Thermodynamics

Question 1

What is energy in science?

Answer 1

The ability to do work or transfer heat.

Question 2

What is potential energy?

Answer 2

Stored energy, e.g. in chemical bonds or due to position.

Question 3

What is kinetic energy?

Answer 3

Energy of motion, e.g. moving particles or electrons.

Question 4

Example of potential vs kinetic energy?

Answer 4

Water in a dam (potential), flowing water (kinetic).

Question 5

State the First Law of Thermodynamics.

Answer 5

Energy cannot be created or destroyed, only transformed or transferred.

Question 6

What is a ‘system’ in thermodynamics?

Answer 6

The part of the universe we are studying (e.g. reaction).

Question 7

What are ‘surroundings’ in thermodynamics?

Answer 7

Everything outside the system.

Question 8

Define open, closed, and isolated systems.

Answer 8

Open: mass + energy can move in/out
Closed: energy ( but not mass) can move in/out
Isolated: neither mass or energy can move in/out

Question 9

What is the difference between heat and temperature?

Answer 9

Heat is energy transfer due to temperature change; temperature is average kinetic energy.

Question 10

What is an extensive property?

Answer 10

a property that depends on the amount of matter or size of the system

i.e. increases or decreases when you add or remove material.

Question 11

What is an intensive property?

Answer 11

Property that changes independent of amount of matter (e.g. temperature, pressure).

Question 12

Define a state function.

Answer 12

A property that depends only on the initial and final state, not the path.

Question 13

Why are only changes in state functions important?

Answer 13

Because absolute values are often unknown; we care about Δ values.

Question 14

What is internal energy (U)?

Answer 14

The total energy of a system (kinetic + potential).

Question 15

What affects internal energy?

Answer 15

Heat (q) and work (w): ΔU = q + w.

Question 16

What is the sign convention for energy?

Answer 16

=q/w: energy into system; -q/w: energy out of system.

Question 17

What is work in thermodynamics?

Answer 17

Energy from motion against an opposing force.

Question 18

What is pressure–volume (PV) work?

Answer 18

Work done when volume changes against external pressure: w = -PΔV.

Question 19

How does PV work relate to gases?

Answer 19

Use ideal gas law: w = -ΔnRT.

Question 20

What is enthalpy (H)?

Answer 20

Internal energy + PV work: H = U + PV.

Question 21

How does enthalpy differ from internal energy?

Answer 21

Enthalpy includes pressure–volume work; internal energy does not.

Question 22

What is heat capacity (C)?

Answer 22

The amount of heat required to change temperature of a substance.

Question 23

What is specific heat (c)?

Answer 23

Heat needed to raise 1 g of a substance by 1°C.

Question 24

What is molar heat capacity?

Answer 24

Heat needed to raise 1 mol of a substance by 1°C.

Question 25

What is the formula for heat transfer?

Answer 25

q = mcΔT (mass × specific heat × temp change).

Question 26

How is heat calculated during phase changes?

Answer 26

q = n × ΔH_fus or ΔH_vap.

Question 27

What is standard state?

Answer 27

Most stable form of substance at 1 bar, 298 K.

Question 28

What is Hess's Law?

Answer 28

The total ΔH of a reaction = sum of ΔH of steps.

Question 29

What is ΔH_f°?

Answer 29

Standard enthalpy of formation: ΔH to form 1 mol of a compound from elements.

Question 30

What is the ΔH_f° of elements in standard state?

Answer 30

Zero.

Question 31

What is bond enthalpy?

Answer 31

Energy required to break a bond in 1 mol of gas-phase molecules.

Question 32

What is a spontaneous process?

Answer 32

A process that occurs without external input.

Question 33

What does entropy (S) measure?

Answer 33

Disorder or the number of microstates in a system.

Question 34

What does the Second Law of Thermodynamics state?

Answer 34

In any E transfer or chemical reaction, some E becomes less useful (often lost as heat), and the overall disorder (S) of the universe inc --> S always inc in a spontaneous process: ΔS_univ > 0.

Question 35

What is the Third Law of Thermodynamics?

Answer 35

The entropy of a perfect crystal is zero at 0 K.

Question 36

What is a reversible process?

Answer 36

A process that can be reversed by infinitesimally small changes, returning the system and surroundings to their original states.

Question 37

Why does heat flow spontaneously from hot to cold?

Answer 37

Because this increases the entropy of the universe.

Question 38

What is the difference between microstates and macrostates?

Answer 38

Microstates are specific arrangements; macrostates are overall distributions.

Question 39

What equation relates entropy to the number of microstates?

Answer 39

Boltzmann Equation: S = k_B ln(W). ## Footnote S: entropy k_B: Boltzmann constant W: # microstates

Question 40

How do you calculate ΔS_sys for a reversible process?

Answer 40

ΔS_sys = q_rev / T. ## Footnote ΔSₛᵧₛ: Change in entropy of the system qᵣₑᵥ: Amount of heat absorbed or released by the system in a reversible process T: Temperature (in Kelvin) at which the process occurs

Question 41

How do you calculate standard entropy change of a reaction?

Answer 41

ΔS⁰ = Σ S⁰(products) − Σ S⁰(reactants).

Question 42

How do you calculate ΔS_surr?

Answer 42

ΔS_surr = -ΔH_sys / T.

Question 43

How does entropy change with number of gas moles or phase changes?

Answer 43

More gas moles or gas phase increases entropy.

Question 44

What is standard molar entropy?

Answer 44

The entropy of 1 mol of a substance at 298 K and 1 atm.

Question 45

What is Gibbs Free Energy (G)?

Answer 45

A state function predicting spontaneity; G = H - T·S.

Question 46

What does ΔG < 0 indicate?

Answer 46

The process is spontaneous.

Question 47

Why does ice melt above 0°C and freeze below 0°C?

Answer 47

Because ΔG becomes negative above 0°C and positive below it.

Question 48

What does ΔG represent in practical terms?

Answer 48

The maximum useful work a system can do.

Question 49

How do you calculate ΔG using thermodynamic quantities?

Answer 49

ΔG = ΔH - TΔS or from tabulated ΔfG° values.

Question 50

Why is S⁰ not zero for elements in their standard states?

Answer 50

Because entropy only becomes zero at 0 K (Third Law).

Question 51

What does the Kirchhoff equation allow you to do?

Answer 51

Calculate ΔH at different temperatures.

Question 52

What is the key difference between thermodynamics and kinetics?

Answer 52

Thermo tells if a reaction will occur; kinetics tells how fast.

Question 53

Why might a spontaneous reaction be slow?

Answer 53

Because it may have a high activation energy, e.g. H2 + O2 → H2O.

Question 54

What does ΔS_univ = 0 mean?

Answer 54

The system is at equilibrium.

Question 55

What equation defines entropy as a state function?

Answer 55

ΔS = S(final) - S(initial).

Question 56

How does entropy change with temperature?

Answer 56

Entropy increases with temperature: S(T2) = S(T1) + Cp·ln(T2/T1).

Question 57

How is entropy involved in phase changes at equilibrium?

Answer 57

ΔS = ΔH / T for reversible phase changes.

Question 58

How do molecular complexity and flexibility affect entropy?

Answer 58

More complexity/flexibility increases entropy.

Question 59

How does Gibbs Free Energy derive from the Second Law?

Answer 59

ΔG = ΔH - TΔS comes from rearranging ΔS_univ = ΔS_sys + ΔS_surr.

Question 60

What is the sign of ΔG when ΔS_univ > 0?

Answer 60

Negative, indicating spontaneity.