Equilibrium Flashcards
(23 cards)
What does dynamic mean
Dynamic means both forward and backward reactions are occurring simultaneously
Two features of dynamic equilibrium
- Forward and backward reactions are occurring at equal rates
- The concentrations of reactants and products stay constant
Le Chatelier’s principle states..
that if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it).
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.
• 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.
What effect would increasing temperature have on the yield of ammonia?
N2 +3H2 2NH3 deltaH= -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.
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
• 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
• 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
H+Cl ⇌ 2HCl
What effect would increasing pressure have on the yield of methanol?
CO +2H ⇌ CHOH (g) 2(g) 3 (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
Effect of pressure on yield
• 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)
Effect of Concentration on Equilibrium
I2 + 2OH- -> I- + IO- + H2O
Browncolourless
I2 + 2OH- -> I- + IO- + H2O
Browncolourless
• Increasing the concentration OH- ions causes the equilibrium to shift to oppose this and move in the forward direction to remove and decrease the concentration of OH- ions
• The position of equilibrium will shift towards the right, giving a higher yield of I- and IO-. ( The colour would change from brown to colourless)
• Adding H+ ions reacts with the OH- ions and reduces their concentration so the equilibrium shifts back to the left giving brown colour.
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
Examples of equilibrium in industry
• Haber process
• Contact process
• Hydration of ethene to produce ethanol
• Production of methanol from CO
Haber process
N +3H ⇌ 2NH
deltaH = -ve 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
Contact process
Stage 1: S(s)+O2 (g) –> SO2 (g)
Stage 2: SO2(g) +1⁄2O(g) ⇌ SO3(g) deltaH = -98kJmol
• T=450*C
• 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
Production of methanol from CO
CO2(g) +2H(g) ⇌ CH3OH(g)
deltaH= -ve 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
Hydration of ethene to produce ethanol
CH2CH2(g) +H2O(g) ⇌ CH3CH2OH(l) deltaH= -ve
• T= 300*C
• 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 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 does recycling unreacted reactants back into the reactor do
Improve the overall yields of these processes
The term carbon neutral refers to..
an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere
Explain why for a reversible reaction used in an industrial process, a compromise temperature and pressure may be used
- Temperature
- higher temps increase rate but if the reaction is exothermic the equilibrium will shift to the left decreasing the yield of products
- lower temps means a higher yield but lower rate so compromise temps are used - Pressure
- high pressures are expensive and dangerous due to the need for strong equipment and energy use
- so a moderate pressure is used that is low enough to keeps costs but high enough to improve yield
In a reversible reaction at equilibrium..
• forward and reverse reactions proceed at equal rates
• the concentrations of reactants and products remain constant
What is a homogenous system
Where all components are in the same phase, physically uniform and have the same properties throughout
Effect of temperature on Kc
Where forward reaction is exothermic:
Increasing temp decreases value of Kc
Where forward reaction is endothermic:
Increasing temp increases value of Kc
Why is Kc not affect by change in concentration or pressure
Because Kc isn’t affect by changes in reactant or product concentrations because the equilibrium will shift to reestablish equilibrium
-> this shift changes the concentrations of all species but not in a way that affects the ratio of products to reactant
-> the equilibrium will shift until the Kc returns to the original value of Kc
When working out equilibrium constant why do we ignore water if it’s in its liquid state
It’s concentration doesn’t change significantly during the reaction because it’s so abundant compared to the other substances involved
