chapter 17: free energy and thermodynamics (again) free energy case 1: ∆h negative, ∆s positive...
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Chapter 17: Free Energy and
Thermodynamics (again)
Mrs. Brayfield
17.2: Spontaneous and Nonspontaneous
Processes
A spontaneous process is one that occurs without
outside intervention
For example, when you drop a book it falls to the floor
In chemistry, we want to know the chemical potential
that will predict the direction of a chemical system
Spontaneous vs Nonspontaneous
DO NOT confuse spontaneity of a reaction with its
speed
Thermodynamics is the direction and the extent to which a
chemical reaction will proceed
Kinetics is the speed – how fast the reaction takes place
Thermodynamic ≠ Kinetic
Spontaneous vs Nonspontaneous
The conversion of diamond to graphite is
thermodynamically spontaneous, but the process is VERY
slow – so diamonds won’t turn to graphite quickly
Also, the rate of a reaction can be increased with a catalyst, but
a nonspontaneous process still will not go with one
A nonspontaneous process is not impossible, it can
happen
You just need to supply enough energy to it
17.3: Entropy and the 2nd Law
Remember entropy back from chapter 6…
Entropy is a function that increases with the number of
energetically equivalent ways to arrange the components
of a system to achieve a particular state (aka disorder)
This is melting ice (endothermic) happens spontaneously…
As the ice melts, the molecules become more “disordered” (or they
have more energy states), so entropy has increased (∴ spontaneous)
Entropy
The definition of entropy can be mathematically defined:
𝑆 = 𝑘𝑙𝑛𝑊 [Ludwig Boltzmann]
Entropy, like enthalpy (H) is a state function:
∆𝑆 = 𝑆𝑓𝑖𝑛𝑎𝑙 − 𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙
A chemical system proceeds in the direction that
increases the entropy of the universe (or towards the
largest number of ways to arrange the components)
Entropy
How many ways can we arrange the following gas
molecules?
2nd Law of Thermodynamics
The second law of thermodynamics states:
For any spontaneous process, the entropy of the universe
increases (∆Suniv > 0)
Entropy Changes with States
Prediction of ∆S
In general, entropy increases (∆S > 0) for each of the
following:
Phase transition from solid to liquid
Phase transition from solid to gas
Phase transition from liquid to gas
An increase in the number of moles of gas during a chemical
reaction
Predicting
Predict the sign of ∆S for each of the following processes:
1. The boiling of water
2. I2(g) → I2(s)
3. CaCO3(s) → CaO s + CO2(g)
positive
positive
negative
Homework Problems: #2, 3, 6, 8, 10, 12
17.4: Heat Transfer and Changes in Entropy
If going from a liquid to a solid is spontaneous, why does
entropy decrease?
Well it doesn’t, even though you would think that.
Why not?
∆𝑆𝑢𝑛𝑖𝑣 = ∆𝑆𝑠𝑦𝑠 + ∆𝑆𝑠𝑢𝑟𝑟
So the entropy of the system CAN decrease, as long as the
entropy of the surroundings increases by a greater amount
So that the entropy of the universe still increases
Spontaneity of Water Freezing
If we look at water freezing at a low temperature:
But if we looked at a high temperature:
This is like giving a rich verses a poor man $1000
Entropy Changes
In general…
A process that emits heat into the surroundings (qsys negative)
increases the entropy of the surroundings (positive ∆Ssurr)
A process that absorbs heat from the surroundings (qsys
positive) decreases the entropy of the surroundings (negative
∆Ssurr)
The magnitude of the change in entropy of the surroundings is
proportional to the magnitude of qsys
We then get the relationship:
∆𝑆𝑠𝑢𝑟𝑟 =−∆𝐻𝑠𝑦𝑠
𝑇 (𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑃, 𝑇)
If at constant pressure qsys = ∆Hsys
Calculating Entropy Changes Example
Consider the following reaction:
2𝑁2 𝑔 + 𝑂2 𝑔 → 2𝑁2𝑂 𝑔 ∆𝐻𝑟𝑥𝑛 = +163.2𝑘𝐽
a. Calculate the entropy change in the surroundings
associated with this reaction occurring at 25˚C.
