Equilibria Flashcards
What does the term dynamic mean
Both forward and backward reactions
are occurring simultaneously
What is le chateliers principle
If an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it).
GIve the Two features of dynamic equilibrium
- Forward and backward reactions are occurring at equal rates.
- The concentrations of reactants and products stay constant
What is the Effect of temperature on equilibrium
*If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.
-And its reverse
*If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.
Typical exam question: What effect would increasing temperature have on the yield of ammonia?
N2 + 3H2 ⇌2NH3 (Delta)H = -ve exo
*If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature.
*The position of equilibrium will shift towards the left, giving a lower yield of ammonia.
EXtra info on temp
Low temperatures may give a higher yield of product but will also result in slow rates of reaction. Often a compromise temperature is used that gives a reasonable yield and rate.
Typical exam question: What effect would increasing
pressure have on the yield of methanol?
CO (g) + 2H2(g) ⇌ CH3OH (g)
*If pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure .
*The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol.
What is the Effect of pressure on equilibrium
- Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure.
-And its reverse - Decreasing pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change and thereby increase the pressure
Extra info on pressure moles
If the number of moles of gas is the same on both sides of
the equation then changing pressure will have no effect on
the position of equilibrium.
H2 + Cl2 ⇌2HCl
Extra info about pressure again
Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs for pumping the gases to make a high pressure) and the equipment is expensive (to contain the high pressures)
What is the Effect of Catalysts on Equilibrium
*A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the
equilibrium is achieved.
*It does not affect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount.
What is the Effect of Concentration on Equilibrium
Using 3H2(g) +N2(g)<=>2NH3(g) as an example
This is only true if the reaction is in homogenous equilibria SearchItUp
*Increasing the concentration of reactants (either H2 or N2) causes the equilibrium to shift to reduce the concentration (the opposite will happen if conc is reduced)
*The position of equilibrium will shift towards the right, giving a higher yield of 2NH3(g)
Haber process
N2 + 3H2 ⇌ 2NH3 H = -ve exo
T= 450oC, P= 200 – 1000 atm, catalyst = iron
Low temp gives good yield but slow rate: compromise temp used
High pressure gives good yield and a high rate:
Too high a pressure would lead to too high energy costs for pumps to produce the pressure
Production of methanol from CO
CO (g) + 2H2(g) ⇌ CH3OH (g)H = -ve exo
T= 400oC, P= 50 atm, catalyst = chromium and zinc
oxides
Low temp gives good yield but slow rate:
compromise temp used
High pressure gives good yield and high rate: too
high a pressure would lead to too high energy costs
for pumps to produce the pressure
Hydration of ethene to produce ethanol
CH2
=CH2 (g) + H2O (g) ⇌ CH3CH2OH(l) H = -ve
T= 300oC, P= 70 atm, catalyst = conc H3PO4
* Low temp gives good yield but slow rate: compromise temp
used
* High pressure gives good yield and high rate: too high
* Pressure would lead to too high energy costs for pumps to produce the pressure
* High pressure also leads to unwanted polymerisation of ethene to poly(ethene)
Contact process
Stage 1 S (s) + O2(g) SO2(g)
Stage 2 SO2 (g) +½O2 (g) ⇌ SO3 (g) H = -98 kJ mol-1 T= 450oC, P= 1 or 2 atm, catalyst = V2O5
Low temp gives good yield but slow rate: compromise moderate
temp used
High pressure only gives slightly better yield and high rate: too
high a pressure would lead to too high energy costs for pumps
to produce the pressure
In all cases catalysts speeds up the rate, allowing a lower temp to be used (and hence lower energy costs), but
have no effect on position of equilibrium
In all cases high pressure leads to too high energy costs for pumps to produce the pressure and too high
equipment costs to have equipment that can withstand high pressures.
What is one way to imporve the overall yield of all these processes
Recycling unreacted reactants back into the reactor can improve the overall yields of all these processes.
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If the carbon monoxide used to make methanol in the above
reaction was extracted from the atmosphere then it could be
classed as carbon neutral.
It would only be carbon neutral, however, if the energy required
to carry out the reaction was not made by combustion of fossil
fuels .
What does The term carbon neutral refers to
“an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”
Kc = equilibrium constant formula
2A+B<=>2C+D
[C]^2 [D]q/[ A]^2 [B]n
A n B would be reactants C n D would be products
[ ] means the equilibrium concentration
Molar numbers are powers in KC
The units are MolDm^3 2A would give two lots of moldm^3
The reactant Moldm3 units are flipped so moldm^-3 turns into Mol^-1dm^3
What is the Effect of temperature on position of equilibrium and Kc
Both the position of equilibrium
and the value of Kc will change
if temperature is altered
In this equilibrium which is exothermic in the forward direction
N2(g) + 3H2(g)<=> 2 NH3
If temperature is increased the reaction will shift to oppose the change and
move in the backwards endothermic direction. The position of equilibrium
shifts left. The value of Kc gets smaller as there are fewer products.
What is the Effect of pressure on position of equilibrium and Kc
The position of equilibrium will
change if pressure is altered but
the value of Kc stays constant as
Kc only varies with temperature
N2(g) + 3H2(g) <=>2 NH3(g)
In this equilibrium which has fewer moles of gas on the product side
If pressure is increased the reaction will shift to oppose the change and
move in the forward direction to the side with fewer moles of gas. The
position of equilibrium shifts right. The value of Kc stays the same though
as only temperature changes the value of
What is the effect of catalysts on the psoition of equilibrium andf Kc
Catalysts have no effect on the value of Kc or the position of equilibrium as they
speed up both forward and backward rates by the same amount.
