Download Thermodynamics

Thermodynamics
tells if a reaction
will occur
Kinetics
tells how fast a reaction
will occur
video
Reaction Rate
• speed of the reaction
• measures change in concentration of
reactant/product over time
• rate =  concentration
 time
reactants disappear as products appear
How do reactions occur?
remember KMT: “Collision Theory”
for reactions to occur: must have effective collisions
between reacting particles so 3 things must happen
1. particles must collide
2. collision must be energetic
3. collision must occur at favorable orientation
–effective collisions lead to formation of products
–ineffective collisions do NOT lead to products
Reaction Rates depend on …
• frequency of collisions
– how often collisions occur
And
• efficiency of collisions
– percentage of collisions that are effective
Particle Diagram of Collision
Activated complex or transition state.
Reactants
Products
NO + O3  NO2 + O2
Activated Complex is NOT in equation!
Effective vs. Ineffective Collision
most collisions are
NOT effective!
due to:
- insufficient energy or
improper orientation
Why Do Collisions Have to be Energetic?
video
Activation Energy
• energy needed to initiate
reaction
• additional energy needed to overcome reaction
barrier (hill)
• difference in energy between what reactants start
with and what need to react (top of hill)
Examples of Activation Energy
• spark plug in car
engine
• using match to
start a fire
PE curve: endothermic rxn
products have more PE
than reactants
start low, end high
PE curve: exothermic rxn
products have less PE than reactants
start high, end low
PE diagram: identify labels
label on both endo & exo PE curves
1)
6)
3)
2)
4)
5)
PE reactants
PE products
PE activated complex
Ea forward reaction
Ea reverse reaction
H
Ea
forward rxn
PE
activated
complex
Ea
reverse
rxn
PE
prods
PE
reacts
Time
What kind of reaction is represented?
H of
reaction
Ea
forward rxn
Ea
reverse rxn
PE
activated
complex
PE reacts
PE prods
Time
What kind of reaction is represented?
H of
reaction
Why does collision have to be energetic?
• colliding reactant particles collect KE from
collisions & use to overcome reaction
barrier (hill)
• KE from collisions transformed into PE
factors that affect reaction rates
1)
2)
3)
4)
5)
6)
nature of reactants
temperature
concentration
pressure (gases only)
surface area
presence of catalyst
Nature of the reactants:
Ions vs Molecules?
• type of particles:
• ionic substances react quicker if in solution
• ions separated individually
• covalent molecules react slower, even if in solution
– atoms must be separated first & takes time to break bonds
to separate atoms!
• phase of particles:
2 gas phase reactants react
more quickly than
2 liquid reactants or
2 solid reactants
Temperature
• measure of average KE of molecules in system
• faster molecules are moving, more often will collide
• faster molecules are moving, more energetic the
collisions
• ↑ temp:
• increases frequency of collisions
• increases percentage of effective collisions that lead to
reaction
Concentration
• increase in concentration:
– more particles per unit volume
– more collisions in given amount time
Pressure
• only pertains to reactions involving GASES only:
– Δ pressure analogous to Δ concentration
•  pressure:
–  # particles per unit volume
–  # effective collisions
• ↓ pressure:
– ↓ # particles per unit volume
– ↓ # effective collisions
Surface Area
• increasing surface area:
– smaller particle size but more
particles exposed for reaction
– increases # effective collisions
• pertains to solids in heterogeneous
reactions only
Vocabulary Interlude
• homogeneous reaction:
– all reactants in same phase
• heterogeneous reaction:
– reactants in different phases
Catalyst
• substance that increases rate of
reaction
–lowers amount activation energy
needed for rxn to occur
BUT
–does not participate in reaction
activation energy is
decreased so
reaction can happen
sooner
does NOT change
PE of reactants or
products
Reaction Mechanism
• series of steps that lead reactants to form products
• also referred to as transition state
• individual steps called elementary steps
• rate-determining step:
• step that must get enough energy or rxn won’t occur
• process during which:
• bonds break
• atoms rearrange &
• new bonds form