Exam 1: Heat, Energy, and Work

First Law of Thermodynamics

Energy is conserved; it only changes forms or transfers between system and surroundings

System vs. Surroundings

The system is the chemical reaction or substance being studied; the surroundings are everything else (water bath, beaker, air)

Internal Energy (DeltaU or DeltaE)

DeltaU=q+w

Pressure-volume work (w) equation

w=-PDeltaV (where DeltaV=Vfinal-Vinitial)

What does “P” stand for?

External Pressure: the constant pressure exerted by the surroundings on the system (or vice versa), usually measured in Pascals (Pa), atmosphere (atm), or kilopascals (kPa)

What does “DeltaV” stand for?

Change in volume: the difference between the final and initial volume (DeltaV=Vfinal-Vinital)

When does Heat (q) have a positive (+) sign?

Endothermic: system absorbs heat (surroundings feel cold)

When does Heat (q) have a negative (-) sign?

Exothermic: system releases heat (surroundings feel hot)

When does Work (w) have a positive (+) sign?

Work done ON the system (compression, DeltaV<0)

When does Work (w) have a negative (-) sign?

Work done BY the system (expansion, DeltaV>0)