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