Chapter 6 key points
Chemistry 121, Spring 2015
Energy as a quantity
• Energy is the ability for an object to move;
there are kinetic and potential energies
• As a simplification, potential energy is stored
in bonds and kinetic energy is expressed as
heat; the units of all energy are the SI unit
Joule (J) or the traditional calorie (cal)
• A food calorie is a kcal = 1000 cal = 4184 J;
that is, 1 cal = 4.184 J
Chemical reaction energy changes
• Breaking a bond requires energy; making a
bond releases energy
• In a rxn, if the amount of energy used to make
bonds exceeds the amount of energy used to
break bonds, then heat is released overall and
the rxn is exothermic; if the opposite is true,
heat is absorbed overall and the rxn is
endothermic
Energy diagrams
• Energy diagrams are graphs that plot the
progress of a reaction on the x-axis and the
energy of the system on the y-axis
• The lower a point on the energy diagram is,
the more stable the system is
• The transition state between reactants and
products is a high-energy state that reduces
the chance of a successful rxn; catalysts are
used to lower the energy of the t.s.
Reaction rates
• The rate of a rxn is measured by how much
product is produced over a length of time, or
how much reactant disappears over time
• Higher temperatures generally increase
reaction rates
• Higher concentrations of reactants generally
increase reaction rates
Equilibrium
• Dynamic equilibrium is the point in a rxn
where the rate of the forward (reactant 
product) rxn is equal to the rate of the
backward (product  reactant) rxn
• The equilibrium constant, K, is calculated
based off the concentrations of products and
reactants at equilibrium and can be used to
predict whether reactants or products are
favored in a rxn
Le Chatelier’s principle
• For a rxn at equilibrium, if a change is imposed
on the system, the system will respond to
oppose the change
• For instance, if a gaseous rxn at equilibrium is
pressurized, the system will shift such that the
total number of gas substance moles is
reduced