CH1 - First Law of Thermodynamics: Conservation of Energy

Question 1

What is a system?

Answer 1

Designated boundary; comprising of any matter which we are interested in (any part of a physical universe)

Question 2

What are conditions for a system?

Answer 2

1. Must be separated from the rest of the universe with boundaries
2. Contains particles in continual motion and interacting with each other

Question 3

What is a closed system?

Answer 3

No matter enters or leaves (no movement of matter)

Question 4

What is the extended/complete 1st law in differential form? (Account for energy in the equation)

Answer 4

δQ + δW = dU + d(PE) + d(KE)

Question 4

What is an open system?

Answer 4

Matter may freely enter or leave

Question 5

What is a quasi-static process?

Answer 5

Also known as a reversible process.

It is a thermodynamic process that occurs so slowly (i.e. under an infinitesimally small driving force) that the system remains in internal equilibrium throughout the entire process.

Question 5

What are macroscopic properties?

Answer 5

Pressure, Temperature, Volume etc, affected by microstate

Question 6

Examples of microscopic properties?

Answer 6

Atomic velocity & position, Brownian motion etc

Question 7

What is the microstate of a system?

Answer 7

Defined by describing atomic/ molecular velocities and positions of the system at any point of time

Question 8

Thermodynamic properties/ variables characterising the macrostate?

Answer 8

Pressure
Temperature
Volume
No of moles
Energy (Kinetic/ Potential energy)

Question 9

If number of moles of various substances are known…?

Answer 9

ANY TWO of the thermodynamic variables is sufficient to specify the macrostate

Question 10

What are the conditions for equilibrium?

Answer 10

If a system is undisturbed for a period, it will settle down until no further change occurs macroscopically

Question 11

What are intensive properties?

Answer 11

Properties independent of the system size and amount of matter present

Question 12

What are extensive properties?

Answer 12

Properties dependent on amount of matter present

Question 12

What are macroscopic properties?

Answer 12

Macroscopic properties are things we can observe. (e.g. T, P, X, V, U)

Question 13

What is a microstate?

Answer 13

Microstate describes the instantaneous velocity/position of atoms/molecules.

Question 13

What is the ideal gas equation?

Answer 13

PV = nRT

Question 13

If heat is GAINED by a system, is heat (Q) positive or negative?

Answer 13

Positive

Question 14

If heat is LOST by a system, is heat (Q) positive or negative?

Answer 14

Negative

Question 15

If work is done BY a system, is work (W) positive or negative?

Answer 15

Negative

Question 16

If work is done ON a system, is work (W) positive or negative?

Answer 16

Positive

Question 17

What are state functions?

(Hint: U, H, and S are state functions)

Answer 17

Change depends only on initial and final states of the system.

Hence dU, dH, and dS represent exact differentials.

Question 18

Derive δW = PdV in isobaric conditions

Answer 18

W = PV
δW = PdV + VdP
In isobaric conditions, dP = 0.
δW = PdV

Question 19

Express U as a state function with state variables P and V.

Answer 19

U = U (P, V)

Question 20

Specific internal energy

Answer 20

U_ = U/m

Question 21

Specific volume

Answer 21

V_ = V/m = 1/ρ

Question 22

Molar volume

Answer 22

V_ = V/mol

Question 23

Derive H = U + PV from 1st law and ideal gas equation. Assume isobaric process.

Answer 23

δW = PdV
∴ ΔU = Q - P(ΔV)
Q = ΔU + PΔV
(U₂ + PV₂) - (U₁ + PV₁) = Qₚ

Since the expression (U + PV) contains only thermodynamic state variables, the expression itself must be a thermodynamic state variable. This is called the enthalpy of the system.

H = U + PV
∴ Qₚ = H₂ - H₁ = ΔH
δQₚ = dH

Question 24

Define what is a “steady state” condition

Answer 24

ΔU = 0

A state or condition of a system or process that does not change (or changes only slightly) over time.

Question 25

What is heat capacity?

Answer 25

The amount of heat to be supplied to an object to produce a unit change in its temperature.

In constant volume:
Cᵥ = (δQ/dT)ᵥ = (δQᵥ/dTᵥ) = (∂U/∂T)ᵥ

In constant pressure:
Cₚ = (δQ/dT)ₚ = (δQₚ/dTₚ) = (∂H/∂T)ₚ

Question 26

What is adiabatic condition?

Answer 26

No thermal energy can pass through the boundary.

Since no heat can flow in or out of the system, δQ = 0.

Question 27

Recall the 1st law for steady state open systems

Answer 27

Q + W = ΣHₒ - ΣHᵢ

When using specific enthalpy where H_ = H/m :
Q + W = ΣH_ₒM_ₒ - ΣHᵢMᵢ

Question 28

What is isenthalpic condition?

Answer 28

Constant enthalpy.

ΔH = 0

Question 29

What is the Joule-Thomson expansion?

Answer 29

ηⱼₜ = (∂T/∂P)ₕ

Question 30

Derive Cₚ - Cᵥ = R

Question 31

Values of Cₚ for a monoatomic and diatomic ideal gas?

Answer 31

Monoatomic: (5/2)R
Diatomic: (7/2)R

Question 32

Values of Cᵥ for a monoatomic and diatomic ideal gas?

Answer 32

Monoatomic: (3/2)R
Diatomic: (5/2)R

Question 33

What is Hess’s Law?