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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: - PowerPoint PPT Presentation
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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.
• 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:
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 (+)
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
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
•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
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.
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 =
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.