3.1.6 Chemical equilibria, Le Chatelier’s principle and Kc Flashcards
What are two features of dynamic equilibrium?
- Forward and backward reactions are occurring at equal rates.
- The concentrations of reactants and products stay constant.
What does Le Chatelier’s principle state?
That if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it).
What is the effect of increasing temperature on equilibrium?
If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try to decrease temperature.
The position of equilibrium will shift towards the right/left, giving a higher yield of X.
- 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.
What is the effect of increasing 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.
The position of equilibrium will shift towards the right/left, giving a higher yield of X.
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 increasing concentration on equilibrium?
Increasing the concentration of a substance causes the equilibrium to shift to oppose this and move in the opposite direction to remove and decrease the concentration of it. The position of equilibrium will shift towards the right/left, giving a higher yield of X (opposite to the substance).
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 effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount.
What is the Haber process and why do we perform it at certain conditions?
N2 + 3H2 ⇌ 2NH3 △H= -ive exo
T= 450°C, P= 200 - 1000 atm, catalyst = iron
- 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
What is the Contact process and why do we perform it at certain conditions?
Stage 1: S(s) + O2(g) → SO2(g)
Stage 2: SO2(g) + ½O2(g) ⇌ SO3(g) △H = -98 kJ mol^-1
T= 450°C, P= 1 - 2 atm, catalyst = V2O5
- Low temp gives good yield but slow rate: compromise 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
What is the process to produce methanol from CO and why do we perform it at certain conditions?
CO(g) + 2H2(g) ⇌ CH3OH(g) △H = -ive exo
T= 400°C, 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
What is the process of hydration of ethene to produce ethanol and why do we perform it at certain conditions?
CH2=CH2(g) + H2O(g) ⇌ CH3CH2OH(l) △H = -ive exo
T= 300°C, P= 70 atm, catalyst = conc H3PO4
- Low temp gives good yield but slow rate: compromise 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
- High pressure also leads to unwanted polymerisation of ethene to poly(ethene)
What else can improve the overall yields of these industrial processes.
Recycling unreacted reactants back into the reactor.
- Both methanol and ethanol can be used as fuels.
- 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 (The term carbon neutral refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”).
- It would only be carbon neutral, however, if the energy required to carry out the reaction was not made by combustion of fossil fuels.
How do you write a Kc expression?
[ ] = equilibrium concentration
Products on top of fraction
Reactants on bottom of fraction
Big numbers become powers
How do you calculate moles at equilibrium to work out equilibrium concentration for a Kc expression?
Create a table with initial, reacted and equilibrium moles
moles of reactant at equilibrium = initial moles - moles reacted
moles of product at equilibrium = initial moles + moles formed
How do you work out the units of Kc?
Sub in moldm^-3 to each value and cancel out
What is the effect of changing conditions on value of Kc?
The larger the Kc the greater the amount of products.
If Kc is small we say the equilibrium favours the reactants
Kc only changes with temperature.
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.
If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try to decrease temperature.
The position of equilibrium will shift towards the right/left, giving a higher yield of X. The value of Kc gets smaller/larger as there are fewer/more 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.
What is the effect of catalysts on position of equilibrium and 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
