thermodynamics the universe is in a state of constant change, the only invariant is energy
TRANSCRIPT
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ThermodynamicsThermodynamics
The universe is in a state of constant change, the only invariant is Energy
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Consider ….Consider ….
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Gravity causes molecules of water move turbine blades
turbines move coils of wire in magnetic fields
moving magnetic fields move electrons
moving electrons drive chemical reactions in a battery
chemical reactions power your phone creating light and sound
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Consider ….Consider …. In this example gravity is responsible for a
working smartphone Gravity does work on the blades The turning blades do work on electrons Electrons do work in chemical reactions Chemical reactions do work on a speaker and power
LEDs in the screen, power radios etc
The ability of something to do work on something else is transferred from gravity to water to electrons to chemical reactions to moving magnetics and moving air and light from the screen
In this example gravity is responsible for a working smartphone Gravity does work on the blades The turning blades do work on electrons Electrons do work in chemical reactions Chemical reactions do work on a speaker and power
LEDs in the screen, power radios etc
The ability of something to do work on something else is transferred from gravity to water to electrons to chemical reactions to moving magnetics and moving air and light from the screen
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Consider ….Consider ….
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Gravity causes hydrogen atoms fuse to make Helium and a little bit of mass is converted into light and heat etc
Photons are absorbed by chlorophyll and used to power photosynthesis
We extract the oil
make biofuel
ignite the biofuel and excess energy released gives the molecules to power to move pistons
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Consider ….Consider …. In this example Gravity causes fusion reactions in
the sun are responsible for a working car Mass is converted to light light moves electrons Electrons do work in chemical reactions to create sugars
oils etc Oils react with oxygen to create fast moving CO2 and H2O Molecules push pistons and drive the car
The ability of something to do work on something else is transferred from the sun to chlorophyll to electrons to chemical reactions to moving molecules and moving pistons and moving wheels
In this example Gravity causes fusion reactions in the sun are responsible for a working car Mass is converted to light light moves electrons Electrons do work in chemical reactions to create sugars
oils etc Oils react with oxygen to create fast moving CO2 and H2O Molecules push pistons and drive the car
The ability of something to do work on something else is transferred from the sun to chlorophyll to electrons to chemical reactions to moving molecules and moving pistons and moving wheels
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Energy: the capacity to do work
Energy: the capacity to do work
In each step in the previous examples a capacity to do work is transferred from one thing to another, this is called energy
Gravitational energy is transferred into kinetic energy, into electrical energy, chemical energy, sound and light energy etc
All dynamic processes in the universe are due to the flow of energy
Thermodynamics is the study of heat flow and the laws that govern it
Since we want to understand chemical transformation we need to understand energy transformation
In each step in the previous examples a capacity to do work is transferred from one thing to another, this is called energy
Gravitational energy is transferred into kinetic energy, into electrical energy, chemical energy, sound and light energy etc
All dynamic processes in the universe are due to the flow of energy
Thermodynamics is the study of heat flow and the laws that govern it
Since we want to understand chemical transformation we need to understand energy transformation 6
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EnergyEnergy
Energy is a universal invariant It can change from one form to another but cannot be
created or destroyed
It is measured in Joules (J)
There is potential energy (energy that something has because of where and what it is) and kinetic energy (the energy is has because of how fast it is moving)
The lower the potential energy the more stable something is. Potential energy can be negative
When some process happens, generally it is to lower the potential energy
The study of energy helps us to predict whether a process is spontaneous or not
Energy is a universal invariant It can change from one form to another but cannot be
created or destroyed
It is measured in Joules (J)
There is potential energy (energy that something has because of where and what it is) and kinetic energy (the energy is has because of how fast it is moving)
The lower the potential energy the more stable something is. Potential energy can be negative
When some process happens, generally it is to lower the potential energy
The study of energy helps us to predict whether a process is spontaneous or not
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What is Thermodynamics?What is Thermodynamics?
Thermodynamics is a branch of physics concerned with energy flow. Historically it had an emphasis on heat, temperature and their relation to energy and work.
