L

Equilibrium

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

energyTypes of collisions

Fruitful – forms a product

Reaction rates

Fast rxn (More collisions) = less time, faster rateSlow rxn (Less collisions) = more time, slower

rate

Slower vs Faster

Change the mechanism Add steps to the process (inhibitor) Remove steps (catalyst)

Change the temperatureChange concentrationPressure (gases)

Less volume to move in, higher concentrationSurface 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 solutionRate of dissolving = rate of precipitation

Phase equilibrium

Rate of melting = rate of freezingRate of vaporization = rate of condensationRate 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

H2O(l) H2O(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 concentrationEx:

2CO2 2CO(g) + O2(g)

Practice

N2O4(g) 2NO2(g)

CO(g) + 3H2(g) CH4(g) + H2O(g)

2H2S(g) 2H2(g) + S2(g)

Solving for Keq

Simply plug and chugFind the equilibrium constant for the

decomposition of 4M CO2, into 2M CO and 1M O2.

2CO2(g) 2CO(g) + O2(g)

Keq = 1

Practice

Calculate Keq given: N2O4(g) 2NO2(g)

CO(g) + 3H2(g) CH4(g) + H2O(g)

Substance

Concentration

N2O4 0.0613M

NO2 0.0627M

CO 0.0613M

H2 0.1839M

CH4 0.0387M

H2O 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.

I2(s) I2(g)

Keq = [I2(g)]

Practice

C10H8(s) C10H8(g)

CaCO3(s) CaO(s) + CO2(g)

C(s) + H(g)O(g) CO(g) + H2(g)

LE CHATELIER’S PRINCIPLE

Life is a see-sawIf 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 productsRemoving reactants shifts towards reactants

Changing Temperature

Alters both Keq and equilibrium positionSince 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

Keq (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 shifts gas

reactions towards the side with fewer moles Increasing Volume/ decreasing Pressure shifts gas

reactions towards the side with more molesIf number of moles is equal on both sides the

system cannot relieve the pressure

More Product More Reactant

Add reactantRemove productChange

temperatureChange pressure

Add productRemove reactantChange

temperatureChange pressure

Making Le Chatelier work for you

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!!!