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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.
• A compound with lower potential energy is more stable than a
compound with higher potential energy.
• Reactions that form products having lower potential energy
than the reactants are favored.
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.
To cleave this bond,
58 kcal/mol must be
added.
Cl
Cl
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 (−).
To cleave this bond,
H = +58 kcal/mol.
Cl
Cl
To form 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:
•If H is positive, the reaction is endothermic:
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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
Endo, +H
C. 2SO2 + O2
2SO3 + heat
Exo, -H
Exo, -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
With Catalyst
(High Ea)
(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
aA + bB
equilibrium
= K =
constant
cC + dD
[products]
[reactants]
[C]c [D]d
=
[A]a [B]b
Note: The coefficient becomes the exponent!
N2 + O2
equilibrium
constant
2 NO
= K
=
[NO]2
[N2] [O2]
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.
•When K is much less than 1 (K < 1):
[products]
[reactants]
The denominator is
larger.
Equilibrium favors the reactants and lies to the right.
•When K is around 1 (0.01 < K < 100):
[products]
[reactants]
Both are similar
in magnitude.
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.
Equilibrium
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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
A2
[B2] = 0.25 M
+ B2
K =
[AB] = 0.50 M
2 AB
[AB]2
[A2][B2]
K =
[AB]2
[A2][B2]
=
=
[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)
What happens if [CO2(g)] is increased?
•The concentration of CO(g) will increase.
•The concentration of O2(g) will increase.
2 CO2(g)
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.
•An exothermic reaction releases heat, so increasing the temperature
favors the reverse reaction.
•Conversely, when the temperature is decreased, the reaction that
adds heat is favored.
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.