Chemical Change: Energy, Rate 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
Exothermic Reaction: Reaction that releases heat to surroundings
Endothermic Reaction: Reaction that absorbs heat from surroundings
Exothermic Rxn Endothermic Rxn
Practice:
Identify each reaction as 1) exothermic or 2) endothermic
A. N2 + 3H2 2NH3 + 22 kcal
B. CaCO3 + 133 kcal CaO + CO2
C. 2SO2 + O2 2SO3 + heat
Exo
Exo
Endo
1st law of thermodynamics: energy of the universe is constant
Enthalpy: heat energy; change in heat energy from reactants to products is Ho
Spontaneous Process: Process that takes place without persuasion, without stimulus or energy input (Ball rolling downhill)
Nonspontaneous Process: Process that requires energy or stimulus to occur (Ball being pushed uphill)
Entropy: Measurement of disorder or randomness of a system; change in entropy from reactants to products is DSo
2nd law of thermodynamics: entropy of universe spontaneously tends to increase
Kinetics
Kinetics: study of the rate, or speed of chemical reactions
Reaction Rate: Speed of a reaction; how quickly or slowly a reactant is used up or a product accumulates
Activation energy: minimum amount of energy required to produce a chemical reaction
Note the Activation Energy (Ea) in Exothermic and Endothermic Rxns
Exothermic Rxn Endothermic Rxn
Factors that Influence Reaction Rates
Structure of Reactants
Attraction between oppositely charged particles; breaking of covalent bonds; size of molecules
Concentration of Reactants
The greater the concentration of reactants, the more collisions leading to a reaction will occur
Temperature of Reactants
Increasing the temperature increases the kinetic energy of the particles, allowing more collision to occur
Physical state of Reactants
Solid, liquid or gas
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)
Chemical Equilibrium
Chemical reactions can go both directions
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
Equilibrium Position Can Change!
H2 + I2 2HI
Le Chatelier’s Principle: Equilibrium position will shift in response to changing conditions in such a way as to minimize or oppose changes
Note: Concentration of chemicals and heat will affect equilibrium; presence of a catalyst will not!
Summary of Changes on Equilibrium