PChem 371: Topic 3 ( 1st Law of Thermodynamics)

1st law of thermodynamics

the overall change in the energy of a system during a process is due to an exchange of heat and/or work with the surrounding

delta U= q+w

Law of conservation of energy

energy cannot be created or destroyed

The energy of a system can only be changed by the

transfer of heat or work between system and surroundings

At constant volume

work is equal to zero

change in energy (delta U) = heat exchange in the surroundings(q)

at constant pressure

delta U= q - Pdelta V

heat exhange (q) in conditions of constant pressure has a special name: enthalpy (delta H)

delta U= delta H - P(delta V)

The temperature of a gas decreases if it expands…

adiabatically

delta U and delta H are

state functions

q and w are

path functions

equation for monatomic ideal gases

(T2/T1)^(3/2) =V1/V2

The internal energy of a closed system can only be altered by

the flow of heat or work across the boundary the system and surroundings

Heat and Work are transitory, meaning that

it is not associated with initial or final states, it only appears during a change in state of the system and surroundings

q is positive(q>0)

when heat is absorbed—> energy increases. (heat is withdrawn from the surroundings and deposited into the system)

q is negative (q<0)

heat is withdrawn from the system and deposited into the surroundings

Only energy, and not work is associated with

the initial and final states of the system

Work is negative (w<0)

mass in the surroundings have been raised

Work is positive (w>0)

work done on the system —> energy increases (mass in the surroundings have been lowered)

positive q and w means

the internal energy increases

q and w are NOT

state functions (state functions are defined only by the current state of the system rather than the changes)

Energy of an isolated system

will remain constant (delta U is zero )

For any cyclical process (start and end in the same note)…

delta U is zero

for any adiabatic process

q will be 0

Two types of expansive work

reversible and irreversible

Reversible process

Infinitesimal (extremely small) change in the direction of the variable that drives the process (e.g. temperature) causes a reversal in the direction of the process

Reversible expansion

It’s slow, gradual and has internal equilibrium (Pext=P)

Irreversible process

Infinitesimal (extremely small) change of the variable that drives the process (e.g. temperature) does not change the direction of the process.

Irreversible expansion

fast and does not have internal equilibrium (Pext does not equal P)

Reversible work is always bigger than irreversible work, why?

In a reversible process entropy remains constant, and entropy is directly related to the dispersal of energy; therefore, minimizing entropy means the maximum amount of energy can be converted into work. On the other hand, in an irreversible process entropy increases.

A system that’s not in internal equilibrium

a gas that is expanding so rapidly that the exchange of energy between molecules through collisions is slow compared with the rate of expansion. (different regions of gas may have different values for density, pressure and temp)

Pext > Pgas

Volume decreases, compressed. w=-Pext(delta V)

since it’s shrinking, delta V is a negative number which is why we need the negative in front in the formula. Pressure should be in Pascals and Volume in m³

Pascals

kg/ms²

Pext< Pgas

gas expands

Pext is only infinitesimally different from P gas, a tiny change in Pext can change the direction of the volume change of the gas

reversible process

In reversible process, the volume changes infinitesimally small but it’s not zero. Which formula should we use?

wrev=- integral from v1 to v2 (PextdV)

Based on the last two questions. a small change in Pext can change the direction in volume, which implies Pext is not constant. What should we change Pext to in the previous formula?

Pgas (and if it’s behaving like an ideal gas we do nRT/V)

atm to Pa

times 101325 Pa/ 1.00 atm

recap: ice melts into water at what temperature?

0C

Lead, Silver, Copper, Iron have lower specific heat capacities than wood, water and polyethylene. What does this mean?

It doesn’t take much energy to cause the lower specific heat capacities materials to become hot than the higher values (would take a lot of energy)

this is why a copper soon get hotter way easier than a wooden spoon. This is also why water takes a long time to boil.