Unit 9: Chemical EquilibriumCollision theoryRates of reactionsCatalystsReversible reactionsChemical equilibriumLe Chatelier’s Principle
ConcentrationTemperatureVolumeCatalysts
A. Collision Theory
Reaction rate depends on the collisions between reacting particles.
Successful collisions occur if the particles...
collide with each other
have the correct orientation
have enough kinetic energy to break bonds
Exothermic Endothermic
Time
En
erg
y
Activation energy
Energy of reaction
Time
En
erg
y
Activation energy: minimum energy required for a reaction to occur
A. Collision TheoryActivation Energy
depends on reactantslow Ea = fast rxn rate
Ea
Chemical kinetics: the study of the rate (the speed) of a reaction
Rate of a chemical reaction depends on:
16.2: Rates of Reactions
3. TEMPERATURE (T) of reactants
2. CONCENTRATION of reactants
4. Presence/absence of a CATALYST
1. SURFACE AREA
SURFACE AREA
Surface Areahigh SA = fast rxn ratemore opportunities for collisionsIncrease surface area by…• using smaller particles• dissolving in water
Concentration:KMT (Kinetic-Molecular theory) states that increasing concentration of reactants results in more collisions.
Effect of Concentration on Rate
More collisions result in more reactions, increasing the rate of the reaction.
Temperature:Increasing T increases particle speed.
Effect of Temperature on Rate
Faster reactants means more collisions have the activation energy, which increases the rate of the reaction.
A catalyst:A chemical that influences a reaction, but is not consumed in the reaction. (It can be recovered unchanged at the end of the reaction.)Lowers the activation energy of the reaction.
Effect of Catalysts on Rate
Activation energy
Time
En
erg
y
Activation energy with catalyst
16.1: Reversible Reactions* Thus far, we have considered only one-way reactions: A + B → C + D
Some reactions are reversible:They go forward (“to the right”) : A + B → C + D
and backwards (“to the left”) : A + B ← C + D
Written with a two-way arrow:A + B ↔ C + D
Boiling & condensingFreezing & meltingRecharging a “rechargeable battery”
Examples:
Examples of irreversible reactions:
Striking a match / burning paperDropping an eggCooking (destroys proteins)
For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established.
Both the forward and backward reactions continue, but there is a balance of products “un-reacting” and reactants reacting.
A + B ↔ C + D
A B+ C D+A B+A B+ C D+C D+
16.3: Chemical Equilibrium
* Le Chatelier’s Principle is about reducing stress – a stress applied to a chemical equilibrium
(1850 – 1936)
Relax! Reduce stress brought on by chemical equilibrium
with me, Henri Le Chatelier!
Le Chatelier’s Principle:When a stress is applied to a system (i.e. reactants and products) at equilibrium, the system responds to relieve the stress.The system shifts in the direction of the reaction that is favored by the stress.A stress is a change in:
ConcentrationTemperatureVolume
16.4: Le Chatelier’s Principle
Ex: Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6
H2O
(pink) (blue)
Stress Result
16.5: Stress: Change Concentration
Add Cl1- Forward rxn favored
Shifts forward to reduce extra Cl1-
More CoCl42- will formAdd H2O Backward rxn favored
Shifts backward to reduce extra H2O
More Co(H2O)62+ will form
Ex: heat + Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O
(pink) (blue)
This reaction is endothermic. For Le Chatelier’s principle, consider “heat” as a chemical.
Stress Result
16.7: Stress: Change Temperature
Decrease T Backward rxn favored; shifts backward to replace “lost” heat
More Co(H2O)62+ will form
Increase T Forward rxn favored; shifts forward to reduce extra heat
More CoCl42- will form
Ex: 1 N2 (g) + 3 H2(g) ↔ 2 NH3(g)
(1 + 3 = 4 moles of gas) ↔ (2 moles of gas)
Stress Result
16.6: Stress: Change Volume
Increase V Backward rxn favored; shifts backward to side with more moles of gas (to fill the larger volume with more molecules)
Decrease VForward rxn favored; shifts forward to side with fewer moles of gas (reduces # of
molecules packed into this smaller volume)
Ex: 2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g)
Since a catalyst increases the forward and backward rates equally, it will not shift the equilibrium.
16.7: Catalysts & Equilibrium
MnO2
Rea
ctio
n R
ate
Time
Crystallization (backward rate) increases…
Dissolving (forward rate) decreases…
Equilibrium is established:
Forward rate = Backward rate
Ex: saturated salt solutionNaCl (s) ↔ Na+ (aq) + Cl- (aq)
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