UNIT 17Review, Part IIIRate, Thermodynamics, Equilibrium, & Electrochemistry

C. Johannesson

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 (activation energy)

B. Factors That Increase Rxn Rate

Surface Area (more SA = more opportunities for collisions)

Concentration (more concentrated = more frequent collisions)

Temperature (hotter = faster moving particles)· This means more frequent collisions AND· More activation energy

Catalysts (lower activation energy)

B. Exothermic Reaction

A reaction thatreleases (gives off)energy

products have lower energy than reactants

Energy is a

PRODUCT

2H2(l) + O2(l) 2H2O(g) + energy

DH(energy

released)

C. Endothermic Reaction

A reaction that absorbs (takes in)energy

reactants have lower energythan products

Energy is a

reactant

2Al2O3 + energy 4Al + 3O2

DH(energy

absorbed)

A. Reaction Pathway Diagram

Catalyst: speeds up the reaction by lowering the Ea (activation energy)

(catalyzed reaction in red)

NOTE:Ereactants,Eproducts,

& DH are unchanged by

a catalyst

Catalyst only affects Ea

Equilibrium

Most reactions are reversible

2 Na + Cl2 2 NaCl + 822 kJThis is ___thermic

The REVERSE reaction:

2 NaCl + 822 kJ 2 Na + Cl2Is endothermic (absorbs energy)

exo (releases energy)

Equilibrium

A reversible reaction becomes an equilibrium reaction if the forward and reverse reaction occur at the same rate.Fe(OH)3 Fe⇌ 3+ + 3OH-

The equilibrium arrow: ⇌

Indicates that the forward and reverse reactions will compete until they are occurring at the same rate.

Equilibrium

If the reverse reaction (iron (III) hydroxide forming from its ions) happens at the same rate as the forward reaction, the reaction will never “finish.”

The concentrations stop changing because

product and reactant are being made and used up

at the same rate.

Le Chatelier’s Principle

A reaction at equilibrium, when

“stressed,” will react to relieve

the stress.

(If you mess with it, it will work

to return to its equilibrium ratio.)

Le Chatelier: Concentration

2SO2 (g) + O2 (g) 2 SO⇌ 3 (g)If the SO2 or O2 concentration was

increased: (i.e., we added more reactants)Equilibrium will shift right to decrease the

concentration of reactants.(Some of the reactant we added gets used

up and more SO3 (product) is produced.)

Le Chatelier: Concentration

2SO2 (g) + O2 (g) 2 SO⇌ 3 (g)If the SO3 concentration was increased:

(i.e., we added more products)Equilibrium will shift left to decrease the

concentration of products.(Some of the product we added gets used

up and more SO2 and O2 (reactants) are made.)

Le Chatelier: Concentration

2SO2 (g) + O2 (g) 2 SO⇌ 3 (g)If the SO2 concentration was decreased:

(i.e., we removed reactants)Equilibrium will shift left to replace the

removed reactants.

Some SO3 will get turned into SO2 & O2.

The amount of SO3 decreases. The amount of O2 increases.

Le Chatelier: Concentration

2SO2 (g) + O2 (g) 2 SO⇌ 3 (g)If the SO3 concentration was decreased:

(i.e., we removed products)Equilibrium will shift right to replace the

removed product.

Some SO2 and O2 will get turned into SO3.

The amount of SO2 and O2 decreases.

Le Chatelier: Temperature

N2 (g) + 3 H2 (g) 2 NH⇌ 3 (g) + 92 kJThe reaction is exothermic. (Heat is a product.)An increase in temperature shifts this reaction to the left.

A decrease in temperature shifts this reaction to the right.

Le Chatelier: Temperature

2 NH3 (g) + 92 kJ N⇌ 2 (g) + 3 H2 (g)

This reaction is endothermic. (Heat is a reactant.)

It is the OPPOSITE of the previous reaction!

An increase in temperature shifts this reaction to the right.

A decrease in temperature shifts this reaction to the left.

Electrochemistry

An electrochemical reaction involves the transfer of electrons:

2 Na0 2 Na1+

Cl2 0 2 Cl1-

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Unit 14: Electrochemistry

+ 2 e-

Loss of Electrons is Oxidation2 e-

+ Gain of Electrons is Reduction

Electrochemistry

Zn + CuSO4 ZnSO4 + CuWhat element was oxidized?

What element was reduced?

Which element LOST electrons?

Which element GAINED electrons?Unit 14: Electrochemistry

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-2+2

-2+2

Zn (0 +2)

Cu (+2 0)

Cu (+2 0)

Zn (0 +2)

Galvanic Cells:

When the half-reactions are separate, electrons produced by the oxidation must travel through a wire (external conductor).

Unit 14: Electrochemistry

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Zn Zn2+ + 2e- 2e- + Cu2+ Cu

e-

e- e-

e-

Galvanic Cells:

The salt bridge completes the circuit by allowing ions to transfer.

Unit 14: Electrochemistry

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Zn Zn2+ + 2e- 2e- + Cu2+ Cu

KCl

Cl- K+

Voltage

Standard Reduction Potential

Unit 14: Electrochemistry

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Given a reduction half-reaction AND an oxidation:• Cu2+ + 2e- Cu• Sn2+ Sn4+ +

2e- Find their potentials (V)

0.34 V-0.15 V

• The oxidation is written opposite from the version on the chart.

• So I “flip” it and change the sign on its voltage.

• Now ADD.

+0.15 V0.49 V

Voltage

Standard Reduction Potential

Unit 14: Electrochemistry

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Given two reduction half-reactions:

Cu2+ + 2e- CuZn2+ + 2e- Zn

Find their potentials (V)

0.34 V-0.76 V

• The more positive voltage will be the reduction. (Cu)

• The less positive voltage will change its sign and run as an oxidation. (Zn)

Voltage

Standard Reduction Potential

Unit 14: Electrochemistry

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Given two reduction half-reactions:

Cu2+ + 2e- CuZn Zn2+ + 2e-

0.34 V+0.76 V

1.10 V

Cu2+ + Zn Zn2+ Cu

Always flip the less positive half-reaction (and change the sign on its voltage).

Then add the voltages.