances that reacts on going to equilibrium
then use the
stoichiometry of the reaction to determine the concentrations of the other
substances in terms of x.
Step 3. Substitute the equilibrium concentrations into the equilibrium equation for
the reaction and solve for x. If you must solve a quadratic equation
choose the mathematical solution that makes chemical sense.
Step 4. Calculate the equilibrium concentrations from the calculated value of x.
Example 4
Step 5. Check your results by substituting them into the equilibrium equation.
Calculations with Imperfect Squares
•The “thousand rule” is an assumption made to
simplify problems:
If the ratio of:
[initial concentration of reactant]/Keq> 1000
then x is very small compared to initial
concentration
so it is considered negligible with
respect to the change from the initial
concentration
and x may be removed from that
part of the calculation.
**Note: this is nota great rule
but acceptable for high school
chemistry
Carbon monoxide is a primary starting material in the
synthesis of many organic compounds
including
methanol
CH
13-14/ 20
At 2000°C
K is 6.40 x 10 for the decomposition of
carbon dioxide into carbon monoxide and oxygen.
Calculate the concentrations of all entities at
equilibrium if 0.250 mol/L of COo
lel is placed in a
closed container and heated to 2000 °C.
2C02 (8)
- 2C0e + Oz(e)
