Energy, Rate, and
Equilibrium
Dr. Michael P. Gillespie
Thermodynamics
• Thermodynamics is the study of energy, work, and
heat.
• We can apply this concept to either chemical or
physical changes.
• We can calculate the amount of heat obtained
from the combustion of one gallon of fuel oil
(chemical).
• We can calculate the energy consumed or
released in the boiling and freezing of water.
Kinetic Molecular Theory
• Molecules and atoms in a reaction mixture are
in constant, random motion.
• These molecules and atoms frequently collide
with each other.
• Only some collisions (those with sufficient
energy) will break bonds in molecules.
• When reactant bonds are broken, new bonds
may be formed and products result.
Chemical Reactions and
Energy
• We cannot measure the absolute value for
energy stored in a chemical system.
• We can only measure the change in energy as
a chemical reaction occurs.
• We must establish a boundary between the
system and the surroundings.
Chemical Reactions and
Energy
• The system contains the process under study.
• The surroundings encompass the rest of the
universe.
• Energy is lost from the system to the
surroundings or energy may be gained by the
system at the expense of the surroundings.
This usually occurs in the form of heat and can
be measured by temperature changes in the
system and surroundings.
Exothermic and
Endothermic Reactions
• An exothermic reaction releases energy to the
surroundings. The surroundings become
warmer.
• An endothermic reaction absorbs energy from
the surroundings. The surroundings become
cooler.
Exothermic and
Endothermic Reactions
Enthalpy
• Enthalpy is the term used to represent heat.
• The change in enthalpy is the energy
difference between the products and
reactants of a chemical reaction and is
symbolized by ∆H.
• Energy released is represented by a negative
sign (exothermic) and energy absorbed is
represented by a positive sign (endothermic).
Enthalpy
Spontaneous and
Nonspontaneous Reactions
• Most, but not all exothermic reactions are
spontaneous.
• Most, but not all endothermic reactions are
not spontaneous.
Entropy
• The second law of thermodynamics states that
the universe spontaneously tends toward
increasing disorder or randomness.
• Entropy is a measure of the randomness of a
chemical system and is represented by the
symbol S.
Entropy
• A random, disordered system has high
entropy.
• A well-organized system has low entropy.
• Disorder or randomness increases when we go
from the solid to liquid to gaseous state.
Entropy
Calorimetry
• Calorimetry is the measure of heat energy
changes in a chemical reaction.
• The device used to measure this is a
calorimeter.
• The technique involves measuring the change
in the temperature of a quantity of water or
solution that is in contact with the reaction of
interest.
Specific Heat
• The specific heat of a substance is defined as
the number of calories of heat needed to raise
the temperature of 1 g of the substance 1
degree Celsius.
• Knowing the specific heat of water or another
aqueous solution enables the experimenter to
calculate the heat released during the
reaction.
Specific Heat
Fuel Value
• Fuel value is the amount of energy per gram of
food.
• The fuel value is generally reported in units of
nutritional calories.
Nutritional Calorie
• One nutritional calorie is equivalent to one
kilocalorie (1000 calories).
• It is also known as one large Calorie
(uppercase C).
Bomb Calorimeter
• A bomb calorimeter is a special type of
calorimeter used to measure the fuel value
(Calories) of foods.
• It measures the heat released upon
combustion of foods.
Bomb Calorimeter
Kinetics
• Thermodynamics help us to decide whether or
not a chemical reaction is spontaneous;
however, knowing that a reaction can occur
spontaneously tells us nothing about the time
that it may take.
• Chemical kinetics is the study of the rate of the
chemical reactions.
Kinetics
• Kinetic information also tells us something
about the mechanism of action.
• Kinetic information can provide information
about the “shelf life” of processed foods.
• The potency of a drug diminishes over time.
Kinetics tells us the rate of decline.
Chemical Reaction
• For a chemical reaction to proceed, sufficient
energy must be available to cause the bonds to
break.
• This energy is provided by the collision of
molecules.
• Bonds will break and the atoms will recombine in a
lower energy arrangement.
• An effective collision is one that produces
product molecules.
Chemical Reaction
Activation Energy
• Activation energy is the minimum amount of
energy required to initiate a chemical reaction.
• Enzymes dramatically lower the required
activation energy for a chemical reaction to
occur.
Activation Energy
Factors That Affect
Reaction Rate
• Structure of the reacting species
• Concentration of reactants
• Temperature of reactants
• Physical state of reactants
• Presence of a catalyst
Structure of the Reacting
Species
• Bond strengths
• Reactions involving ions in solution are usually
rapid. Ionic compounds in solution are
dissociated, their bonds are already broken.
The activation energy should be low.
• Reactions involving covalent bonds may
proceed more slowly. Covalent bonds must be
broken before new bonds are formed. The
activation energy is higher.
Structure of the Reacting
Species
• The size and shape of the reactant molecules
• Large molecules with bulky groups may block
the reactive site.
Concentration of
Reactants
• The rate of a chemical reaction generally
increases as the concentration of reactants
increases because more reactant molecules
results in a greater number of collisions.
Temperature of
Reactants
• The rate of a reaction increases as the
temperature increases.
• The average kinetic energy of the reacting
particles is directly proportional to the Kelvin
temperature.
• Increasing the speed of the particles increases the
likelihood of a collision.
• A 10 degree Celsius rise in temperature often
doubles the reaction rate.
Physical State of
Reactants
• In the solid state, atoms, ions, and molecules
are restricted in their motion.
• In the liquid and gaseous states the particles
have free motion and close proximity to each
other.
• Reactions tend to happen fastest in the liquid
state and slowest in the solid state.
Presence of a Catalyst
• A catalyst is a substance that increases the
reaction rate.
• The catalyst undergoes no net change.
• The catalyst creates an alternate chemical
pathway for the reactants that requires a
lower activation energy.
Equilibrium
• Equilibrium reactions are ones that do not go
to completion. Not all reactants have been
converted to products.
Equilibrium
LeChatelier’s Principle
• LeChatelier’s principle states that if a stress is
placed upon a system, the system will respond
by altering the equilibrium composition in such
a way as to minimize the stress.