∆𝑆𝑠𝑢𝑟𝑟 =−∆𝐻
𝑇=−(163.2𝑘𝐽)
298𝐾= −548𝐽/𝐾
b. Determine the sign of the entropy change for the
system.
ΔSsys is negative (less moles on product side)
c. Determine the sign of the entropy change for the
universe. Will the reaction be spontaneous?
∆𝑆𝑢𝑛𝑖𝑣 = ∆𝑆𝑠𝑦𝑠 + ∆𝑆𝑠𝑢𝑟𝑟
∆𝑆𝑢𝑛𝑖𝑣 = − + −
Therefore ΔSuniv is negative -> process is not spontaneous
17.5: Gibbs Free Energy
Using the equations for this chapter, we can solve for a
new thermodynamic function: (see page 653)
∆𝐺 = ∆𝐻 − 𝑇∆𝑆
Where G is Gibbs free energy
If you know the sign of G (positive or negative), you can
tell if the process is spontaneous or not
If ∆G is negative, the process is spontaneous
If ∆G is positive, the process is nonspontaneous
Gibbs Free Energy
Gibbs Free Energy
Case 1: ∆H negative, ∆S positive
G will be negative at all temperatures (∴ spontaneous)
Case 2: ∆H positive, ∆S negative
G will be positive at all temperature (∴ nonspontaneous)
Gibbs Free Energy
Case 3: ∆H negative, ∆S negative
Case 4: ∆H negative, ∆S positive
Predicting Spontaneity Example
Consider the following reaction:
𝐶2𝐻4 𝑔 + 𝐻2 𝑔 → 𝐶2𝐻6 𝑔 ∆𝐻 = −137.5𝑘𝐽; ∆𝑆 = −120.5𝐽/𝐾
Calculate ∆G at 25˚C and determine whether the
reaction is spontaneous. Does ∆G become more negative
or more positive as the temperature increases?
∆𝐺 = ∆𝐻 − 𝑇∆𝑆 = −317 × 103𝐽 − 298𝐾 −120.5𝐽
𝐾
∆𝐺 = −101.6𝑘𝐽
Reaction is spontaneous. As temperature increase, G
becomes more positive
Homework Problems: #14, 15, 17, 22, 24, 26, 28, 32, 38
17.6: Entropy Changes in Reactions
To find the change in entropy for any reaction, use an
equation that is similar to one we’ve already seen:
∆𝑆𝑟𝑥𝑛° = ∆𝑆𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠
° + ∆𝑆𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠°
Just use the standard molar entropy values (pg. 657 and
appendix IIB)
The third law of thermodynamics states:
The entropy of a perfect crystal at absolute zero (0K) is zero.
Entropy Changes Example
Calculate ∆Srxn˚ for the following reaction:
2𝐻2𝑆 𝑔 + 3𝑂2 𝑔 → 2𝐻2𝑂 𝑔 + 2𝑆𝑂2(𝑔)
∆𝑆𝑟𝑥𝑛°= 𝑛𝑝𝑆
°(𝑝𝑟𝑜𝑑) − 𝑛𝑟𝑆° 𝑟𝑒𝑎𝑐𝑡
∆𝑆𝑟𝑥𝑛°= 2𝑚𝑜𝑙 188.8
𝐽
𝑚𝑜𝑙𝐾+ 2𝑚𝑜𝑙 248.2
𝐽
𝑚𝑜𝑙𝐾
− 2𝑚𝑜𝑙 205.8𝐽
𝑚𝑜𝑙𝐾+ 3𝑚𝑜𝑙 205.2
𝐽
𝑚𝑜𝑙𝐾
∆𝑆𝑟𝑥𝑛°= 874
𝐽
𝐾− 1027
𝐽
𝐾= −153.2
𝐽
𝐾
17.7: Free Energy Changes
Using all of the thermodynamics information (and tables),
we can calculate the free energy change of a reaction:
∆𝐺𝑟𝑥𝑛° = ∆𝐻𝑟𝑥𝑛
° − 𝑇∆𝑆𝑟𝑥𝑛°
Example:
For the following reaction:
𝑁𝑂 𝑔 +1
2𝑂2(𝑔) → 𝑁𝑂2(𝑔)
Compute ∆Grxn˚ at 25˚C and determine if the process is
spontaneous.
