6.2 THE NATURE OF ENERGY
Energy: the capacity to do work.
Law of conservation of energy: Energy can neither be created nor destroyed.
6.3 THE FIRST LAW OF THERMODYNAMICS
Thermodynamics is the study of energy and its interconversions.
THE FIRST LAW OF THERMODYNAMICS: the energy of the universe
is constant; it cannot be created nor destroyed, only transferred.
Internal energy of a system (U): the sum of the kinetic and potential energies of all of
the particles that make up the system.
The change in internal energy is the sum of the heat transferred and the work done:
U = q + w
q: heat transferred
w: work done
Internal energy is a state function - it only depends on the current state of the system
and not on how it arrived at that state.
U (+): energy flowing into(-): energy flowing out of
the system the system
q (+): thermal energy (-): thermal energy leaving
entering the system the system
w (+): work done on the (-): work done by the
r = U products reactants
The first law of thermodynamics states that energy must be conserved. Therefore,
U sys -U surr
ENERGY FLOW PROCESS:
If the internal energy of reactants is greater than the internal energy of the
products, energy flows out of the system (U sys0, taking reactants as the system)
If the internal energy of reactants is less than the internal energy of the
products, energy flows into the system. (U sys0, taking reactants as the system).
6.4 QUANTIFYING HEAT AND WORK
The exchange of thermal energy between a system and its surroundings caused by a
Energy transfer can occur via the heat associated with a temperature change.
Temperature: the amount of thermal energy in a sample of matter.
Thermal energy is transferred from a hotter object to a colder object until thermal
equilibrium is reached (no net transfer of heat and the two objects are at the same
The amount of heat absorbed (q) is directly proportional to temperature change
The constant of proportionality between q and T is heat capacity (C): the
amount of heat required to change an objects temperature by 1 C.
Units of heat capacity are those of heat (J) divided by temperature (C)
C = T = C
AKA molar heat capacity - amount of heat required to raise the temperature of
1 mol of a substance by 1C
Heat is an extensive property - it depends on the amount of matter.
Specific heat capacity: the measure of an objects intrinsic capacity to absorb heat;
the amount of heat required to raise the temperature of 1 gram of a substance by 1 C.
Units: J g C -1 -1
Molar heat capacity and specific heat capacity are both intensive properties - depend
on the kind of substance being heated; do not depend on the amount of matter.
q = m C Ts
q: heat (J)
m: mass (g)
C : specific heat capacity (J g C )1 -1