1. CHEMICAL EQUILIBRIUM

2. Collision Model
Molecules must collide in order for a reaction to occur.
Rate depends on concentrations of reactants and temperature.
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3. Temperature – increases rate. Why?
Concentration – increases rate because more molecules lead to more collisions.
Temperature – increases rate.
Why?
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4. Activation Energy Minimum energy required for a reaction to occur.
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5. Catalyst A substance that speeds up a reaction without being consumed.
Enzyme – catalyst in a biological system
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6. HC2H3O2 + H2O  H3O+ + C2H3O2-
Equilibrium occurs when two opposing processes occur at the same rate.
The rates are equal in both directions.
Once equilibrium is reached, the concentrations don’t change, but the reaction constantly takes place.
Almost all systems come to equilibrium.
Composition of the equilibrium mix does not depend on which side of equation we begin.

7. Graph of Equilibrium

8. The mass action expression equals
[Products]x/[Reactants]y . This manner of expression is a convention.
[ NaCl] indicates concentration of NaCl.
The equilibrium constant (Keq) is the value of the mass action expression when equilibrium concentrations are used. It is only valid at a given temperature.

9. Keq or Kc Consider the general chemical equation
The exponents in the mass action expression are the same as the stoichiometric coefficients

10. MEANING OF Keq VALUES
K is very large -- reaction proceeds far to completion; position of equilibrium is far toward product side.
K = 1 -- Concentration of products & reactants about same at equilibrium; equilibrium is midway between products & reactants.
K is very small -- little product formed; position of equilibrium is far toward reactants.

11. Kc and Kp

12. HETEROGENEOUS EQUILIBRIUM
When more than one phase exists in a reaction, it is called a heterogeneous reaction.
The concentration of a pure solid (sodium carbonate) or a pure liquid (water) is a constant.
Equilibrium law expressions are written without concentration of pure solids and liquids.

13. Heterogeneous Equilibria
The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present
Write the equilibrium expression for the reaction:
PCl5(s)  PCl3(l) + Cl2(g)
Pure
solid
Pure
liquid

14. ICE CHART Concentration must be in moles/L
Concentration must be at equilibrium
In the change row products and reactants must have opposite signs.
In the change row products and reactants are in the same ratio as the coefficients for the reaction’s equation.
A + B ↔ C + D
Reaction A B C D
Initial concentration
Change
Equilibrium

15. Reaction Quotient (Q)
The reaction quotient (Q) is the value of the mass action expression when nonequilibrium concentrations are used.
If Q < Keq, products will form
If Q> Keq, reactants will form
If Q = Keq, reaction is at equilibrium

16. If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.
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17. Effect of a Change in Concentration
When a reactant or product is added the system shifts away from that added component.
If a reactant or product is removed, the system shifts toward the removed component.
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18. Effect of a Change in Volume
The system is initially at equilibrium.
The piston is pushed in, decreasing the volume and increasing the pressure. The system shifts in the direction that consumes CO2 molecules, lowering the pressure again.
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19. Effect of a Change in Volume
Decreasing the volume
The system shifts in the direction that gives the fewest number of gas molecules.
Increasing the volume
The system shifts in the direction that increases its pressure.
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20. Effect of a Change in Temperature
The value of K changes with temperature. We can use this to predict the direction of this change.
Exothermic reaction – produces heat (heat is a product)
Adding energy shifts the equilibrium to the left (away from the heat term).
Endothermic reaction – absorbs energy (heat is a reactant)
Adding energy shifts the equilibrium to the right (away from the heat term).
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21. Ksp = [M+][X] = solubility product constant
The equilibrium conditions also applies to a saturated solution containing excess solid, MX(s).
Ksp = [M+][X] = solubility product constant
The value of the Ksp can be calculated from the measured solubility of MX(s).
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22. Solubility Equilibria
Solubility product (Ksp) – equilibrium constant; has only one value for a given solid at a given temperature.
Solubility – an equilibrium position.
Bi2S3(s) Bi3+(aq) + 3S2–(aq)
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