Equilibrium

L

Remember this?

  Collision theory: reactants only react when they collide at the correct angle and with sufficient energy to do so.

 Once they collide correctly and with enough energy they form an activation complex (unstable)  transition state – physical arrangement of particles during the activation complex  How much energy they require is the activation energy Types of collisions  Fruitful – forms a product

Reaction rates

  Fast rxn (More collisions) = less time, faster rate Slow rxn (Less collisions) = more time, slower rate

Slower vs Faster

     Change the mechanism  Add steps to the process (inhibitor)  Remove steps (catalyst) Change the temperature Change concentration Pressure (gases)  Less volume to move in, higher concentration Surface area (solids)  Particle size controls how face a reaction happens

Equilibrium

  State of rate balance between two opposing changes      Ex: Escalader Treadmill Haircuts Nails Money  3 types

Reaction equilibrium

 Rate of Synthesis = Rate of decomposition

+

Solution equilibrium

  Saturated solution Rate of dissolving = rate of precipitation

Phase equilibrium

   Rate of melting = rate of freezing Rate of vaporization = rate of condensation Rate of sublimation = rate of deposition

Changing equilibrium

 Anything that will change the reaction rate will shift the equilibrium    Types of reactants  More reactive = faster reaction Concentrations  Increased reagents = more chances for collisions Surface area  Increased exposure to reagents   Temperature  Increased kinetic energy Addition of a catalyst  Decreases activation energy

Properties of equilibrium

   Closed system = no change in anything that would affect equilibrium A system at equilibrium will stay that way forever unless some property of the system changes Dynamic – ALWAYS in motion

H

2

O

(l)

H

2

O

(g)

So what…

 Equilibrium is about RATES, not concentration  At equilibrium the concentration of products does NOT have to equal the concentration of reactants [p] ≠ [r]  Keq = measures how far a reaction gets  Equilibrium constant    Ratio of products to reactants Can be calculated by setting up a mass action expression Keq =[p]/[r]

Mass action expression

• Keq =[p]/[r]  + = multiply   Coefficients = exponents Use brackets to signal molar concentration Ex: 2CO 2 2CO

(g)

+ O 2(g)

Practice

 N 2 O 4(g)  2NO 2(g)  CO (g) + 3H 2(g)  CH 4(g) + H 2 O (g)  2H 2 S (g)  2H 2(g) + S 2(g)

Solving for Keq

  Simply plug and chug Find the equilibrium constant for the decomposition of 4M CO 2 , into 2M CO and 1M O 2 .

2CO 2(g) 2CO

(g)

+ O 2(g)

Keq = 1

Practice

 Calculate Keq given:   N 2 O 4(g) CO (g)  + 3H 2(g) 2NO 2(g)  CH 4(g) + H 2 O (g)

Substance Concentration

N 2 O 4 NO 2 CO H 2 CH 4 H 2 O 0.0613M

0.0627M

0.0613M

0.1839M

0.0387M

0.0387M

Quick note on Keq

   Till now we’ve only looked at Homogenous equilibria (all one phase) Since concentration is density ( Mol / L ) in solids or liquids the density of a PURE substance doesn’t change it remains constant and can be negated In a heterogenous equilibrium (mixed phase) write only substances in the gas phase.

I 2(s)  I 2(g)

Keq = [I 2(g) ]

Practice

 C 10 H 8(s)  C 10 H 8(g)  CaCO 3(s)  CaO (s) + CO 2(g)  C(s) + H (g) O (g)  CO (g) + H 2(g)

LE CHATELIER’S PRINCIPLE

  Life is a see-saw If stress is applied to a system at equilibrium, the system will shift to relieve that stress   Stress = Change in temperature, concentration, volume or pressure Keq doesn’t change if concentration, volume or pressure changes

Changing Concentration

    SYSTEM WILL ALWAYS SHIFT TO UNDO THE STRESS!

Adding reactants causes a shift towards the products Removal of products, more products Removing reactants shifts towards reactants

Changing Temperature

   Alters both Keq and equilibrium position Since Heat can be thought of as either a product or a reactant (endo/exothermic)   Addition of heat will shift an endothermic reaction towards the products Addition of heat will shift an exothermic reaction towards the reactants K eq (equilibrium constant) increases in value when the temperature is lowered and decreases when the temperature is raised

Changing Volume/Pressure

   Together because you can’t change volume without changing pressure Only cause a shift if the number of moles of gaseous product is different then the total moles of gaseous reactants   Decreasing Volume/ increasing Pressure towards the side with fewer moles shifts gas reactions Increasing Volume/ decreasing Pressure shifts gas reactions towards the side with more moles If number of moles is equal on both sides the system cannot relieve the pressure

Making Le Chatelier work for you

More Product

    Add reactant Remove product Change temperature Change pressure

More Reactant

 Add product    Remove reactant Change temperature Change pressure

Practice: Suppose you’re an evil scientist bent on world domination and to power your death ray you need a lot of silver chloride. What are 3 ways you could bend this reaction to your evil will, and then the WORLD!

Given: AgCl + Heat <-->Ag +1 + Cl -1 3 things that would increase [AgCl]    Add Ag +1 Add Cl -1 Drop the temperature  Fire the LAZER!!!