∆𝐻𝑟𝑥𝑛°= 33.2
𝑘𝐽𝑚𝑜𝑙 − 91.3
𝑘𝐽𝑚𝑜𝑙 +
1
20
∆𝐻𝑟𝑥𝑛°= −58.1𝑘𝐽
∆𝑆𝑟𝑥𝑛°= 240.1
𝐽𝑚𝑜𝑙𝐾
− 210.8𝐽𝑚𝑜𝑙𝐾 +
1
2205.2𝐽𝑚𝑜𝑙𝐾
∆𝑆𝑟𝑥𝑛°= −73.2
𝐽𝐾
∆𝐺𝑟𝑥𝑛°= ∆𝐻𝑟𝑥𝑛
° − 𝑇∆𝑆𝑟𝑥𝑛°
∆𝐺𝑟𝑥𝑛°= −58.1 × 103𝐽 − 298𝐾 −73.2
𝐽𝐾
= −36.3𝑘𝐽 ∴ 𝑟𝑥𝑛 𝑖𝑠 𝑠𝑝𝑜𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠
17.8: Free Energy Changes with
Nonstandard States
Just use the equation:
∆𝐺𝑟𝑥𝑛 = ∆𝐺𝑟𝑥𝑛° + 𝑅𝑇𝑙𝑛𝑄
Where Q is the reaction quotient (equilibrium)
T in K
R is gas constant
8.314 J/mol•K
Example
Consider the following reaction at 298K:
2𝐻2𝑆 𝑔 + 𝑆𝑂2 𝑔 → 2𝑆 𝑠 + 2𝐻2𝑂 𝑔 ∆𝐺𝑟𝑥𝑛
°= −102𝑘𝐽 Compute ∆Grxn under the following conditions:
𝑃𝐻2𝑆 = 2.00𝑎𝑡𝑚 𝑃𝑆𝑂
2= 1.50𝑎𝑡𝑚 𝑃𝐻
2𝑂 = 0.0100𝑎𝑡𝑚
Is the reaction more or less spontaneous under these conditions than under standard states?
𝑄 =𝑃𝐻2𝑂
2
𝑃𝐻2𝑆2𝑃𝑆𝑂2=(0.1)2
2 2(1.5)= 1.67 × 10−5
∆𝐺𝑟𝑥𝑛 = ∆𝐺° + 𝑅𝑇𝑙𝑛𝑄
= −102kJ + (8.314𝐽
𝑚𝑜𝑙𝐾)(298𝐾)(ln 1.67 × 10−5
= −129𝑘𝐽 𝑡ℎ𝑒 𝑟𝑥𝑛 𝑖𝑠 𝑚𝑜𝑟𝑒 𝑠𝑝𝑜𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠
17.9: Free Energy and Equilibrium
If we know that at equilibrium, ∆Grxn = 0 and Q = K,
∆𝐺𝑟𝑥𝑛° = −𝑅𝑇𝑙𝑛𝐾
When K < 1, lnK is negative, ∆Grxn˚ is positive
Reaction is spontaneous in reverse direction
When K > 1, lnK is positive, ∆Grxn˚ is negative
Reaction is spontaneous in forward direction
When K = 1, lnK is zero, ∆Grxn˚ is zero
Reaction is at equilibrium
Example
Compute ∆Grxn˚ at 298K for the following reaction:
𝐼2 𝑔 + 𝐶𝑙2 𝑔 ↔ 2𝐼𝐶𝑙 𝑔 𝐾𝑝 = 81.9
∆𝐺𝑟𝑥𝑛°= −𝑅𝑇𝑙𝑛𝐾
∆𝐺𝑟𝑥𝑛°= −(8.314
𝐽
𝑚𝑜𝑙𝐾)(298𝐾)(ln 81.9
= −10.9𝑘𝐽𝑚𝑜𝑙
Homework Problems: #40, 42, 46
Review: #50, 51, 53
Entropy:
http://www.youtube.com/watch?v=ZsY4WcQOrfk&index
=21&list=PL8dPuuaLjXtPHzzYuWy6fYEaX9mQQ8oGr
Electrochemistry:
http://www.youtube.com/watch?v=IV4IUsholjg&index=38
&list=PL8dPuuaLjXtPHzzYuWy6fYEaX9mQQ8oGr