Chem 300 - Ch 19/#1 Today’s To Do List

Start Chapter 19: 1st Law• P-V work• State Functions• 1st Law• Adiabatic Processes

Thermodynamics

Based on 3 fundamental laws• Natural laws• Summaries of experimental facts• No known exceptions

Macroscopic Concerned with change in a

system

System & Surroundings

System• Part of world we are looking at

Surroundings• Rest of the universe

1st Law of Thermo

Deals with:• Conservation of Energy• Changes in energy of a system• expressible in terms of work and heat

work & heat

Methods of energy transfer between a system and its surroundings:

Heat:• due to a temperature difference

Work:• due to unbalanced forces

Heat Transfer

Surroundings ---->>> System• Positive quantity

• Tsurr > Tsys

• System --->>> Surroundings• Negative quantity

• Tsurr < Tsys

• Example: “hot” coffee cup, “cool” surroundings…• heat flow: cup-->surroundings

Work is a Process: (a) by the system, (b) on the system

PV Work

Consider a gas in a container (system)

apply an external force (in surroundings) to compress the gas

work (w) = force x displacement pressure (P) = force/area

PV work

w = -PextdV

at constant Pext

• w = - Pext (Vfinal - Vinit)

• If compression, Vfinal < Vinit & w > 0

• If expansion, Vfinal > Vinit & w < 0

Work

depends upon the path PV work depends upon value of Pext

PV work: isothermal const-P compression at 2 different P’s

Reversible work: minimum amount for compression

Ideal Gas & PV Work

In general w = -PextdV for any reversible process

• P = f(V) in order to integrate for IG

• P = RT/Vm

w = -PextdV = -(RT/V)dV If T = const (isothermal)

Isothermal Reversible PV work for an IG

w = -RTdV/V = - RT ln(Vfin/Vinit)

Value of w depends on the path between Vinit & Vfin

2-Stage compression at constant-P

Energy

A property of the system A state function

• Path independent

1st Law

U = q + w• U is state function (path independent)• q & w not state functions (they are path

functions)• A system contains an amount of energy

(U) but no work or heat.• For a process where q is transferred & w is

done, the energy change for the system is U = q + w

3 paths to the same end-point

Adiabatic Process

Adiabatic process: q = 0• No heat transfer

Example: styrofoam cup

Energy & Ideal Gas

For IG, U only depends on T• U = f (T) (prove this later)

• Specifically: dU = Cv dT• C = heat capacity

U = Cv (Tf - Ti) For isothermal process, U for IG is

constant

3 paths to the same end-point for an IG

Next Time

Adiabatic Processes & T Enthalpy More on Heat Capacity Heats of Transition