Predicting and Calculating Entropy15.3.115.3.215.3.3
The Laws of Thermodynamics The First Law
The total energy of the universe (which hates you) is constant.○ This is similar to the law of conservation of
energy.It can be written as
○ ΔEuniverse = ΔEsystem + ΔEsurroundings = 0Energy can only be transferred
○ The flow of heat is considered one such transfer
The Laws of Thermodynamics
The Second LawAll processes that occur spontaneously move in the
direction of an increase in entropy of the universe (system + surroundings).○ The probability of a state existing is known as its
entropy (S)○ In general terms the less order there is in a state, the
greater the probability of the state and the greater its entropy.
○ Therefore this term describes the disorder present in a system
ΔSuniverse = ΔSsystem + ΔSsurroundings > 0
Entropy as a form of energy
Entropy (S) is a term coined by Rudolph Clausius in the 19th century.
Clausius was convinced of the significance of the ratio of heat delivered and the temperature at which it is delivered, q
T
Entropy as a form of energy
Like total energy, E, and enthalpy, H, entropy is a state function.
Therefore, S = Sfinal Sinitial
Entropy as a form of energy
For a process occurring at constant temperature (an isothermal process like the melting of ice):
qrev = the heat that is transferred when the process is carried out reversibly at a constant temperature.T = temperature in Kelvin.
Linking S and H: Phase changes
A phase change is isothermal (no change in T).
Ent
ropy
syst
em
For water:Hfusion = 6 kJ/molHvap = 41 kJ/mol
If we do this reversibly: Ssurr = –Ssys
Q - What do you notice happening to S when T=O?
Third Law of ThermodynamicsThe entropy of a pure crystalline
substance at absolute zero is 0.
15.3.1- State and explain the factors that increase the entropy in a system. These rules can be used to determine
the net entropy change for a system.1. Entropy increases when the number of
molecules increases during a reaction.2. Entropy increases with an increase in
temperature.3. Entropy increases when a gas is formed from
a liquid or solid.4. Entropy increases when a liquid is formed
from a solid.
Is this system exhibiting an overall increase or decrease of entropy?
Is this system exhibiting an overall increase or decrease of entropy?
Is this system exhibiting an overall increase or decrease of entropy?
How about for the universe?
Entropy practice problem 1 Would the change in entropy (ΔS)
for the following process be increasing (+) or deceasing (-)? Why?
Br2 (l) Br2(g) Positive, because entropy
increases as you move from a low energy state to a higher one. (# of moles of gas increased)
15.3.2 - Predict whether the entropy change (∆S) for a given reaction or process is positive or negative.
Entropy practice problem 2 Would the change in entropy (ΔS)
for the following process be increasing (+) or deceasing (-)? Why?
Ag+ (aq) + Cl- (aq) AgCl (s) Negative, because entropy
decreases as you move from a high energy state to a lower one. (# of moles of solid increased)
Entropy practice problem 3 Would the change in entropy (ΔS)
for the following process be increasing (+) or deceasing (-)? Why?
2NO2 (g) N2O4 (g) Negative, because the number
of moles of gas decreased
Entropy practice problem 4 Would the change in entropy (ΔS) for
the following process be increasing (+) or deceasing (-)? Why?
2OH- (aq) + CO2 (g) H2O (l) + CO32- (aq)
Negative, because the number of moles of gas decreased (1 to none) as well as the number of moles of total products decreased compared to the reactants (3 to 2)
Entropy practice problem 5 Would the change in entropy (ΔS)
for the following process be increasing (+) or deceasing (-)? Why?
H2 (g) + Cl2 (g) 2 HCl (g) Negative, because the number
of moles of gas decreased (2 to 1)
15.3.3 - Calculate the standard entropy change for a reaction (ΔSo) using standard entropy values (So). ΔSo = the change in standard molar
entropy of an element This is the entropy associated with 1 mol of a
substance in its standard state. Values can be found in Table 11 of the IB
Data Booklet. ΔSo for any element in its
standard state is zero!!
Standard Entropies These are molar entropy
values of substances in their standard states.
Standard entropies tend to increase with increasing molar mass.
Note that the units are different than those used for standard enthalpies!
Standard EntropiesLarger and more complex molecules have greater entropies.
Entropy Changes
Entropy changes for a reaction can be calculated the same way we used for H:
Note for pure elements:
Entropy practice problem 6 ΔSo = ΣSo products - ΣSo reactants
ΔSo of H2O=188.7 J/mol K ΔSo of H2=131.0 J/mol K ΔSo of O2=205.0 J/mol K
Find the ΔSo for the reaction of Hydrogen and Oxygen Gasses to form Water.
H2(g) + ½ O2(g) H2O(g) Q – Should your answer be positive or
negative? A – Negative!
Answer ΔSo = ΣSo products - ΣSo reactants
ΔSo of H2O=188.7 J/mol K ΔSo of H2=131.0 J/mol K ΔSo of O2=205.0 J/mol K
H2(g) + ½ O2(g) H2O(g) 188.7 J/mol K – [131.0 + ½(205.0)] J/mol K = -44.8 J/mol K
HW You should now have finished your
calorimetry IA so….Do the evens for the section 15.3 Exercises
on page 156Due Wednesday
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