1. Chemical equilibrium By/ BATAA EL GAFAARY

2. Equilibrium:
Equilibrium is a state in which there are no observable changes as time goes by.
Chemical equilibrium is achieved when:
• the rates of the forward and reverse reactions are equal and
• the concentrations of the reactants and products remain constant
Physical equilibrium H2O (l) H2O (g)
Chemical equilibrium N2O4 (g) NO2 (g)
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3. N2O4 (g) NO2 (g)
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5. Homogenous equilibrium
applies to reactions in which all reacting species are in the same phase.
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6. Ex.
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7. The equilibrium concentrations for the reaction between carbon monoxide and molecular chlorine to form COCl2 (g) at 740°C are [CO] = M, [Cl2] = M, and [COCl2] = 0.14 M. Calculate the equilibrium constants Kc and Kp.
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8. The equilibrium constant Kp for the reaction 2NO2 (g) NO(g) O2(g) is 158 at 1000K. What is the equilibrium pressure of O2 if the PNO2 = atm and PNO = atm?
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9. Heterogenous equilibrium
Heterogenous equilibrium applies to reactions in which reactants and products are in different phases.
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10. Consider the following equilibrium at 295 K: NH4HS(S) NH3(g) + H2S(g) The partial pressure of each gas is atm. Calculate Kp and Kc for the reaction?
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12. Writing Equilibrium Constant Expressions
1. The concentrations of the reacting species in the condensed phase are expressed in M. In the gaseous phase, the concentrations can be expressed in M or in atm.
2. The concentrations of pure solids, pure liquids and solvents do not appear in the equilibrium constant expressions.
3. The equilibrium constant is a dimensionless quantity.
4. In quoting a value for the equilibrium constant, you must specify the balanced equation and the temperature. 5. If a reaction can be expressed as a sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions.
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13. Chemical Kinetics and Chemical Equilibrium
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14. The reaction quotient (Qc) is calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (Kc) expression.
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15. Calculating Equilibrium Concentrations
1. Express the equilibrium concentrations of all species in terms of the initial concentrations and a single unknown x, which represents the change in concentration.
2. Write the equilibrium constant expression in terms of the equilibrium concentrations. Knowing the value of the equilibrium constant, solve for x.
3. Having solved for x, calculate the equilibrium concentrations of all species.
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16. Le Chatelier’s Principle
If a stress is applied to a system at equilibrium, the system will change to relieve that stress and re –establish equilibrium. It is like the “undo” button on your computer!
Factors that Affect Equilibrium :
Concentration
Temperature
Pressure For gaseous systems only!
The presence of a catalyst
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17. Concentration Changes
Add more reactant Shift to products
Remove reactants Shift to reactants
Add more product Shift to reactants
Remove products Shift to products
Reaction Quotient
The reaction quotient for an equilibrium system is the same as the equilibrium expression, but the concentrations are NOT at equilibrium!
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18. Changes in concentration are best understood in terms of what would happen to “Q” if the concentrations were changed.
•Q = Keq at equilibrium
•If Q< K then there are too many reactants, the reaction will shift in the forward direction (the products)
•If Q>K then there are too many products, the reaction will shift to the reactants
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19. Temperature Changes Consider heat as a product in exothermic reactions
– Add heat Shift to reactants
– Remove heat Shift to products
A + B = AB + Heat
• Consider heat as a reactant in endothermic reactions
A + B + heat = AB
– Add heat Shift to products
– Remove heat Shift to reactants
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20. Pressure Changes
Only affects equilibrium systems with unequal moles of gaseous reactants and products.
Increase Pressure *
Stress of pressure is reduced by reducing the number of gas molecules in the container
• There are 4 molecules of reactants vs. 2 molecules of products.
– Thus, the reaction shifts to the product( ammonia ).
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21. There are two product gas molecules vs. one reactant gas molecule.
• Decrease Pressure
Stress of decreased pressure is reduced by increasing the number of gas molecules in the container.
There are two product gas molecules vs. one reactant gas molecule.
Thus, the reaction shifts to the products
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22. Presence of a Catalyst
• A Catalyst lowers the activation energy and increases the reaction rate.
• It will lower the forward and reverse reaction rates,
• Therefore, a catalyst has NO EFFECT on a system at equilibrium!
• It just gets you to equilibrium faster!
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23. Presence of an Inert Substance
An inert substance is a substance that is not- reactive with any species in the equilibrium system.
These will not affect the equilibrium system.
If the substance does react with a species at equilibrium, then there will be a shift!
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