Equlibrium Flashcards
what does equilibrium mean
a state of balance
what is a reversible reaction
the forward and backwards reaction take place at the same time
what is dynamic equilibrium
the forward and backward reaction occur at the same rate and the concentrations of the reactants and product do not change
what are the conditions of eq
-the eq can only be reached in a closed system - where products and reactants cant escape
-eq can be approached from either direction and the final eq positon will be the same
-dynamic process - reached when the rates of two opposing processes which go on at the same time and are the same
-conc, pressure, density and colour do not change
where is the positon of eq if there is more reactants than products
to the left
where is the positon of eq if there is less reactants than products
to the right
what is Le Chateliers principle
the position of eq of a system changes to minimise the effect of any imposed change on the conditions - at dynamic eq
how does equilibrium shift temperature
increase
-favour endothermic direction and shifts towards this - positive enthalpy
decrease
-favours exothermic directions and shifts in this direction - negative enthalpy
how does equilibrium shift pressure
increase
-favour the side with fewer molecules/moles
decrease
-favour the side with more molecules/ moles
how does equilibrium shift conc
increase
-shift right to reduce effect of increase of reactants
decrease
-shift left to reduce effect of decrease of reactant/increase in products
effect of catalyst on equilibrium
no effect
effect of surface area on eq
no effect
how to describe change in eq
eq shifts to counteract/oppose increase/decrease in temp,pressure or conc
what is the eq constant Kc
The equilibrium expression links the equilibrium constant, Kc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
what is an overall expression for Kc
aA + bB = cC+ dD
[C]c [D]d/ [A]a [B]b
examples of Kc
Deduce the equilibrium expression for the following reactions:
Ag+ (aq) + Fe2+ (aq) ⇌ Ag (s) + Fe3+ (aq)
[Fe3+]/[Ag+] [Fe2+]
Ag (s) not included as is a solid
how to work out units using Kc
all molecules are moldm-3
then look at how many of each molecule and see if cancel
if not then add remaining moldm-3 together
if all cancel out then write no units
if the values are in mols and u are given a volume what do you do for Kc equations
use conc =mol/vol
then put in Kc expression
solve and work out units
what changes do not affect Kc
conc, pressure , catalyst and surface area
how does temp affect Kc
for an increase in temp
forward reaction is endo
-then products increase and reactants decrease causing kc to increase
forward exo
-products decrease and reactants increase meaning kc decreases
how is le Chatelier’s principle used to maximise the yield of ammonia in the haber process
Haber process
The Haber process involves the synthesis of ammonia according to:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔHr = -92 kJ mol-1
Le Chatelier’s principle is used to get the best yield of ammonia
Maximising the ammonia yield
Pressure
An increase in pressure will result in the equilibrium shifting in the direction of the fewest molecules of gas formed to reduce the pressure
In this case, the equilibrium shifts towards the right so the yield of ammonia increases
An increase in pressure will cause the particles to be closer together and therefore increasing the number of successful collisions leading to an increased reaction rate
Very high pressures are expensive to produce therefore a compromise pressure of 200 atm is chosen
Temperature
To get the maximum yield of ammonia the position of equilibrium should be shifted as far as possible to the right as possible
Since the Haber process is an exothermic reaction, according to Le Chatelier’s principle the equilibrium will shift to the right if the temperature is lowered
A decrease in temperature will decrease the energy of the surroundings so the reaction will go in the direction in which energy is released to counteract this
Since the reaction is exothermic, the equilibrium shifts to the right
However, at a low temperature the gases won’t have enough kinetic energy to collide and react and therefore equilibrium would not be reached therefore a compromise temperature of 400-450oC is used in the Haber process
A heat exchanger warms the incoming gas mixture to give molecules more kinetic energy such that the gas molecules collide more frequently increasing the likelihood of a reaction
Removing ammonia
Removing ammonia by condensing it to a liquid causes the equilibrium position to shift to the right to replace the ammonia causing more ammonia to be formed from hydrogen and nitrogen
The recovered ammonia is stored at very low temperatures and there is no catalyst present with the stored ammonia so the decomposition reaction of ammonia to decompose back into hydrogen and nitrogen will be too slow to be a problem
Catalysts
In the absence of a catalyst the reaction is so slow that hardly anything happens in a reasonable time!
Adding an iron catalyst speeds up the rate of reaction
how is le Chatelier’s principle used in the contact process
Contact process
The Contact process involves the synthesis of sulfuric acid according to:
2SO2(g) + O2(g) ⇌ 2SO3(g) ΔHr = -197 kJ mol-1
SO3 + H2SO4 → H2S2O7
H2S2O7 + H2O → 2H2SO4
Le Chatelier’s principle is used to get the best yield of sulfuric acid
Maximising the sulfuric acid yield
Pressure
An increase in pressure will result in the equilibrium shifting in the direction of the fewest molecules of gas formed to reduce the pressure
In this case, the equilibrium shifts towards the right so the yield of sulfur trioxide increases
In practice, the reaction is carried out at only 1 atm
This is because Kc for this reaction is already very high meaning that the position of the equilibrium is already far over to the right
Higher pressures than 1 atm will be unnecessary and expensive
Temperature
The same principle applies to increasing the temperature in the Contact process as in the Haber process
A compromise temperature of 450 oC is used
Removing sulfuric acid
In practice, SO3 is removed by absorbing it in 98% sulfuric acid
This is because adding SO3 directly to water produces an uncontrollable and hazardous exothermic reaction
The SO3 reacts with the sulfuric acid to make oleum, H2S2O7, which is then carefully diluted to make more H2SO4
Catalysts
The Contact process uses vanadium(V) oxide as a catalyst to increase the rate of reaction
how to find eq moles when only given starting moles
ICE
intial moles = the ones given in equation - if none given for the products assume 0
eq moles= the one that will be given in the equations is used
change = work out the change in the moles by looking at start and final eq moles
if products are +ve then reactants will be same val but -ve
and vice versa
what is homogenous eq
all reactants and products in the same state
what is heterogenous eq
reactants and products in different states
