Chemistry Unit 12: Reaction Rates and Equilibrium

Reaction Rate

the rate at which reactants change into products

Reaction Rate Equation

Δ[A]/Δt

Products have a ________ rate.

positive

Reactants have a ________ rate.

negative

What is Kinetics?

the study of how fast a reaction happens

What must happen for a reaction to occur?

-Particles must collide

-Must have enough energy (to break bonds)

Effective Collisions

Meet two conditions and result in a reaction.

Activated Complex

-an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier.

-Between reactants and products.

Potential Energy

Energy that is stored and held in readiness

Activation Energy

-Energy required to start a reaction

-Difference in potential energy

The larger the activation energy...

the slower the reaction

How is energy lost in a reaction

heat

If energy of products is lower than the energy of reactants...

exothermic (-ΔH)

If energy of products is higher than the energy of reactants...

endothermic (+ΔH)

How many factors affect reaction rate?

7

What are the factors that affect reaction rate?

Nature of Reactants, Concentration, Particle Size (SA), Agitation, Temperature, Pressure, and Catalyst.

Nature of Reactants

Some substances react faster than others

Elementary steps

Individual steps in a reaction (reactants to products)

All elementary steps together

Reaction Mechanism

Which step determines the reaction rate?

Slowest

Rate Determining Step

The slowest step determines the reaction rate

The slowest step determines the reaction rate

Rate Determining Step

Intermediate

products in one step, reactants in another

Concentration

Larger concentration = more collisions = faster reaction

Particle Size (Surface Area)

More particles on surface = more collisions = more activated complex = more product

Agitation

Particles more exposed = more correct collisions = more activated complex = product faster

Pressure

More pressure = less volume = particles closer = more collisions = more activated complex = product faster

Temperature

-Higher temp = more KE = more collisions

-Collisions are harder (overcome Activation Energy easier)

Catalyst

-lowers activation energy

-Is not used up in reaction

Most effective factor to increase reaction rate?

Temperature

Reversed Reaction

products can turn back into reactants (goes both ways)

Chemical Equilibrium

-rate(forward)=rate(reverse)

-Only occurs in a closed system

-Only solutions and gases

Chemical Equilibrium can only occur in...

A closed system

Chemical Equilibrium Equation

K(eq)=[C]c[D]d/[A]a[B]b

Capital letters in chemical equilibrium equation

Reactants/Products

Lowercase letters in chemical equilibrium equation

Coefficients

Chemical Equilibrium equation must be

Products/Reactants

Chemical Equilibrium > 1

Products are favored at equilibrium

Chemical Equilibrium < 1

Reactants are favored at equilibrium

Le Châtelier's Principle

If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

3 Main Stressors

Concentration, Temperature, and Pressure!

What effect does the addition of a catalyst have on equilibrium?

no effect

More product is added (Equilibrium)

Shifts left

More reactant is added (Equilibrium)

Shifts right

Product removed (Equilibrium)

Shifts right

Reactant removed (Equilibrium)

Shifts left

Exothermic reactions (Equilibrium)

Shifts left (away from heat)

Endothermic reactions (Equilibrium)

Shifts right (away from heat)

Cooling (Equilibrium)

Has opposite effects (moves towards heat)

Increased Pressure (Equilibrium)

Shifts towards the side with fewer gas molecules

Decreased pressure (Equilibrium)

Shifts towards the side with more gas molecules

20. What information is NOT given by an overall equation for a chemical reaction?

a. the relative numbers of molecules used

b. the probable order of the reaction

c. the number of atoms participating in the reaction

d. the reaction mechanism

d. the reaction mechanism