Study of energy changes accompanying chemical and physical changes to a system
Defines systems using a few macroscopic (measurable) variables, such as internal energy, entropy, temperature and pressure
Statistical treatment of microstates (atom positions and velocities) to obtain macrostates
In chemistry, thermodynamics predicts if reactions occur, how the equilibrium constant changes with temperature
Thermodynamics is a branch of physics concerned with energy flow. Historically it had an emphasis on heat, temperature and their relation to energy and work.
Study of energy changes accompanying chemical and physical changes to a system
Defines systems using a few macroscopic (measurable) variables, such as internal energy, entropy, temperature and pressure
Statistical treatment of microstates (atom positions and velocities) to obtain macrostates
In chemistry, thermodynamics predicts if reactions occur, how the equilibrium constant changes with temperature
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First Law of ThermodynamicsFirst Law of Thermodynamics
you can’t get something for nothing
First Law of Thermodynamics: Energy cannot be Created or Destroyed the total energy of the universe cannot change though you can transfer it from one place to another
ΔEuniv = 0 = ΔEsys + ΔEsurr (1)
you can’t get something for nothing
First Law of Thermodynamics: Energy cannot be Created or Destroyed the total energy of the universe cannot change though you can transfer it from one place to another
ΔEuniv = 0 = ΔEsys + ΔEsurr (1)
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First Law of ThermodynamicsConservation of Energy
For an exothermic reaction, “lost” heat from the system goes into the surroundings
two ways energy “lost” from a system, converted to heat, q used to do work, w
Energy conservation requires that the internal energy E change in the system equal the heat released (q) + work done (w)
ΔE = q + w (2)ΔE = ΔH + PΔV (3)
E is the total energy of everything in the system (the kinetic and potential energy of the atoms)
ΔE (ΔU)is a state function internal energy change independent of how this change
occurs
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The first law and time reversal
The first law and time reversal
The first law tells us that only processes where there is no net change in the total energy are allowed (energy is conserved)
The first law tells us that only processes where there is no net change in the total energy are allowed (energy is conserved)
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The first law and spontaneityThe first law and spontaneity
In all observed phenomena the total energy is always the same
The energy at t, E(t) is equal to the energy at time time t+dt,
E(t) = E(t+dt) So if that is the case why do we always see some
processes only going one way?
In all observed phenomena the total energy is always the same
The energy at t, E(t) is equal to the energy at time time t+dt,
E(t) = E(t+dt) So if that is the case why do we always see some
processes only going one way?
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✓
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The first law and spontaneityThe first law and spontaneity
Clearly the first law isn’t the end of the story regarding energy and what happens in processes
Clearly the first law isn’t the end of the story regarding energy and what happens in processes
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✓
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Factors Affecting Whether a Reaction Is Spontaneous
It turns out that there are two factors that determine the thermodynamic favorability are the enthalpy H and the entropy S.
The enthalpy is a comparison of the bond energy of the reactants to the products. bond energy = amount needed to break a bond. statistical model of collective behavior ΔH
The entropy factors relates to the randomness/orderliness of a system ΔS
The enthalpy factor is generally more important than the entropy factor
Let’s look at these
It turns out that there are two factors that determine the thermodynamic favorability are the enthalpy H and the entropy S.
The enthalpy is a comparison of the bond energy of the reactants to the products. bond energy = amount needed to break a bond. statistical model of collective behavior ΔH
The entropy factors relates to the randomness/orderliness of a system ΔS
The enthalpy factor is generally more important than the entropy factor
Let’s look at these
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Enthalpy related to the internal energy E, the energy change measured at
constant P is ΔH = ΔΔgenerally kJ/mol)
ΔHrxn is related to the breaking and forming of chemical bonds. Stronger bonds = more stable molecules
if products more stable than reactants, energy released exothermic ΔH = negative
if reactants more stable than products, energy absorbed endothermic ΔH = positive
The enthalpy is favorable for exothermic reactions and unfavorable for endothermic reactions.
