Chemical Equilibrium and Reaction Rates

Chapter 14 & 16

In an equilibrium reaction, the rate of the reaction of one direction will equal the rate of the reaction in the opposite direction.

Chemical Equilibrium

Chemical Equilibrium

Reversible reactions: Products take part in a separate reaction to reform the reactants.

Some reactions can reverse direction; however, some need added energy to proceed in the reverse direction.

A double arrow is used to express a reversible reaction.

Chemical Equilibrium

The rate of a reaction is determined by the following factors: concentration, temperature, pressure.

As the rate of the forward reaction is equal to the rate of the reverse reaction, the reaction has reached a state of equilibrium.

↔ or

Chemical Equilibrium

Chemical Equilibrium: is the state in which the concentration of the reactants and products remain constant with time because the rate at which they are formed in each reaction equal the rate they are consumed in the opposite reaction.

Chemical equilibrium is called dynamic because it is always changing.

Chemical Equilibrium

Equilibrium

Concentration becomes Constant NOT Equal

Rates become Equal

1. What is in the reaction vessel at time = 0? H2 + N2

2. Write the forward reaction: 3H2 + N2 → 2NH3

3. What kind of reaction is the forward reaction? synthesis 4. Write the reverse reaction: 2NH3 → 3H2 + N2

5. What kind of reaction is the reverse reaction? decomposition6. Over time the concentration of which substance(s)

decreases? H2 + N2

7. Over time the concentration of which substance(s) increases?

NH3

8. Mark on the graph with a dashed line when equilibrium is reached.

9. At equilibrium which substance(s) is(are) present in the greater concentration?

H2 + N2

10. Is the forward or reverse reaction favored? reverse

Equilibrium System and Stress LeChatelier’s Principle states that if a change in

conditions is imposed on a system at equilibrium, the equilibrium position will shift on a system that tends to reduce that change in conditions.

Pure solids and liquids are not affected by changes in equilibrium.

Changes in Concentration Adding a substance to a system drive the

system to consume the substance.

Removing a substance from a system at equilibrium drives the system to the production of the substance.

N2O4(g) NO2

Changes in Pressure

If the pressure is increased, the reaction will shift in the direction that produces the least molecules.

2NO(g) N2O2

2H2O O2 + 2H2

H2 + Cl2 2HCl No shift

Shift to the left

Shift to the right

Changes in TemperatureExothermic Reactions

H2(g) + I2(g) 2HI(g) + heat

Lowering the temperature will produce a higher yield of HI

Increasing the temperature will produce a higher yield of H2 & I2

Changes in TemperatureEndothermic Reactions Heat + NH4Cl(s) NH3(g) + HCl

Lowering the temperature will produce a higher yield of NH4Cl

Increasing the temperature will produce a higher yield of NH3 & HCl

Homogeneous and Heterogeneous Equilibria Homogeneous equilibria: reactions where

reactants and products are in the same states.

Heterogeneous equilibria: reactions where the reactants and products are in different states.

Collision Theory

Collision Theory – Molecules must collide with each other in order to react.

Particle must collide with another particle or the container wall.

Types of collisionsEffective Collisions: Lead to the formation of

products Ineffective Collisions: Do not lead to the

formation of products

Factors which determine effectiveness of collisions: 1) Energy of particles

2) Orientation of particles

In a chemical reaction, the kinetic energy of the reactants is converted to potential energy for the products.

Ineffective Collision – Insufficient Energy - No Products

Effective Collision – Sufficient Energy – Forms Products

Effective collisions have enough energy, and the correct orientation to form products. Ineffective collisions revert to the original products.

Activation Energy (EA)

Particles must possess a minimum amount of energy in order to react.

Analogy: Rolling a ball up a hill

Activation Energy: Energy needed to start a reaction; difference between the energy at the peak and the energy of the reactants

Activation Energy

Reverse ReactionForward Reaction

Energy Diagram for a Chemical Rxn.

Energy of Reactants

Activated Complex

Energy of Products

Reaction Pathway

Ea forward reaction

(80 kJ)

Ea reverse reaction

(140 kJ)

Pot

enti

al(k

J)

(Exo) (Endo)

∆H = -60 kJ (for.)+60 kJ (rev.)

Added Catalyst

Endothermic Exothermic

Activated Complex

Transition State: A short-lived complex which lives as neither a reactant or product. This state occurs when old bonds are broken and new bonds are formed.

This complex is called the activated complex.

Activation energy is the energy needed to form the activated complex.

Activated Complex (cont.) Most reactions occur in a series of steps called

a reaction mechanism.A BB CC D

Often one step is slower than the others. It is called the rate determining step.

A substance that increases the rate of a chemical reaction by providing a mechanism with a lower energy of activation is called a catalyst.

Factors Affecting Reaction Rates Reaction rate is determined by measuring a

change in concentration of reactants and products in a certain amount of time. Reaction rates are determined experimentally.

Rate-influencing factors are those factors that affect rate of reactions by altering the frequency, orientation, or energy levels at which the particles collide.

Factors affecting the rate of reaction

A) Nature of Reactants1) Structure: The more complex, the slower it

reacts.

A) Nature of Reactants2) State: Gases react the fastest and solids

react the slowest.

B) Temperature: As temperature increases, the rate of the reaction increases.

Rule of Thumb: Rate doubles every 10ºC increase

C) Concentration: As concentration increases, the rate of the reaction increases.

D) Surface Area: As surface area increases, the rate of the reaction increases.

E) Catalyst increases the rate of the reaction without being used in the reaction.

Catalysts speed up the reaction rate by reducing the activation energy needed for the reaction to occur.

Uncatalyzed reaction (slow)

Catalyzed reaction (fast)

Inhibitors: slow the rate of the reaction

Energy diagram involving a catalyst: