1. The 4-stroke Auto Engine
Intake
Compression
Ignition
Power
Exhaust
piston
q (neg) heat released
...heat warms engine, passengers and surroundings on cold days E
+ w
= q
w (neg) work done on surroundings
....moves car against frictional forces and gravity
Automotive engineers design engines that release as little heat as possible. Why?

2. State Functions and Your Checkbook Balance.
October
November
December
January
500
1000
$200 less than in October
Not saving any money!
What happens between October and January isn’t important.
You are $200 behind in your savings and that is what matters.

3. State Functions: E, P, V
State functions: the path or prior history doesn’t matter....
...What does matter is where the system begins and ends.
20.0 MJ Chemical Energy
12.0 Mega Joules (work)
20.0 Mega Joules (Heat)
8.0 Mega Joules (Heat)
Begin: 20 MJ
Fire: Heat
Engine: Heat & work
End: 0 MJ
End: 0 MJ

4. Enthalpy: How we measure energy changes in chemistry
E = q w
 E > 0 (i.e. positive ) system gains energy
Work done by system w = - PV
 E < 0 (i.e. negative) system loses energy
E = q PV
e.g. Explosion (boom!)
... System volume increases...
... V > 0
... Lowers the system’s energy.
Destructive V > 0
Constructive V > 0
...  Esystem < 0
Explosive’s E < 0
Fuel’s E < 0

5. Enthalpy: How we measure energy changes in chemistry
Need to measure E for chemical reactions...
...and this requires us to measure both q and pV
...measuring q is easy but...
... Measuring V is difficult.
Reformulate... and assume P is constant...
heat at constant pressure
Esystem = qP PV
qP = Esystem + PV
H = Esystem + PV