Chemical Thermodynamics Flashcards
Define Internal Energy
Total Capacity of a system to do work
Translational Motion (Equation)
Etrans = 1/2*mv^2
Rotational Motion (Equation)
Erot = 1/2*Iw^2, where w is the rotational frequency and I is moment of inertia
Equipartition Theorem
Etotal for each mode of motion per mole of molecules = (1/2)RT
Avg KE (Degree of Freedom for one molecule)
(1/2)kT, where k is the boltzmann constant
Translational contribution (Gases)
A gas molecule has translational contribution to internal energy (U) per mole = (3/2)RT
Rotational Contribution (Linear Molecule)
Linear molecules rotate about any two axis perpendicular to bonds therefore the rotational contribution = RT
Rotational Contribution (Non-linear molecule)
Molecules rotate about three axis therefore rotational contribution = (3/2)RT
First Law of Thermodynamics
The internal energy of an isolated system is constant meaning deltaU = 0
Work & Heat
w = work
q = heat
If w +ve work done on system and -ve if done by.
If q +ve heat supplied to system and -ve if released by.
Therefore: deltaU = w + q
Types of work
Expansion (Changes in Volume)
Non-Expansion (No change in Volume)
Irreversible Expansion (equation)
w = Pex*deltaV
Equation is negative if work is done by gas
Reversible expansion (equation)
Wrev = nRT*ln(Vf/Vi) where it is negative if work is done by gas.
At constant Temperature, area under Boyle’s curve
Calorimetry
Insulated container so that any change in heat by reaction is directly related to change in temperature of water.
As insulated, flow of head is directly proportional to internal energy change.
Delta U = -Ccal*deltaT
Negative as heat being released
Introducing Enthalpy with Internal Energy
H = U + pV
At constant pressure, Delta H = q
Delta H = H(Products)-H(Reactants)
Heat Capacity
1) Heating causes changes in Temperature
2) Different substances heated at T do not give same dT
Heat Capacity (Definition)
Heat Capacity, C, of object measures how much heat must be supplied to raise the temperature by a given amount.
Specific Heat Capacity (Cs)
Cs = q/m*dT
This relates to 1g of an object
Use g not Kg in this equation
Specific Heat Capacity (Cm)
Cm = q/n*dT
This relates to moles
General Heat Capacity (equation)
C = q/dT
Enthalpy Change (Definition)
dH is the heat transferred by a chemical reaction at constant pressure
Molar Enthalpy of Melting
dH which occurs when 1 mol of a solid melts to form a liquid
Molar Enthalpy of Vaporization
dH which occurs when 1 mol of liquid boils to form a gas
Open System and Constant Volume (Enthalpy Change Equation)
dH = dU + p*dV
Hess’s law
drH = Sum of all drH
drH = Sum(dfH(products)) - Sum(dfH(reactants)) drH = Sum(bonds broken) - Sum(bonds formed)
Kirchoff’s Law
Heat Capacity at const. pressure = drH[T2] = drH[T1] + dCp(T2 - T1)
Second Law of Thermodynamics
Spontaneous processes are those that increase the total entropy of the universe
Third Law of Thermodynamics
The entropy of a perfect crystal is zero at T=0K
Entropy
dS = dS(system) + dS(surroundings)
if dS > 0 spontaneous, if < 0 then non spontaneous and if =0 then process is at equilibrium
Entropy with dT (NOT GIVEN IN EXAM)
S[T2] = S[T1] +Cv*ln(T2/T1)
Phase Transition in terms of entropy
dS = dH / T
Crystal Disorder
Disorder due to possible rotations therefore residual entropy. Where, S = Kb*lnW, where W = 2Na for 2 molecules.
Where Kb is the boltzmann constant
