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Equilibria (6.1: The Idea of Equilibrium (Chemical Equilibria (A + B -->…
Equilibria
6.1: The Idea of Equilibrium
In a closed container products form and then react together to form the reactants again so you get a mixture of both
Equilibrium mixture when proportions of all 3 components remain constant
(Water) in a closed container. It will evaporate but as it happens more the vapour will start to condense so after time the rate of evap and condens is the same and they are still going on. This is a Dynamic Equilibrium
Conditions
Can only be reached in a closed system (a beaker may be if reaction takes place in solvent as long as nothing evaporates)
Can be approached from either direction and final position will be the same
Dynamic process - Reached when rates of 2 opposing processes (going on at same time) are the same
Equilibrium has been reached when the macroscopic properties don't change over time (properties that don't depend on total quantity of matter like density, concentration, colour, pressure)
Chemical Equilibria
A + B --> C + D
Start - Forward rate is fast as there is lots of A and B. No revers reaction as no C and D
As conc. of C and D increase, reverse reaction speeds up and A and B decrease so forward reaction slows
Exactly the same no. of particles are changing from A+B to C+D (equilibrium)
Equilibrium is not necessarily half p and half r and proportions can be changed depending on conditions of reaction
6.4: Equilibrium Constant Kc
Initial
Change
Equilibrium
Homogeneous - where all reactants and products are in same phase e.g. liquids
Changes with temp and work out units by cancelling out
6.6: Effect of Changing Conditions on Equilibria
Temperature
Whether Kc increases or decreases depends on whether the reaction is endothermic or exothermic
Table (see textbook)
If Kc increases, moves to right (forward + more product)
If decreases, moves left (backward + less product)
Exothermic: Increasing temp decreases Kc (left)
Endothermic: Increasing temp increases Kc (right)
Only temp has an effect
Kc and Position of Equilibrium
Size of Kc tells us about composition:
-If much greater than 1, products predominate over reactants and eqm to right
-If much less than 1, reactants predominate and eqm is left
Gaseous Equilibria: Use partial pressures rather than concentration
6.2: Changing the Conditions of an Equilibrium Reaction
Equilibrium Mixture
If products increase, equilibrium moves to the right (forward)
If reactants increase, equilibrium moves to the left (backward)
You can change proportion of reactants to obtain greater yield (change position of Eqm)
Le Chatilier's Principle
If a system is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance
Changing Concentration: Adding more A uses up more B, produces more C and D and moves Eqm to the right . End up with greater proportions of products and same if add more B
Remove C - Eqm moves right to produce more C using up A and B and same if remove D
Changing Pressure: Increase, move to left as fewer molecules = less pressure. If decrease, move to right
If there is same no. of moles of gases on both sides then pressure has no effect
Changing Temp: if heat is given out then heat is absorbed in reverse reaction (left)
If you increase temp reaction moves in direction which cools system down
If we cool mixture eqi moves right so increases product
Catalysts: No effect on position as affects forward and backward reactions equally. They allow equilibrium to be reached more quickly
6.3: Equilibrium Reactions in Industry
Ethanol C2H5OH
-Made by hydration to ethene
-Reaction is reversible
-Phosphoric acid absorbed on silica (catalyst)
-Everything is gaseous at temp used
Le Chatilier's Principle Applied
-High pressure so moves right where fewer molecules
-Low temp so moves right to give out heat
-Excess steam so moves right to reduce steam conc.
Practical Problems
-High pressure - ethene polymerises
-High pressure - Increase cost of building and running
-Low temp - reduce rate though partially compensated for by catalyst
-Too much steam dilutes catalyst
Conditions of about 570K and 6500kPa give conversion to ethanol of only 5% but unreacted ethene is recycled until aboout 95% is obtained
6.5: Calculations Using Equilibrium Constant Expressions
See textbook