Hess’ Law
related to the internal energy E, the energy change measured at constant P is ΔH = ΔΔgenerally kJ/mol)
ΔHrxn is related to the breaking and forming of chemical bonds. Stronger bonds = more stable molecules
if products more stable than reactants, energy released exothermic ΔH = negative
if reactants more stable than products, energy absorbed endothermic ΔH = positive
The enthalpy is favorable for exothermic reactions and unfavorable for endothermic reactions.
Hess’ Law
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Spontaneity: Enthalpy Driven ProcessesSpontaneity: Enthalpy Driven Processes
• In many cases, the direction of spontaneity can be determined by comparing the potential energy of the system at the start and the end
• Cellulose and O2 have a bigger potential energy than the equivalent amount of carbon dioxide and water
• The transformation lowers the overall potential energy, C-O and H-O bonds are more stable than C-C and C-H bonds
• exothermic reactions are spontaneous
• The extra energy leaves as heat
• All transformations have accompanying energy changes.
• Can we tell which transformations will occur spontaneously by studying the energy change?
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Spontaneity: Entropy Driven ProcessesSpontaneity: Entropy Driven Processes
• But some processes are spontaneous but not exothermic!
• These are entropy driven processes
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Entropy S
Entropy, S, is a thermodynamic function that increases as the number of equivalent ways of arranging the atoms/molecules (positions and velocities) in a system to give the appropriate V, U and T increases S generally J/(K.mol)
S = k ln W = Q/T (6)
k = Boltzmann Constant = 1.38 x 10-23 J/K W is the number of energetically equivalent ways
accessible, unitless (measure of our lack of knowledge about the system)
Entropy is the energy dispersal per unit temperature
Random systems require less energy than ordered systems
Measure of the unavailability of a system to do work
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WW
Energetically Equivalent States for the Expansion of a Gas
Energetically Equivalent States for the Expansion of a Gas
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Macrostates → MicrostatesMacrostates → Microstates
This macrostate can be achieved throughseveral different arrangements of the particles
This macrostate can be achieved throughseveral different arrangements of the particles
These microstates all have the same
macrostate
So there are 6 different particle arrangements that result in the same
macrostate
These microstates all have the same
macrostate
So there are 6 different particle arrangements that result in the same
macrostate
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Macrostates and ProbabilityMacrostates and Probability
There is only one possible arrangement that gives State A and one that gives
State B
There is only one possible arrangement that gives State A and one that gives
State B
There are 6 possible arrangements that give State C
There are 6 possible arrangements that give State C
Therefore State C has higher entropy than either State A or State B
Therefore State C has higher entropy than either State A or State B
The macrostate with the highest entropy also has the greatest dispersal of energyThe macrostate with the highest entropy also has the greatest dispersal of energy
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Changes in Entropy, ΔSChanges in Entropy, ΔS
entropy change is favorable when the result is a more random system. ΔS is positive
Some changes that increase the entropy are: reactions whose products are in a more disordered state.
(solid > liquid > gas) reactions which have larger numbers of product molecules
than reactant molecules. increase in temperature solids dissociating into ions upon dissolving
entropy change is favorable when the result is a more random system. ΔS is positive
Some changes that increase the entropy are: reactions whose products are in a more disordered state.
(solid > liquid > gas) reactions which have larger numbers of product molecules
than reactant molecules. increase in temperature solids dissociating into ions upon dissolving
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Increases in EntropyIncreases in Entropy
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The 2nd Law of Thermodynamics: Spontaneity
"Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so.”
The total entropy change of the universe must be positive for a process to be spontaneous for reversible process ΔSuniv = 0,
for irreversible (spontaneous) process ΔSuniv > 0
ΔSuniv = ΔSsys + ΔSsurr (7)
if the entropy of the system decreases, then the entropy of the surroundings must increase by a larger amount when ΔSsys is negative, ΔSsurr is positive
the increase in ΔSsurr often comes from the heat released in an exothermic reaction
"Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so.”
