Chem 14B - Thermodynamics Flashcards
If no other energy is transferring:
∆U = w
Formula for Volume change, no heat change:
w = P∆V or w = -P∆V (expansion, work)
If energy is transferred ONLY as heat:
∆U = q
Exothermic
Releases heat to surroundings, ∆H < 0
Endothermic
Absorbs heat from surroundings, ∆H > 0
If heat capacity is known:
q = C∆T; C = heat capacity
∆U = q + w
U = internal energy. This formula is always true.
At constant volume, heat transfer is interpreted as:
∆U;
q = ∆U;
U = total energy in a system
At constant pressure, heat transfer is interpreted as:
∆H;
q = ∆H;
H = tracks losses & gains of energy as expansion or compression work during heat at constant pressure
H = U + PV -> PV = nRT
∆H = ∆U + ∆n(gas)RT
Isothermal:
No change in temperature. ∆T = 0
∆U = 0
∆U = 0 = q + w; q = -w or -q = w
“Compresses reversibly”
Constant Pressure
“in a bomb calorimeter”
Constant Volume
Bomb Calorimeter Equation
q(cal) = C(cal)∆T
Equation for reversible, isothermal expansion/compression:
w = -nRT ln (v2/v1)
Equation for when system expands/compresses with constant external pressure:
w = -P∆V
Equation if you know mass, specific heat and temperature change:
q = mC(s)∆T
Equation if you know moles, specific heat, and temperature change:
q = nC(m)∆T
Equation for melting (fusion) point phase change:
q(fus) = n∆H(fus)
Equation for freezing point phase change:
q(freez) = -n∆H(fus) -> opposite of fusion
Equation for vaporization phase change:
q(vap) = n∆H(vap)
Equation for condensation phase change:
q(cond) = -n∆H(vap) -> opposite of vaporization
Molar Heat Capacity: Constant Volume C(v,m), Atom
3/2R
Molar Heat Capacity: Constant Volume C(v,m), Linear Molecules
5/2R
Molar Heat Capacity: Constant Volume C(v,m), Nonlinear Molecules
3R
Molar Heat Capacity: Constant Pressure C(p,m), Atom
5/2R
Molar Heat Capacity: Constant Pressure C(p,m), Linear Molecule
7/2R
Molar Heat Capacity: Constant Pressure C(p,m), Nonlinear Molecule
4R
Entropy; S
State function;
Measure of disorder - Positional or Thermal
Positional Disorder (entropy)
Disorder related to location of molecules; “will occupy empty space”
Thermal Disorder (entropy)
Disorder due to thermal motion of molecules
First Law of Thermodynamics
Change in internal energy of isolated system is zero
Second Law of Thermodynamics
Entropy of isolated system increases during spontaneous process
Equation for entropy change due to heating:
∆S = C ln (Tf/Ti)
Equation for entropy change due to isothermal expansion:
∆S = nR ln (Vf/Vi)
Equation for entropy change accompanying phase change (for 1 mole of substance):
∆S(vap) = q(vap)/T = ∆H(vap)/T
What type of disorder (entropy) occurs at absolute zero?
Positional disorder (residual disorder)
Third Law of Thermodynamics
Entropies of all perfect crystals approach zero as absolute temperature approaches zero.
Equation for Residual Entropy if given molecules:
S = k(b) ln W^n;
k(b) is Boltzmann’s Constant (1.381 x 10^-23 J per K^-1
Equation for Residual Entropy if given moles:
S = nR ln W
List from least to greatest entropy: Gas, Liquid, Solid
Solid, Liquid, Gas
Equation for Standard Reaction Entropy:
∆Sº = 𝚺nSº(m)(P) - 𝚺nSº(m)(R)
Is reaction entropy positive or negative when gas is consumed?
Negative
Is reaction entropy positive or negative when gas is created?
Positive
