Fundamentals of Thermodynamics Flashcards
(21 cards)
What is the mean free path (λ) of a gas molecule?
The average distance a molecule travels before colliding with another.
Formula: λ = 1 / (√2 ρₙπd²)
What assumptions are made in the simple model of mean free path?
Molecules collide at a uniform rate; all have the same speed; no intermolecular forces; correction is needed for λₘ𝒻𝓅.
Why is gas density (ρ) low compared to liquids and solids?
Molecules are far apart, with minimal interaction, moving in random directions.
How do molecular speeds in liquids compare to those in gases?
Liquid molecules have lower average speeds than gas molecules due to stronger intermolecular interactions.
What defines a crystalline solid?
A 3D periodic atomic arrangement with high density and fixed atom positions.
Name the four classifications of solids with examples.
Molecular: Ar, CO₂ - no sharing of electrons
Ionic: NaCl - coulombic attraction of cations and anions as ionic bonds
Covalent: Si, carbon nanotubes - electrons shared in bonds between neighbouring atoms
Metallic: Au, Cu - delocalised electrons are shared between atoms
What distinguishes a closed system from an open system?
In a closed system, the number of particles (N) is constant; in an open system, it can change.
Define state variables and give examples for 1D, 2D, and 3D systems.
Properties defining system state at equilibrium:
1D: Force (ℱ), Length (L)
2D: Surface tension (𝒯), Area (A)
3D: Pressure (P), Volume (V)
What is hydrostatic pressure?
Pressure from a fluid acting uniformly on all surfaces; calculated as P = ρgh.
What happens at the triple point on a phase diagram?
Solid, liquid, and gas phases coexist in equilibrium.
What is the critical point on a phase diagram?
The point beyond which liquid and gas phases become indistinguishable.
Why doesn’t temperature change during a phase transition?
Energy is used to change the state (enthalpy of fusion/vaporisation), not raise the temperature.
What is enthalpy, and what are its units?
The energy required for a phase transition at constant pressure; units are J/kg or J/mol.
Equation for mean free path (corrected):
Formula: λ = 1 / (√2 ρₙπd²)
Hydrostatic/barometric pressure equations:
P = ρgh or P = ℱ/A
Surface tension on a Wilhelmy plate:
𝒯 = ℱ / (2w cos θ)
Convert Celsius to Kelvin:
K = °C + 273.15
Surface Tension equation against a plate
T = F/2w
Volume occupied by an atom
1 / p
Radius of an atom
r0 =√Vat
Relationship between temperature and surface tension
Higher temperature, weaker attractive forces, lower surface tension
