CHEMICAL EQUILIBRIUM - RATES OF REACTION

1
CHEMICAL EQUILIBRIUM - RATES OF REACTION
kF Reactants
?? products
kB Chemical reactions are a dynamic process,
that is, reactions involve both forward and
reverse processes. Chemical Equilibrium is
reached by as reaction mixture when the rates of
forward and reverse reactions becomes
equal. kF kB NO net change appears
obvious although the system is still in constant
motion.
2
LAW OF MASS ACTION k ? equilibrium
constant aA bB ?? cC dD k
Products Reactants k Cc Dd
Q reaction quotient Bb Aa Write
the equilibrium equation for a. HC2H3O2 ?? H
C2H3O2- b. H2O H2O ?? H3O OH- c.
4NH3(g) 302(g) ?? 2N2(g) 6H2O(g) d.
N2(g) 3H2(g) ?? 2NH3(g)
3
Predicting the direction of reaction Q gt K
forms more reactants ? Q K
equilibrium Q lt K forms more products
? Note 1. kf 1 kr 2. k kn If
the balanced equation is multiplied by a
factor then the K( Q) is multiplied by the
exponent.
4
K as either Kc or Kp Kc the equilibrium
constant using concentrations. Kp the
equilibrium constant using pressure P n ?
P x n RT v
v Kp Kc(RT) ?ngas Write Kp the Kc for 1.
N2(g) 3H2(g) ?? 2NH3(g) 2. N2O4(g) ??
2NO2(g) 3. Calculate kp if kc 0.105 for 1 4.
2SO3(g) ?? 2SO2(g) O2(g) if Kc 4.07 x
10-3, what is kp?
5
N2 O2 ?? 2NO a. Since Q is very small
Qltltlt1, very little, NO will form _at_ 25C. The
equilibrium lies to the left favoring
reactants. N2 O2 2NO b. Kp (PNO)2
?n 2 - 2 0 PN2PO2
kp Kc Kc NO2
N2O2 c. Kp PN2 PO2 (PNO)2 d.
?n 2 - 2 0 kp kc kc
N2O2 NO2
6
DIRECTION OF REACTIONS AND Keg 1. The
following reaction is a means of fixing
nitrogen N2(g) O2(g) ?? 2 NO(g) A. If
the value for Q at 25C is 1 x 10-30, describe
the feasibility of this reaction for
Nitrogen fixation. B. Write the
equilibrium expression, Kc C. Write the
equilibrium expression for 2NO(g) ?? N2(g)
O2(g) D. Determine the Kc for C
7
HOMOGENEOUS EQUILIBRIA H2(g) I2(g) ??
2HI(g) Kp (PHI)2 (PI2)(PH2)
Kc ? 2O3(g) ?? 3O2(g) Kp
(PO2)3 (PO3)2 Kc ?
HETEROGENEOUS EQUILIBRIA 2H2O(l) ??
H3O(aq)_ OH-(aq) Kc H3OOH- C2H5OH(l
) 3O2(g) ?? 2CO2(g) 3H2O(g) Kc ?
8
HETEROGENEOUS EQUILIBRIA Substance in more
then 1 phase 1. CaCO3(s) ?? CaO(s)
CO2(g) Kc CaOCO2 CaCO3 How
do the of solid express? A. D
g/cm3 mol MW g/mol
cm3 Pure solids liquid have constant
Kc constant CO2 constant Kc Kc
con CO2 con
9
Each of the mixtures listed below was placed in a
closed container and allowed to stand. Which of
these mixtures is capable of attaining the
equil, expressed by 1 a) pure CaCO3 b) CaO
PCO2 gt Kp c) solid CaCO3 PCO2 gt Kp d) CaCO3
CaO
10
CALCULATING THE Keq 1. In one experiment,
Haber introduced a mixture of H2 N2 into a
reaction vessel and allowed the system to attain
chemical equilibrium at 472C. The equilibrium
mixture of gases were analyzed and found to
contain 0.1207M H 2, 0.0402M N2, and 0.00272M
NH3. Calculate Keq. 2. Nitryl Chloride,
NO2Cl, is in equilibrium in a closed container
with NO2 and Cl2. 2 NO2Cl(g) ?? 2 NO2(g)
