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Level 144

Thermodynamics V

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Molar volume
What does Vm stand for?
Enthalpy change
Open - can exchange matter and energy with the surroundings.
What is an open system, a closed system and an isolated system?
What is the internal energy of a molecule?
The sum of all kinetic and potential energies for the molecule (U = U translation + U rotation + U vibration etc)
What is the equipartition principle?
For each degree of freedom we get ¹/₂ kT of energy (¹/₂ RT per mol).
What is the definition of enthalpy, in words and the equation?
The enthalpy of a substance is a measure of the total energy of a thermodynamic system. It includes internal energy, and pV - the energy needed to make room for the substance by displacing its surroundings.
H = ³/₂ RT + RT
What is the enthalpy for 1 mol of an ideal gas?
What is the first law of thermodynamics, in words and the equation?
In chemical changes, energy can be converted from one form to another, but not destroyed.
from system to surrounding?
q is positive and ΔU increases.
w is negative and ΔU decreases.
What happens to w if work is done by the system on the surroundings (eg gas expanding)?
How to calculate work for a gas expanding?
Work = force x distance = -pressure x volume change
Heating then expanding.
Heat and work will depend on the path taken. What are the two possible paths?
Constant temperature change
What is heat capacity?
The amount of heat needed to raise temperature of a substance by one degree.
In terms of a graph, what is heat capacity (C) equal to?
Heat capacity is equal to the slope of the graph of a graph of q against T.
With an ideal gas?
Ideal gas: U = 1.5RT
ΔU = Cv ΔT
ΔH in terms of heat capacity?
What does ΔH > 0 mean?
ΔfH⁰(298.15 K)?
Standard enthalpy of reaction at 298.15 K.
Define standard enthalpy of formation?
The enthalpy change when one mole of a compound in its standard state is made from its constituent elements in their standard states.
Hess's law of constant heat summation?
ΔH is independent of the path taken. Any path from reactant to product can be taken and the ΔH will the same.
Calculate the change in Cp.
How to use Kirchoff's law to calculate a change in enthalpy, with temperature?
State the second law of thermodynamics.
The entropy of a system and surroundings increases in the course of a spontaneous change.
What is ΔS(tot) for a reversible change?
ΔS(tot) = ΔS(system) + ΔS(surroundings) = 0
ΔS = Cp ln(T2/T1)
ΔS in relation to heat capacity?
How to represent ΔS on a graph?
ΔS in the area under a curve on a graph of Cp against Temp.
Ice to water:
How to work out ΔS at phase changes?
State Troutons rule?
ΔvapS = 85 J K⁻¹ mol⁻¹ for most liquids.
Isotherm so ΔU = 0.
ΔS for reversible expansion along an isotherm.
What is ΔS(tot) for an irreversible change?
ΔS(tot) = ΔS(system) + ΔS(surounding) > 0
How to calculate ΔS of an irreversible change?
Find a reversible pathway. The path doesn't matter.
ΔS(surroundings) = ΔH(T)/T
How to calculate ΔS(surroundings) of an irreversible change?
Does entropy increase or decrease when going from a solid to aqueous ions?
Would guess increase, but it decreases. This is because the ions are solvated, increasing the order of the system.
State the third law of thermodynamics?
A perfect crystal at 0 K as zero entropy.
qrev = TdS
Using entropy what can ΔU be rewritten as?
For a non-isolated system?
dS system > 0
Clausius inequality?
dS > dq/T for spontaneous irreversible change
What is free energy?
A measure of the capacity of a system to do work.
Helmholtz free energy?
G = H - TS
Negative - dG
What value must dG take for a spontaneous change?
dA ≤ 0
What condition does A have for a spontaneous change? (free energy)
Equation for K (equilibrium constant)?
[product 1] [product 2].../[reactant 1][reactant 2]...
Small K?
Products favoured.
What fundamental equation does this lead to?
dG = dU + pdV +Vdp - TdS - Sdt
Graph of G against T.
How to calculate S from a graph?
ΔG = nRT ln(p₂/p₁)
How to calculate the change in G with pressure, at constant temperature?
Use ΔG = nRT ln(p₂/p₁)
How to find out how ΔG will vary from standard state, at constant temp?
GA = GA⁰ + RTlnpA
How to link G to partial pressure?
ΔG = ΔG⁰ + RTlnK
How to associate G and K?
ΔG⁰ = - RTlnK
What key equation does this lead to?
K = a(products)/a(reactants)
An equation for K linking a?
ΔG⁰ = ?
ΔG⁰ = -RTlnK
How to work out S from a graph?
s is the slope of a graph of g against T.
For exothermic reactions?
Positive, as ΔH⁰ is positive.
Graph of lnK against T.
How to work out ΔH from a graph, if it is independent of T?
ΔG = 0
Therefore what does ΔS equal at phase transitions?
ΔfusS < ΔvapS
Which is smaller, ΔfusS or ΔvapS?
Describe the features of a pV phase diagram?
The shaded part is the two phase transition.
Top left - solid.
Describe the features of a pT phase diagram?
What is sublimation?
Transition directly from solid to gas.
Increase in pressure favours a higher melting point .
What does increase in pressure mean for the melting point, in the usual case?
What does a solid-liquid line leaning slightly left mean?
Solid has lower density than liquid - solid floats on the liquid.
(δp/δT) = ΔfusH/TΔfusV
What is Clapevron Equation? (links ΔfusH, ΔfusV)
One; p = 1
One; C = 1
F = C - p + 2
F is the degree of freedom that we can change independently, eg temperature, pressure, composition. What is the phase rule?
p = 2
What does this mean?
p = 1
What does this mean?
What are C, p and F in the liquid?
The 2 phase region consists of liquid and vapour in equilibrium.
Describe the graph of temperature against composition.
Tuc is at the top of the curve.