6. Equilibria Flashcards
Define a reversible reaction
- All reversible reactions reach a dynamic equilibrium state
State examples of reversible reactions
Define dynamic
- Both forward and backward reactions are occurring simultaneously
State two features of dynamic equilibrium
- Forward and backward reactions are occurring at equal rates
- The concentrations of reactants and products stay consent
We use the expression ‘position of equilibrium’ to describe the composition of the equilibrium mixture.
If the position of equilibrium favours the reactants (also described as “towards the left”) then the equilibrium mixture will contain mostly reactants.
State Le Chatelier’s Principle
- If an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it)
State the four things that can effect equilibrium
- Concentration
- Temperature
- Pressure
- Catalysts
- Increase concentration of a reactant
- Decrease concentration of a product
- Shift to the right
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- Increase conc of product
- Decrease conc of reactants
- Shift to left
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
Describe what happens to the equilibrium when you increase pressure in a reversible reaction
- Cause equilibrium to shift towards the side with fewer moles of gas to oppose the change the change and thereby reduce the pressure
(Increase = want to decrease, less moles, less collisions, shift to less moles so more)
Describe what happens to the equilibrium when you decrease pressure in a reversible reaction
- Cause the equilibrium to shift towards the side with more moles of gas to oppose the change, and thereby increase the pressure
- (Decrease = want to increase, more moles, more collisions, shift to more moles, so more)
Describe the effect of pressure on equilibrium when the no of moles are the same
- If the no 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
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)
Typical exam question: What effect would increasing pressure have on the yield of methanol?
CO +2H ⇌ CHOH
(g) 2(g) 3 (g)
Exam level answer : must include bold points
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.
Describe the effect of increase in temp on equilibrium in a reversible reaction
- Equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.
- (increase = we want to decrease, endothermic takes heat from surrounding, shift to endo so less heat)
Describe the effect of decrease in temp on equilibrium in a reversible reaction
- Equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.
- (Decrease = we want to increase, exothermic releases heat, shift to exo so more heat)
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 temperature have on the yield of ammonia?
N2 +3H2 2NH3 H=-veexo Exam level answer : must include bold points
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.
Describe the effect of catalysts on equilibrium
- No effect on position of equilbrium
- Will speed of the rate at which the equilibrium is achieved
Explain why catalysts do not have an effect on equilibrium
- It speeds up the rates of the forward and backward reactions by the same amount
Haber Process
- N2 + 3H2 ⇌ 2NH3
- 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 high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure
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Contact Process
Contact process
Stage 2 SO + 1⁄2 O ⇌ SO H = -98 kJ mol 2 (g) 2 (g) 3 (g)
Stage 1 S (s) + O2 (g)SO2 (g)
-1
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
