Energy Changes, Reaction Rates and Equilibrium

Thermodynamics: study of energy, work and heat

Kinetic energy: energy of motion

Potential energy: energy of position, stored energy

Chemical reactions involve changes in energy.

Types of energy include:

Heat, sound, electricity, light, motion, etc.

Example: 2H + O2 2H2O + energy

Energy

Law of conservation of energy: the total energy in a system does not change. (Energy cannot be created or destroyed during chemical reactions.)

• Chemical bonds store potential energy.

• Reactions that form products having lower potential energy than the reactants are favored.

• A compound with lower potential energy is more stable than a compound with higher potential energy.

The Units of Energy

calorie (cal): the amount of energy needed to raise the temperature of 1 g of water by 1 oC.

Joule (J): is another unit of energy; 1 cal = 4.184 J

•Both joules and calories can be reported in the larger units kilojoules (kJ) and kilocalories (kcal).

1,000 J = 1 kJ 1,000 cal = 1 kcal

Energy Changes in Reactions

When molecules come together and react, bonds are broken in the reactants and new bonds are formed in the products.

•Bond breaking always requires an input of energy.

•Bond formation always releases energy.

ClCl

To cleave this bond, 58 kcal/mol must be added.

To form this bond, 58 kcal/mol is released.

Energy Changes in Reactions

Enthalpy change (H): the energy absorbed or released in a reaction; it is also called the heat of reaction

•When energy is absorbed, the reaction is endothermic; H is positive (+).

•When energy is released, the reaction is exothermic; H is negative (−).

ClClTo form this bond, H = −58 kcal/mol.

To cleave this bond,H = +58 kcal/mol.

Energy Diagrams

The difference in energy between reactants and the products is H.

•If H is negative, the reaction is exothermic:

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•If H is positive, the reaction is endothermic:

Exothermic Reaction: Reaction that releases heat to surroundings

Endothermic Reaction: Reaction that absorbs heat from surroundings

Exothermic Rxn H is negative (−)

Endothermic Rxn H is positive (+)

Summary of Energy Changes in Reactions

Practice:

Identify each reaction as exothermic or endothermic, and indicate the if the H is positive or negative.

A. N2 + 3H2 2NH3 + 22 kcal

B. CaCO3 + 133 kcal CaO + CO2

C. 2SO2 + O2 2SO3 + heat

Exo, -H

Exo, -H

Endo, +H

Energy of activation (Ea): the minimum amount of energy necessary for a reaction to occur

Note the Activation Energy (Ea) in Exothermic and Endothermic Rxns

Exothermic Rxn Endothermic Rxn

Activation Energy (Ea)

•The Ea is the minimum amount of energy that the reactants must possess for a reaction to occur.

•Ea is called the energy barrier and the height of the barrier determines the reaction rate.

•When the Ea is high, few molecules have enough energy to cross the energy barrier, and the reaction is slow.

•When the Ea is low, many molecules have enough energy to cross the energy barrier, and the reaction is fast.

Factors that Influence Reaction Rates

Temperature of Reactants

Increasing the temperature increases the kinetic energy of the particles, allowing more collision to occur

Concentration of Reactants

The greater the concentration of reactants, the more collisions leading to a reaction will occur

Presence of Catalysts

Catalyst: Substance that increases rate of a reaction without being used up in the reaction

Catalysts provide alternate way for reaction to occur, with a lower activation energy than the normal way

Effect of Catalyst on Activation Energy

Without Catalyst

(High Ea)

With Catalyst

(Lower Ea)

•The uncatalyzed reaction (higher Ea) is slower.

•The catalyzed reaction (lower Ea) is faster.

H is the same for both reactions.

Chemical Equilibrium

Chemical reactions can go both directions (forward and reverse)

H2 + I2 2HI

Equilibrium: Condition when rate of forward reaction equals rate of reverse reaction

Equilibrium Concentrations: Unchanging concentrations of products and reactants in a reaction that is at equilibrium

The Equilibrium Constant

Equilibrium constant, K: relationship between concentration of products and concentration of the reactants; concentration of products divided by concentration of reactants

a A + b B c C + d D

equilibriumconstant = K =

[products][reactants] =

[C]c [D]d

[A]a [B]b

N2 + O2 2 NO

equilibriumconstant

= K =[N2] [O2]

[NO]2

Note: The coefficient becomes the exponent!

What does the Equilibrium Constant Tell Us?

•When K is much greater than 1 (K > 1):

[products]

[reactants]The numerator is

larger.

Equilibrium favors the products and lies to the left.Equilibrium favors the products and lies to the left.

•When K is much less than 1 (K < 1):

[products]

[reactants] The denominator is larger.

Equilibrium favors the reactants and lies to the right.Equilibrium favors the reactants and lies to the right.

•When K is around 1 (0.01 < K < 100):

[products]

[reactants]Both are similarin magnitude.

Both reactants and products are present.Both reactants and products are present.

2 H2(g) + O2(g) 2 H2O(g) K = 2.9 x 1082

The product is favored because K > 1.

The equilibrium lies to the right.

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Equilibrium

Calculating the Equilibrium Constant

Example: Calculate K for the reaction between the general reactants A2 and B2. The equilibrium concentrations are as follows:

[A2] = 0.25 M [B2] = 0.25 M [AB] = 0.50 M

A2 + B2 2 AB

[AB]2

[A2][B2]K =

[AB]2

[A2][B2]K = =

[0.50]2

[0.25][0.25]

=0.25

0.0625= 4.0

Le Châtelier’s Principle

If a chemical system at equilibrium is disturbed or stressed, the system will react in a direction that counteracts the disturbance or relieves the stress.

Some of the possible disturbances:

•concentration changes

•temperature changes

•pressure changes

Le Châtelier’s Principle: Concentration Changes

2 CO(g) + O2(g) 2 CO2(g)

What happens if [CO(g)] is increased?

•The concentration of O2(g) will decrease.•The concentration of CO2(g) will increase.

2 CO(g) + O2(g) 2 CO2(g)

What happens if [CO2(g)] is increased?

•The concentration of CO(g) will increase.•The concentration of O2(g) will increase.

What happens if a product is removed?

•The concentration of ethanol will decrease.

•The concentration of the other product (C2H4) will increase.

Le Châtelier’s Principle: Temperature Changes

•When the temperature is increased, the reaction that absorbs heat is favored.

•An endothermic reaction absorbs heat, so increasing the temperature favors the forward reaction.

•Conversely, when the temperature is decreased, the reaction that adds heat is favored.

•An exothermic reaction releases heat, so increasing the temperature favors the reverse reaction.

Le Châtelier’s Principle: Pressure Changes

•When pressure increases, equilibrium shifts in the direction that decreases the number of moles in order to decrease pressure.

•When pressure decreases, equilibrium shifts in the direction that increases the number of moles in order to increase pressure.