Equilibrium has 4 characteristics.
1. It is dynamic
2. Forward & reverse reactions occur at the same rate
3. Concentration of reactants and products stay the same in equi
4. It requires a closed system
2 of 19
Reversible reactions
Equilibrium
1 of 19
Definition; A reaction where the products can react to reform the reactants is
called a reversible reaction
Reversible reactions often do not have the highest yield formed because the
products are reacting together to reform the reactancts, and vice versa. This is
called an equilibrium reaction.
Definition; dyanmic equi means that molecules of reactants and products are
constantly reacting.
Dynamic Equi
3 of 19
Same rate of fwd & bckwd reactions
Rate of fwd reaction = rate of bckwd reaction
Equi can be reached by approached from both directions of the equation
4 of 19
Both temperature and concentration of products affects the position of the equi
"Position of equi" = relative to the amounts of products & reactants that are
present in an equi mixture.
6 of 19
Closed systems
Position of equi
5 of 19
Definition; A closed system is where none of the reactants or products can
escape from the reaction mixture.
If the reaction takes place in a liquid where no gas is lost, equilbrium of this
reaction can be studied in an open system.
Definition; If one or more factors that affect equi are changed, then equi shifts in
the direction to oppose/reduce this change.
Le Chatelier's Principle
7 of 19
Conc. affects on equi
When conc of the REACTANTS is increased;
Posi of equi moves to the RIGHT away from the reactants
More products are produced as a result of this
When conc of the PRODUCTS is increased;
Posi of equi moves to the LEFT
More reactants are formed as a result of this
8 of 19
Special note; if there are EQUAL moles of gas on each side, this change in
pressure does not affect the equi in this system
Only affects gases
When pressure in a system is INCREASED
Molecules move closer together
Equi shifts to minimise this increase
Shifts to the direction of fewer gas molecules
Pressure affects on equi
9 of 19
Temp. affects on equi
10 of 19
When temperature of equi is INCREASED;
The reaction will go in the direction that opposes the change, so it will go in
the direction where energy is ABSORBED - this is the endothermic reaction
When temperature of equi is DECREASED;
The reaction will go in the direction that opposes the change, so it will go in
the direction where energy is RELEASED - this is the exothermic reaction
If all other conditions in the equi stay constant, then the value of Kc does not
change when the conc. of reactants or products is altered
12 of 19
Equi constant, Kc
Kc & conc. changes
11 of 19
Definition; The Kc is the equilibrium constant. The equi constant is a number that
expresses the relationship between the amount of products and reactants that
are present in a mixture.
The way to work out Kc is to divide the products by the reactants
Each species is raised to a power (that is the number of moles)
Units of Kc are dependant on the form of the equi expression
(c)C + d(D) = A(a) + (b)B
Eg. Kc = A(a) B(b)
C(c) D(d)
Special note; in equations that have a solid in, you ignore the solid's
concentration because it pretty much stays constant.
If all other conditions in an equi stay constant, chaging the pressure does not
have an affect on the Kc.
Kc & pressure changes
13 of 19
Kc & temperature changes
14 of 19
ENDOTHERMIC REACTION; Increasing the temperature in an endothermic
reaction means that the Kc will increase.
EXOTHERMIC REACTION; Increasing the temperature in an exothermic
reactiona means that the Kc will decrease.
Removing ammonia as it is formed means that the postition of equilibrium goes
to the right to replace the ammonia that has been removed, so MORE ammonia
is produced to keep the Kp value constant.
Kp value INCREASES when the TEMPERATURE DECREASES as the reaction goes
in the direction in which energy is released, the fwd reaction is exothermic so
the reaction favours ammonia production.
Ammonia is made by the Haber process
N2 + 3H2 = 2NH3
Yield of ammonia INCREASES if the PRESSURE also INCREASES because the
position of equilibrium shifts to the right hand side because there are less moles
(there are 4 molecules of has on the left hand side and 2 molecules of gas on
the right hand side)
16 of 19
Equi constant, Kp
Equi & ammonia production
15 of 19
Total pressure of gases in a container is due to the molecules of the gas hitting
the sides of the container.
Definition; pressure exerted by one gas is called a partial pressure. Each partial
pressure adds up to the total pressure of gases.
Ptotal = Pa + Pb + Pc
Pa, Pb and Pc are each partial pressures of gases in the mixture.
Partial pressure equilibrium expressions are written in a similar way to
concentration expressions are written.
Eg. N2 + 3H2 = 2NH3
Kp = p2 NH3
.
p N2 x p3 H2
The products are written on the top and the reactants are written on the
bottom.
The moles of each species in the equation are also written as powers in the
equations, next to each value
Units for Kp are dependant on the form of equilibrium expression (they are
worked out by deducing the units of each species)
Bronsted-Lowry acid; a proton donor
Bronsted-Lowry base; a proton acceptor
Amphoteric substances; a substance eg. water that can act both as an acid AND
a base
18 of 19
Equi & sulfuric acid production
Bronsted-Lowry theory
17 of 19
SA is made by the conact process
2SO2 + O2 = 2SO3
When PRESSURE is INCREASED, more sulfuric acid is produced as there are less
moles on the right hand side compared to the left hand side, (there are 2
molecules of gas on the right hand side whereas there are 3 molecules of gas on
the left hand side)
Despite this, the reaction is carried out at just above atp as the value of Kp is v.
high, and position of equi is already far over to the right. Very high pressure is
uneccessary because it is expensive and not needed
If the TEMPERATURE is DECREASED, it shifts the position of equilibrium over to
the right because the reaction goes to where energy is released, and this is an
exothermic reaction so equi favours the production of SO3, increasing the value
of Kp
Conjugate acids & bases
19 of 19
If a reactant and a product is linked by the transfer of a proton, this is called a
conjuagte pair.
In a conjugate pair, the acid has one more proton than the base
Eg. NH3 + H20 = NH4+ + OH-