Chapter 2: 2.3 Entropy and Spontaneous Change
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True or False: Enthalpy along cannot predict reaction spontaneity
True
Define: Spontaneous process
A process that occurs in the absence of any ongoing outside intervention
Why is the 1st Law of Thermodynamics insufficient?
Sometimes exothermic reactions will not occur spontaneously, and sometimes endothermic reactions will occur spontaneously
What do you have to consider in order to determine whether a process will occur spontaneously?
Entropy
Define: Entropy
A measure of the randomness of a system
What does a higher entropy mean?
There is a higher number of possible arrangements of a system
True or False: Mixing is a spontaneous process
True, as mixing increases the entropy of a system
True or False: Spontaneous reactions are fast
False, processes that occur spontaneously are not necessarily fast
State: Notation for Entropy and Entropy Change
S and ΔS
State the formula for: Entropy change
ΔS = S (final) - S (initial)
What does ΔS > 0 and ΔS < 0?
• ΔS > 0 corresponds to an increase in the number of possible arrangements • ΔS < 0 corresponds to a decrease
What is the effect on entropy when heat is added to the system?
Entropy increases with temperature
What processes increase entropy (temperature-wise)?
Vapourization, melting, sublimation (S gas > S liquid > S solid)
What is the 3rd Law of Thermodynamics?
The entropy of a perfect crystal solid is defined as zero at 0K
Entropy increases with temperature, which is measure of what?
Kinetic energy of molecular motion in the system (translational, vibrational, rotational)
True or False: At temperature of absolute zero, the substance has no motion
Not necessarily. Due to quantum effect known as zero-point vibrational energy, atoms are still vibrating (but no translational or rotational motion)
How do chemical reactions affect entropy?
Increasing the number of molecules in the gas-phase increases entropy, as can many other factors
State: 2nd Law of Thermodynamics
Every spontaneous process increases the entropy of the universe
State the formula for: 2nd Law of Thermodynamics
• ΔS univ > 0 • ΔS univ = ΔS sys + ΔS surr > 0
What notation and units are standard molar entropies typically reported in?
• S^o • J/mol K • Refers to standard states, where all gases have partial pressures of 1 atm and all species in solution have concentration of 1 M • Listed values usually at 25 degrees Celsius
State the formula for: Standard entropy change (ΔS^o)
ΔS^o = ΣS^o (products) - ΣS^o (reactants)