cmh 121 luca preziati chapter 7: reaction rates and chemical equilibrium chemical kinetics: chemical...

CMH 121 Luca Preziati

Chapter 7: Reaction Rates and Chemical Equilibrium

Chemical kinetics:Chemical kinetics: The study of the rates of chemical reactions.

• Consider the reaction that takes place when chloromethane and sodium iodide are dissolved in acetone; the net ionic equation for this reaction is:

• To determine the rate of this reaction, we measure the concentration of iodomethane at periodic time intervals, say every 10 minutes.

CH3-Cl I- CH3-I Cl-+ +Chloromethane Iodomethane



• The rate of reaction is the increase in concentration of iodomethane divided by the time interval.

• For example, the concentration might increase from 0 to 0.12 mol/L over a 30 minute time period.

• The reaction rate over this period is:

• This unit is read moleper liter per minute.

30 min(0.12 mol CH3I/L) - (0 mol CH3I/L)

=0.0040 mol CH3I/L

min



The rates of chemical reactions are affected by the following factors:

On a microscopic scale:

1. Molecular collisions

2. Activation energy

On a macroscopic scale:

1. Nature of the reactants

2. Concentration of the reactants

3. Temperature

4. Presence of a catalyst


Chapter 7: Molecular collisions

Effective collision: Effective collision: A collision that results in a reaction

An effective collision happens when two conditions are met:

1.The colliding particles must be properly oriented properly oriented for bond breaking and bond making

2.The colliding particles must have enough kinetic energyenergy

+ +

H2O + HCl H3O+ Cl-+

Activation energy:Activation energy: The minimum energy required for a reaction to take place.


Chapter 7: Reaction Rates and Energy

Energy diagram for an exothermic reaction.

Energy diagram for an endothermic reaction.

Transition state:Transition state: A maximum on an energy diagram.


Chapter 7: Reaction Rates

Nature of reactants• In general, reactions between ions in aqueous solution

are very fast (activation energies are very low).• In general, reaction between covalent compounds,

whether in water or another solvent, are slower (their activation energies are higher).

Concentration• In most cases, the reaction rate increases when the

concentration of either or both reactants increases.• For many reactions, there is a direct relationship

between concentration and reaction rate. When concentration doubles the rate doubles.



Temperature• In virtually all reactions, rate increases as temperature

increases.• An approximate rule for many reactions is that for a

10°C increase in temperature, the reaction rate doubles.

• When temperature increases, molecules move faster (they have more kinetic energy), which means that they collide more frequently. More frequent collisions mean higher reaction rates.

• Not only do molecules move faster at higher temperatures, but the fraction of molecules with energy equal to or greater than the activation energy also increases.



Catalyst:Catalyst: A substance that increases the rate of a chemical reaction without itself being used up.


Chapter 7: Chemical Equilibrium

Reversible reaction:Reversible reaction: A reaction that can be made to go in either direction.

forward reactionreversereaction

CO(g) + H2O(g) CO2(g) + H2(g)

Equilibrium:Equilibrium: A dynamic state in which the rate of the forward reaction is equal to the rate of the reverse reaction.

•At equilibrium there is no change in concentration of either reactants or products.•Reaction, however, is still taking place. Reactants are still being converted to products and products to reactants, but the rates of the two reactions are equal.



Equilibrium constant, Equilibrium constant, KK:: The product of the concentrations of products of a chemical equilibrium divided by the concentrations of reactants, each raised to the power equal to its coefficient in the balanced chemical equation.• For the general reaction:

• The equilibrium constant expression is:

aA + bB cC + dD

ba

dc

[B][A]

[D][C]K

There is no relationship between a reaction rate and the value of K.



Problem:Problem: When H2 and I2 react at 427°C, the following equilibrium is reached:

• The equilibrium concentrations are [I2] = 0.42 mol/L, [H2] = 0.025 mol/L, and [HI] = 0.76 mol/L. Using these values, calculate the value of K.

Solution:Solution:

This K has no units because molarities cancel.

[HI]2

[I2][H2]K = = (0.76 M)2

(0.42 M) x (0.025 M)= 55


Chapter 7: Le Chatelier’s Principle

Le Chatelier’s Principle:Le Chatelier’s Principle: When a stress is applied to a chemical system at equilibrium, the position of the equilibrium shifts in the direction to relieve the applied stress.

We are going to look at three types of stress that can be applied to a chemical equilibrium:• addition of a reaction component• removal of a reaction component• change in temperature



Problem:Problem: When acid rain (H2SO4(aq))attacks marble (calcium carbonate), the following equilibrium can be written:

How does the fact that CO2 is a gas influence the equilibrium?

Solution:Solution: CO2 gas diffuses from the reaction site, and is removed from the equilibrium mixture. The equilibrium shifts to the right and the marble continues to erode.

+H2SO4(aq)Sulfuric

acid

CaCO3(s) CaSO4(s) +CO2(g) + H2O(l)Calciumcarbonate

Calciumsulfate

Carbondioxide

Water



Reaction type

exothermic

endothermic

Change intemperature

increase

decrease

increase

decrease

Direction equilibriumis driven

to the left; toward reactants

to the right; toward products

to the right; toward products

to the left; toward reactants

cmh 121 luca preziati chapter 7: reaction rates and chemical equilibrium chemical kinetics: chemical...

Documents