Cl2(g) Calculate Keq if NO2Cl
0.00106M NO2 0.0108M Cl2 0.00538M
11
3. For the Haber process N2(g) 3H2(g) ??
2NH3(g) Kp 1.45 x 10-5 at 500C If an
equilibrium mixture of the three gas started
with partial pressures of 0.928 atm for H2 and
0.432 atm for N2, what is the partial pressure
of NH3? 4. A 1.00 L flask is filled with 1.00
mol of H2 and 2.00 mol I2 at 448C is
50.5. What are the equilibrium concentrations
of H2, I2 HI?
12
CALCULATING Keq 1. A mixture of 5.0 x 10-3 mol
of H2 and 1.0 x 10-2 mol of I2 is placed in a
5.0L container at 448C and allowed to come to
equilibrium. Analysis of this equilibrium
mixture shows that the HI is 1.87 x 10-3
M. Calculate Kc H2(g) I2(g) ?? 2HI(g)
2. At 448C the equilibrium constant Kc for the
reaction below is 50.5. H2(g) I2(g) ??
2HI(g) Predict how the reaction will proceed to
reach equilibrium if the initial amount of HI is
2.0 x 10-2 mol, H2 is 1.0 x 10-2 mol, and I2 is
3.0 x 10-2 mol in a 2.00 L container.
13
APPLICATION OF Keq 1. Predicting the
direction of reaction Q reaction
quotient at equil Q K Q gt K ? species on Rt
(prod) (no net Rx) react to form left K
Equil Q Non Equil Q lt K ? forms
more products Goal Calculate Q to determine
state of Rx, equil, more product or more
reaction
14
1. At 448C the equilibrium constant Kc for the
reaction is 50.5. H2(g) I2(g) ??
2HI(g) Predict how the Rx will proceed to reach
equil at 448C if the initial amount of HI is 2.0
x 10-2 mol, 1.0 x 10-2 mol, H2 and 3.0 x 10-2 mol
I2 in at 2.0 L container. HI 2 x 10-2
mol/2.0L 1.0 x 10-2M H2 1.0 x
10-2mol/2.0L 5.0 x 10-3M I2 3.0 x
10-2mol/2.0L 1.5 x 10-2M Q Prod
HI2 (1.0 x 10-2)2
1.3 React H2I2 (5 x
10-3)(1.5 x 10-2) since K 50.5, Q 1.3,
Q lt K HI will need to increase and
H2I2 will decrease to reach equilibrium.
15
2. At 1000K the value of Kc for the
reaction 2S03(g) ?? 2SO2(g) O2(g) is
4.07 x 10-3. Calculate the value for Q and
predict the direction in which the reaction will
proceed towards equil if the initial
concentration of reactants are SO3 2 x
10-3 M SO2 5 x 10-3 M O2 3 x 10-2
M Q 0.2 reaction will proceed from Rt to
left forming SO3.
16
CALCULATING Keq 1. Sulfur Trioxide
decomposes at High temperature in a sealed
container. 2 SO3(g) ?? 2 SO2(g) O2
(g) Initially the vessel is filled at 1000K
with SO3(g) at a concentration of 6.09 x 10-3M.
At equilibrium, the SO3 is 2.44 x 10-3M.
Calculate Kc. 2. Calculate the value for Q and
predict the direction in which the reaction
will proceed towards equilibrium if the initial
concentrations are SO3 2.0 x
10-3M SO2 5.0 x 10-3M O2 3.0 x
10-2M
17
LE CHATELIERS PRINICIPLE If a system at
equilibrium is disturbed by a change in
temperature, pressure, or concentration of one
of its components, that system will shift its
equilibrium position as to counteract the
effect of the disturbance. Equilibrium can be
disturbed by - adding or removing
components - a change in pressure - a change
in volume - a change in temperature
18
PREDICTING THE DIRECTION OF THE SHIFT I.
