CHM 113 - 6.1 The Nature of Energy
• Energy is the capacity to do work (or to produce heat).
a. Work is a force acting over a distance (moving an object).
b. Heat is actually a form of energy.
(1) chemicals may store potential energy in their bonds that can be released as heat energy.
B. Law of Conservation of Energy
• Energy can be converted from one form to another, but cannot be created or destroyed.
a. Potential energy
(1) Energy due to position or composition
b. Kinetic energy
(1) energy due to the motion of an object
(2) KE=1/2mv 2
C. Heat and Temperature
• Temperature reflects random motion of particles in a substance.
• Temperature indicates the direction in which heat energy will flow.
• Heat is a measure of energy content.
• Heat is what is transferred during a temperature change.
D. State Functions
• Aproperty of a system that depends only on its present state.
• State functions do not depend on what has happened in the system, or what might happen in the system
in the future.
• State functions are independent of the pathway taken to get to that state.
E. Chemical Energy
• Exothermic reactions
a. Reactions that give off energy as they progress.
b. Some of the potential energy stored in the chemical bonds is converted to thermal energy
(random KE) through heat.
c. Products are generally more stable (stronger bonds) than reactants.
• Endothermic reactions
a. Reactions in which the energy is absorbed from the surroundings.
b. Energy flows into the system to increase the potential energy of the system.
c. Products are generally less stable (weaker bonds) than the reactants.
F. Thermodynamics • System energy
Δ E = q + w
a. q = heat
(1) q is positive in endothermic reactions
(2) q is negative in exothermic reactions
b. w = work
(1) w is negative if the system does work
(2) w is positive if work is done on the system
Work done by gases
W = -P ΔV
a. by a gas (through expansion)
(1) ΔV is positive
(2) w is negative
a. to a gas (by compression)
(1) ΔV is negative
(2) ΔW is positive
• In systems at constant pressure, where the only work is PV, the change in enthalpy is due only to energy
flow as heat (ΔH = heat of rxn)
• H = E + PV
ΔH = H -H
a. ΔH is negative for exothermic rxns
b. ΔH is