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