CHMB20 Chapter 2
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Chapter 2: The First Law
The Basic Concepts
•Thermodynamics: the study of the transformations of energy.
•System: the part of the world in which we have a special interest.
•Surroundings: the region outside the system and are where we make our
oOpen: if matter can be transferred through the boundary between the
system and its surroundings.
oClosed: if matter cannot pass through the boundary.
oBoth open and closed systems can exchange energy with their
oIsolated System: a closed system that has neither mechanical nor
thermal contact with its surroundings.
•Work: done to achieve motion against an opposing force; the transfer of
energy that makes use of organized motion.
•Energy: capacity to do work; when the capacity to do work is increased,
energy is increased but when a system does work, the energy of the system is
reduced and can do less work.
•Heat: the transfer of energy that makes use of disorderly molecular motion.
oDiathermic: boundaries that permit the transfer of energy as heat.
oAdiabatic: boundaries that don’t permit the transfer of energy as heat.
oExothermic Process: a process that releases energy as heat into its
oEndothermic Process: a process in which energy is acquired from its
surroundings as heat (vaporization of water).
Endothermic and diathermic container results in energy flowing
into the system as heat to restore the temperature to that of
Exothermic and diathermic container results in a release of
energy as heat into the surroundings.
Endothermic and adiabatic container results in a lowering of
temperature of the system.
Exothermic and adiabatic results in a rise of temperature of the
•Thermal Motion: disorderly motion of molecules in the surroundings
(movement of heat).
•Internal Energy (U): the total energy (kinetic and potential) of a system, a
o∆U = Uf – Ui
oState Function: its value depends only on the current state of the
system and is independent of how that state has been prepared.
oMolar Internal Energy (Um): the internal energy divided by the amount
of substance in a system, Um = U/n.
•Equipartition Theorem: used to estimate the contribution to the internal
energy of classical modes of motion; the average energy of each quadratic
contribution to the energy is ½kT.
oFor monatomic gas (translation only), Um(T) = Um(0) + (3/2)RT, where
Um(-) is the molar internal energy at T = 0.
oInternal energy of a perfect gas increases linearly with temperature.
oFor a linear molecule (translation and rotation only), Um(T) = Um(0) +
oFor a nonlinear molecule (translation and rotation only), Um(T) =
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Um(0) + 3RT.
oFor a gas consisting of 1 mol of nonlinear molecules to undergo the
same rise in temperature as 1 mol of monatomic gas, twice as much
energy must be supplied.
oThe internal energy of a perfect gas is independent of the volume it
•First Law of Thermodynamics: internal energy of an isolated system is
constant (q & w = 0).
oHeat and work are equivalent ways of changing a system’s internal
oChange in Internal Energy (∆U) = q + w
oAcquisitive Convention: w and q are positive if energy is transferred to
the system as work or heat and are negative if energy is lost from the
•Expansion Work: proportional to external pressure; work arising from a
change in volume.
oWork Done (dw) = -Fdz, where F is the magnitude of opposing force.
oExpansion Work Equation: dw = -pex(dV), where pex is external
pressure and dV is the change in volume.
•Free Expansion: expansion against zero opposing force; does no work (pex =
0, w = 0).
•The work of expansion against constant pressure is proportional to that
pressure and to the change in volume.
oExpansion of Work Against Constant External Pressure: w = -pex(∆V).
oIndicator Diagram: a p,V-graph used to illustrate expansion work.
•To achieve reversible expansion, the external pressure is matched at every
stage to the pressure of the system.
oReversible Change: a change that can be reversed by an infinitesimal
modification of a variable (thermal equilibrium of two systems with the
oSystems at equilibrium are poised to undergo reversible change.
oReversible Expansion Work: dw = -pex(dV) = -p(dV) because pex = p.
•Work of reversible, isothermal expansion of a perfect gas is a logarithmic
function of volume.
oReversible, Isothermal Expansion Work of a Perfect Gas: w =
oIf Vf > Vi, the ln equation is positive and w < 0.
oIf temperature increases, work done is greater.
oR = 8.3145 J/K(mol)
•Energy transferred as heat at constant volume is equal to the change in
internal energy of the system.
oHeat Transferred at Constant Volume: dU = dq, ∆U = qv, where qv
implies there is a change in heat at constant volume.
•Calorimetry: the measurement of heat transactions; study of heat transfer
during physical and chemical processes.
oCalorimeter: a device for measuring energy transferred as heat.
oAdiabatic Bomb Calorimeter: the most common device for measuring
oq = C∆T, where C is the calorimeter constant to convert ∆T to qv
oq = It∆Φ , where ∆Φ is the potential difference, I is a constant current
in amperes and t is time in seconds.
•Heat Capacity at Constant Volume: the slope of internal energy with respect
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