The total entropy change of the universe must be positive for a process to be spontaneous for reversible process ΔSuniv = 0,
for irreversible (spontaneous) process ΔSuniv > 0
ΔSuniv = ΔSsys + ΔSsurr (7)
if the entropy of the system decreases, then the entropy of the surroundings must increase by a larger amount when ΔSsys is negative, ΔSsurr is positive
the increase in ΔSsurr often comes from the heat released in an exothermic reaction
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Temperature Dependence of ΔSsurrTemperature Dependence of ΔSsurr
when a system process is exothermic, it adds heat to the surroundings, increasing the entropy of the surroundings
when a system process is endothermic, it takes heat from the surroundings, decreasing the entropy of the surroundings
the amount the entropy of the surroundings changes depends on the temperature it is at originally
the higher the original temperature, the less effect addition or removal of heat has
when a system process is exothermic, it adds heat to the surroundings, increasing the entropy of the surroundings
when a system process is endothermic, it takes heat from the surroundings, decreasing the entropy of the surroundings
the amount the entropy of the surroundings changes depends on the temperature it is at originally
the higher the original temperature, the less effect addition or removal of heat has
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Gibbs Free Energy, ΔG
For a spontaneous process ΔSuniv > 0
maximum amount of energy from the system available to do work on the surroundings at constant temperature T
(9)
when ΔG < 0, there is a decrease in free energy of the system that is released into the surroundings; therefore a process will be spontaneous when ΔG is negative
For a spontaneous process ΔSuniv > 0
maximum amount of energy from the system available to do work on the surroundings at constant temperature T
(9)
when ΔG < 0, there is a decrease in free energy of the system that is released into the surroundings; therefore a process will be spontaneous when ΔG is negative
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Thermodynamics and SpontaneityFree Energy
spontaneity is determined by comparing the free energy G of the system before the reaction with the free energy of the system after reaction, it includes both the enthalpy and entropy change of a process
ΔG = ΔH – T∙ΔS (9)
if the system after reaction has less free energy than before the reaction, the reaction is thermodynamically favorable
spontaneity ≠ fast or slow
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Gibbs Free Energy, ΔG
process will be spontaneous when ΔG is negative
ΔG will be negative when ΔH is negative and ΔS is positive
exothermic and more random ΔH is negative and large and ΔS is negative but small ΔH is positive but small and ΔS is positive and large
or high temperature
ΔG will be positive when ΔH is + and ΔS is − never spontaneous at any temperature
when ΔG = 0 the reaction is at equilibrium
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ΔG, ΔH, and ΔSΔG, ΔH, and ΔS
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Chemical Potential EnergyChemical Potential Energy
The chemical potential – is a form of free energy used for chemical reactions, in spontaneous reactions the chemical potential decreases
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Thermodynamics vs. KineticsThermodynamics vs. Kinetics
• Kinetics describes how fast things change• Thermodynamics is concerned if they will change and if so
what changes we will see in internal energy, temperature, pressure etc
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Example: Diamond → GraphiteExample: Diamond → Graphite
Graphite is more stable than diamond, so the conversion of diamond into graphite is spontaneous – but don’t worry, it’s so slow that your ring won’t turn into pencil lead in your lifetime (or through many of your generations).
Graphite is more stable than diamond, so the conversion of diamond into graphite is spontaneous – but don’t worry, it’s so slow that your ring won’t turn into pencil lead in your lifetime (or through many of your generations).
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Reversibility of ProcessReversibility of Process
any spontaneous process is irreversible it will proceed in only one direction
a reversible process will proceed back and forth between the two end conditions equilibrium results in no change in free energy
if a process is spontaneous in one direction, it must be nonspontaneous in the opposite direction
any spontaneous process is irreversible it will proceed in only one direction
a reversible process will proceed back and forth between the two end conditions equilibrium results in no change in free energy
if a process is spontaneous in one direction, it must be nonspontaneous in the opposite direction
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Entropy Change and State ChangeEntropy Change and State Change
Phase changes, melting boiling etc these are endothermic changes driven by entropy concerns not enthalpy concerns
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Entropy Change in State ChangeEntropy Change in State Change
when materials change state, the number of macrostates it can have changes as well for entropy: solid < liquid < gas because the degrees of freedom of motion
increases solid → liquid → gas
when materials change state, the number of macrostates it can have changes as well for entropy: solid < liquid < gas because the degrees of freedom of motion
increases solid → liquid → gas
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Heat Flow, Entropy, and the 2nd LawHeat Flow, Entropy, and the 2nd Law
Heat must flow from water to ice in order for the entropy of the universe to increase
But why that way round? The 1st law is not violated if more ice was formed?
