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Lecture 11 CH131 Summer 1 2020 6/11/2020 12:12 PM
Copyright ยฉ 2020 Dan Dill [email protected] 1
Lecture 11 CH131 Summer 1Thursday, June 11, 2020
โข Standard enthalpy of formation of X, ฮf๐ปยฐ Xโข Example thermochemistry problems.
Begin ch13: Spontaneous Processesโข The essence of change: Blind chance and dumb luckโข Arrangements particles entropyโข Heat energy flow entropy change
Next lecture: Complete ch13
Standard enthalpy of formation, ฮfHยฐ, of XForm one mole of X โฆ
โฆ from the elements it contains, โฆโฆ each in their standard state.
The standard state of an element is its most stable form.
Copyright ยฉ 2020 Dan Dill [email protected] 11 CH131 Summer 1 2020
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ฮfHยฐ of glucose, C6H12O6(s)Form one mole of glucose โฆ
C6H12O6 ๐ โฆ from the elements it contains, โฆ
C, H, and Oโฆ each in their standard state,
C ๐ , H2 ๐ , and O2 ๐
Copyright ยฉ 2020 Dan Dill [email protected] 11 CH131 Summer 1 2020
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ฮfHยฐ of glucose, C6H12O6(s)The standard enthalpy of formation of glucose is defined as the enthalpy change when one mole of sugar is formed from its elements, each in their standard states.
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Lecture 11 CH131 Summer 1 2020 6/11/2020 12:12 PM
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ฮfHยฐ of glucose, C6H12O6(s)Task: Write down the balanced chemical equation whose enthalpy change is the standard enthalpy of formation of sugar, C6H12O6 ๐
The enthalpy change of the chemical reaction6 C ๐ 6 H2 ๐ 3 O2 ๐ C6H12O6 ๐
is the standard enthalpy of formation of sugar.
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Copyright ยฉ 2020 Dan Dill [email protected]
ฮfHยฐ of water, H2O(l)Task: Write down the balanced chemical equation whose enthalpy change is the standard enthalpy of formation of water, H2O ๐
The enthalpy change of the chemical reaction
H2 ๐ O2 ๐ H2O ๐
is the standard enthalpy of formation of water.
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Copyright ยฉ 2020 Dan Dill [email protected]
ฮfHยฐ of steam, H2O(g)Task: Write down the balanced chemical equation whose enthalpy change is the standard enthalpy of formation of steam, H2O ๐ .
The enthalpy change of the chemical reaction
H2 ๐ O2 ๐ H2O ๐
is the standard enthalpy of formation of steam.
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Copyright ยฉ 2020 Dan Dill [email protected]
ฮfHยฐ of water and steamIs the standard enthalpy of formation of water ๐กโ๐ ๐ ๐๐๐ as that of steam?
The enthalpy changes of the chemical reactions
H2 ๐ O2 ๐ H2O ๐
H2 ๐ O2 ๐ H2O ๐
are different, since โฆH2O ๐ H2O ๐ requires energy.
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Example thermochemistry problems
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Heat of combustionCalculate โ๐ปยฐ for the combustion of methane, CH ๐ .
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Heat of combustionCalculate โ๐ปยฐ for the combustion of glucose, C H O ๐ .
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Problem 7e & 8e: 12.39Lecture 11 CH131 Summer 1 2020
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Problem 7e & 8e: 12.43Lecture 11 CH131 Summer 1 2020
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Problem 7e & 8e: 12.47Lecture 11 CH131 Summer 1 2020
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Copyright ยฉ 2020 Dan Dill [email protected]
TP The enthalpy diagram shows changes associated with the reaction Na2 ๐ Br2 ๐ 2 NaBr ๐ .
The uppermost line corresponds to the species โฆ
1. 2 Na ๐ Br2 ๐2. 2 Na ๐ 2 Br ๐3. Na2 ๐ Br2 ๐
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Chapter 13: Spontaneous Processes
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Lecture 11 CH131 Summer 1 2020 6/11/2020 12:12 PM
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The essence of spontaneous changeWhy does a drop of ink in water disperse?
Why do salt water and fresh water mix?
