Download - Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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Page 1: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

Unit 9: Chemical EquilibriumCollision theoryRates of reactionsCatalystsReversible reactionsChemical equilibriumLe Chatelier’s Principle

ConcentrationTemperatureVolumeCatalysts

Page 2: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 3: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 4: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

A. Collision TheoryActivation Energy

depends on reactantslow Ea = fast rxn rate

Ea

Page 5: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 6: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

SURFACE AREA

Surface Areahigh SA = fast rxn ratemore opportunities for collisionsIncrease surface area by…• using smaller particles• dissolving in water

Page 7: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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.

Page 8: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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.

Page 9: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 10: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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:

Page 11: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

Examples of irreversible reactions:

Striking a match / burning paperDropping an eggCooking (destroys proteins)

Page 12: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 13: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

* 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!

Page 14: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 15: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 16: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 17: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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)

Page 18: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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

Page 19: Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration.

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)