CATALYST A catalyst increases the rate at which
equilibrium is achieved but not the composition
of the equilibrium mixture. II. THE REACTION
QUOTIENT Q lt K the reaction shifts to the
products Q gt K the reaction shifts to the
reactants III. CHANGES IN VOLUME Reducing the
volume of a gas at equilibrium causes the system
to shift in the direction that reduces the
number of moles of gas.
19
IV. CHANGES IN TEMPERATURE When heat is added to
a system, the equilibrium shifts in the
direction that absorbs heat. Cooling has the
opposite effect and shifts the equilibrium toward
s the side which produces heat. Endothermic
Reactions heat Reactants ? Products An
increase in temperature leads to a
shift towards the products, a decrease leads to
a shift towards the reactants. (Keq
increases) Exothermic Reactions Reactants ?
Products heat An increase in temperature
leads to a shift towards reactants. (Keq
decreases)
20
Example N2O4(g) ?? 2 NO2(g) ?H
58 kJ In which direction will the equilibrium
shift when each of the following changes are made
to a system at equilibrium? A) add N2O4 B)
remove NO2 C) increase the total pressure by
adding N2 D) increase the volume of the
container E) decrease the temperature
21
Example PCl5(g) ?? PCl3(g) Cl2(g)
?H 88 kJ In which direction will the
equilibrium shift when each of the following
changes are made to a system at equilibrium? A)
add Cl2 B) temperature is increased C) the
volume of the reaction system is decreased D)
PCl5 is added E) a catalyst is added
22
N2O4 ? 2NO2 A) The system will adjust so to
decrease N2O4 shifts to ? products B)
Shifts to ? reactants (NO2 removed) C) N2 will
increase total pressure but since it is not
involved in Rx the partial pressures of N2O4
and NO2 are unchanged, no shift. D) ? volume
shifts ? to more moles of gas E) ? Temp - Rx
is endo heat N2O4 ?? 2 NO2 Temp ? shifts so
more heat produced ? K is affected
23
1. The equilibrium constant for the Haber
process at 472C is Kc 0.105. A 2.00 L flask
is filled with 0.500 mol of ammonia and is then
allowed to reach equilibrium at 472C. What
are the equilibrium concentrations? N2(g)
3 H2(g) ?? 2 NH3(g) 2. For the
reaction PCl5(g) ?? PCl3(g) Cl2(g) at a
certain temperature Kc equals 450. What
will happen when 0.10 mol of PCl5, 1.0 mol of
PCl3, and l.5 mol of Cl2 are added to a 2.0-L
container and the system is brought to the
temperature at which Kc450. What are the
equilibrium concentrations?
24
EFFECT OF VARIOUS DISTURBANCES ON AN
EQUILIBRIUM SYSTEM DISTURBANCE NET DIRECTION OF
REACTION EFFECT ON VALUE OF
K Concentration Increase (reactant) Toward
formation of product None Decrease
(reactant) Toward formation of reactant None Pre
ssure (volume) Increase P Toward formation
of lower amount (mol) of gas1 None
Decrease P Toward formation of higher amount
(mol) of gas None Temperature Increase
T Toward absorption of heat Increases
?Hrxngt0 Decreases if ?Hrxnlt0
Decrease T Toward release of heat Increases
?Hrxnlt0 Decreases ?Hrxngt0 Catalyst
added None rates of forward and
reverse reactions increase equally None
25
VANT HOFF EQUATION Changes in K due to T In
K2 - ?Hrxn ( 1 - 1) K1 R
T1 T2 R 8.314 J/mol K T
Kelvin The formation of methanol is an important
industrial reaction in the processing of new
fuels. At 298K, Kp 2.25 x 104 for the
reaction CO(g) 2 H2(g) ?? CH3OH(l) If
?Hrxn -128 kJ/mol CH3OH, calculate kp at
0C.
26
CH4(g) CO2(g) ?? 2CO(g) 2
H2(g) A. What is the percent yield of H2 when
equimolar mixture of CH4 and CO2 with a total
pressure of 20.0 atm reaches equilibrium at
1200K at which Kp 3.548 x 106? B. What is
the percent yield of H2 for this system at
1300K, at which Kp 2.626 x 107? C. Use
the Vant Hoff equation to find ?Hrxn.