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The essence of spontaneous changeโthings happen simply because they can happen
and because they are statisticallymost likely to happen.โ
Michael Munowitz, "Principles of Chemistry," W. W. Norton, 2000
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A gas fills its containerLecture 11 CH131 Summer 1 2020
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A gas fills its containerLecture 11 CH131 Summer 1 2020
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A gas fills its containerLecture 11 CH131 Summer 1 2020
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A gas fills its containerLecture 11 CH131 Summer 1 2020
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Copyright ยฉ 2020 Dan Dill [email protected]
A gas fills its container
Gas all on left of container
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A gas fills its container
Gas evenly distributed throughout container
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Pressure in a gas is uniformCopyright ยฉ 2020 Dan Dill [email protected] 11 CH131 Summer 1 2020
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moveable barrier
๐ proportional to ๐/๐ โlattice gasโ๐ higher on the right
Pressure in a gas is uniform
๐ proportional to ๐/๐ โlattice gasโ๐ the same on the left and right
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moveable barrier
Gases mix evenlyCopyright ยฉ 2020 Dan Dill [email protected] 11 CH131 Summer 1 2020
One gas on left, another on right
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permeable barrier
Gases mix evenlyCopyright ยฉ 2020 Dan Dill [email protected] 11 CH131 Summer 1 2020
Equal amounts throughout
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permeable barrier
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The essence of spontaneous changeThe essential similarity in all of these behaviorsโฆ
gas particles spontaneously spread uniformly thought their container, so that pressure is everywhere the same;an ink drop spontaneously disperses throughout water;gases spontaneously mix uniformly, so that their partial pressures are everywhere the same; โฆ
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The essence of spontaneous changeThe essential similarity in all of these behaviorsโฆ
is that all spontaneous change results in the possible arrangements ๐ of particles that is as large as it can be:
๐ โ ๐ ๐
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The essence of spontaneous changeSpontaneous change always results in the possible arrangements ๐ of particles that is as large as it can be.For example,
gas particles spontaneously spread uniformly thought their container, so that pressure is everywhere the same .
๐ 1
๐ !! !
84
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Copyright ยฉ 2020 Dan Dill [email protected]
The essence of spontaneous changeโthings happen simply because they can happen
and because they are statisticallymost likely to happen.โ
Michael Munowitz, "Principles of Chemistry," W. W. Norton, 2000
All spontaneous change results in the possible arrangements ๐ of particles that is as large as it can be:
๐ โ ๐ ๐
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Lecture 11 CH131 Summer 1 2020 6/11/2020 12:12 PM
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Arrangements โ entropy๐ ๐ ln ๐
๐B 1.4 10 23 J/K
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Copyright ยฉ 2020 Dan Dill [email protected]
S = kB ln(W )Why natural log?
So that doubling the size of the system double the value of the spontaneity measure:
๐ โถ ๐ ๐ ๐
๐ โ ๐ ln ๐ 2๐ ln ๐ 2๐, so โฆ
Boltzmannโs definition in terms of natural log makes ๐ extensive โฆS scales with size of system
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Copyright ยฉ 2020 Dan Dill [email protected]
The essence of spontaneous changeGas particles spontaneously spread uniformly thought their container, so that pressure is everywhere the same:
๐ 1
๐ !! !
84
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Copyright ยฉ 2020 Dan Dill [email protected]
TP What is the value of โ for ๐ 1 โ ๐ 84?
1. 12. 4.43. 834. 845. Something else
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Lecture 11 CH131 Summer 1 2020 6/11/2020 12:12 PM
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The essence of spontaneous changeGas particles spontaneously spread uniformly thought their container, so that pressure is everywhere the same:
๐ 1
๐ !! !
84
ฮ๐ ๐ โ ๐ ๐B ln ๐B ln ๐ 4.4 0
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Copyright ยฉ 2020 Dan Dill [email protected]
TP Boltzmannโs definition of entropy is proportional to the natural logarithm of the number of arrangements, ๐~ln ๐ . Could Boltzmannโs definition of entropy just as well be defined in terms of base-10 logarithm, ๐~ log ๐ ?
1. Yes2. No3. Not sure
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S = kB ln(W )The extensive property of entropy is true for any logarithm base.
For example, if base-10 logarithm were used, the only change needed would be the numerical value of the proportionality constant.
๐ ๐ ln ๐ ๐ 2.303 log ๐
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Copyright ยฉ 2020 Dan Dill [email protected]
Heat (energy) flow entropy changeAdding energy increases the units of energy in the system.
More units of energy mean more arrangements of energy ๐
This means that adding energy increases entropy, ๐ ๐ ln ๐ .
Does the entropy increase depend on how much energy is initially present?
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TP How does โ๐ for adding 10 J to 1 mol at 300 K
compare with โ๐ for adding 10 J to 1 mol at 600 K?
1. โ๐ โ๐2. โ๐ โ๐3. โ๐ โ๐
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Heat (energy) flow entropy changeฮ๐ is larger the less energy already present
ฮ๐ is larger the lower ๐ at which the energy is added.
โ๐~
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