PETE 315 Midterm 2

isothermal process

• constant temp

• internal energy and enthalpy = 0

• Q = -W

Isobaric process

• constant pressure

• Q=H

Isochoric

• constant volume

• Q=U

• W=0

Adiabatic Process

• no Q

• W = U

irreversable processes

• produces work-multiply by efficiency

• requires work- divide by efficiency

compressibility factor (Z)

• measure of the deviation of the real-gas molar value from its ideal-gas value

• ideal-gas state- V=V(ig), Z=1

• moderate temps Z < 1

• elevated temps Z > 1

Van Der Waals equation

sensible heat effect

• heat transfer with no phase transitions, chemical reactions, or composition

• temperature of the system changes

latent heats

• amount of heat absorbed or released during a phase change

• constant temp and pressure

• types: fusion, vaporization

Standard State

• if in standard state, known as standard heat of reaction

• state of a substance at specified pressure, composition, and physical condition, e.g., gas, liquid, or solid

• based on pressure of 1 bar(10⁵ Pa)

• can be calculated if the standard heats of formation at the same temp are known

formation reaction

• a reaction that produces a single compound from its constituent elements

• based on 1 mol of compound formed

heat of combustion

• chemical reaction where substance reacts with oxygen and releases heat

• usually forms carbon dioxide and water

Entropy

• intrinsic property

• reflects how energy is dispersed and spread out in a system

• transferred along with heat

• if you add heat, entropy increases (more randomness)

• removing heat (to make work as in steam engines), not all of the heat is used for the work

• in thermo: measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work

• “Tax paid whenever you try to convert heat into motion”

• high entropy: energy spread

• low entropy: concentrated, bundled up energy

• over time systems move towards a state of maximum entropy because entropy can only increase (remain in constant reversible process)

• ex: melting of ice, spreading perfume, hot object cooling

• heat flows from hot to cold

heat engines

• rely on high temperature source of heat and discard heat to the environment

• ex: internal combustion engine and steam power plant

• second law imposes restrictions on how much of the heat intake can be converted to work

• since engines operate in cycles, U=0, W= - (Q_H+ Q_C)

• energy that flows from the higher temp reservoir, must flow out as work or as heat transfer to the low temp reservoir

thermal efficiency of a heat engine

• ratio of the work produced to the hear supplied to the engine

Carnot engine

• heat engine that operates in a completely reversible manner