Flowing hot to cold we increase energy randomization.
Heat flowing into the hot concentrated energy so S decreases
Heat must flow from water to ice in order for the entropy of the universe to increase
But why that way round? The 1st law is not violated if more ice was formed?
Flowing hot to cold we increase energy randomization.
Heat flowing into the hot concentrated energy so S decreases
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The reaction C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) has ΔHrxn = -2044 kJ at 25°C.
Calculate the entropy change of the surroundings.
The reaction C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) has ΔHrxn = -2044 kJ at 25°C.
Calculate the entropy change of the surroundings.
combustion is largely exothermic, so the entropy of the surrounding should increase significantly
ΔHsystem = -2044 kJ, T = 298 K
ΔSsurroundings, J/K
Check:
Solution:
Concept Plan:
Relationships:
Given:
Find:
ΔST, ΔH
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Free Energy Change and SpontaneityFree Energy Change and Spontaneity
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The reaction CCl4(g) C(s, graphite) + 2 Cl2(g) has ΔH = +95.7 kJ and ΔS = +142.2 J/K at 25°C. Calculate ΔG and determine if it is spontaneous.
The reaction CCl4(g) C(s, graphite) + 2 Cl2(g) has ΔH = +95.7 kJ and ΔS = +142.2 J/K at 25°C. Calculate ΔG and determine if it is spontaneous.
Since ΔG is +, the reaction is not spontaneous at this temperature. To make it spontaneous, we need to
increase the temperature.
ΔH = +95.7 kJ, ΔS = 142.2 J/K, T = 298 K
ΔG, kJ
Answer:
Solution:
Concept Plan:
Relationships:
Given:
Find:
ΔGT, ΔH, ΔS
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The reaction CCl4(g) C(s, graphite) + 2 Cl2(g) has ΔH = +95.7 kJ and ΔS = +142.2 J/K.
Calculate the minimum temperature it will be spontaneous.
The reaction CCl4(g) C(s, graphite) + 2 Cl2(g) has ΔH = +95.7 kJ and ΔS = +142.2 J/K.
Calculate the minimum temperature it will be spontaneous.
The temperature must be higher than 673K for the reaction to be spontaneous
ΔH = +95.7 kJ, ΔS = 142.2 J/K, ΔG < 0
Answer:
Solution:
Concept Plan:
Relationships:
Given:
Find:
TΔG, ΔH, ΔS
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The 3rd Law of ThermodynamicsAbsolute Entropy
The 3rd Law of ThermodynamicsAbsolute Entropy
the absolute entropy of a substance is the amount of energy it has due to dispersion of energy through its particles
the 3rd Law states that for a perfect crystal at absolute zero, the absolute entropy = 0 J/mol∙K
therefore, every substance that is not a perfect crystal at absolute zero has some energy from entropy
therefore, the absolute entropy of substances is always +
the absolute entropy of a substance is the amount of energy it has due to dispersion of energy through its particles
the 3rd Law states that for a perfect crystal at absolute zero, the absolute entropy = 0 J/mol∙K
therefore, every substance that is not a perfect crystal at absolute zero has some energy from entropy
therefore, the absolute entropy of substances is always +
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Standard EntropiesStandard Entropies
S°
Extensive (depends on the system size)
entropies for 1 mole at 298 K for a particular state, a particular allotrope, particular molecular complexity, a particular molar mass, and a particular degree of dissolution
S°
Extensive (depends on the system size)
entropies for 1 mole at 298 K for a particular state, a particular allotrope, particular molecular complexity, a particular molar mass, and a particular degree of dissolution
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Relative Standard EntropiesStates
Relative Standard EntropiesStates
the gas state has a larger entropy than the liquid state at a particular temperature
the liquid state has a larger entropy than the solid state at a particular temperature
the gas state has a larger entropy than the liquid state at a particular temperature
the liquid state has a larger entropy than the solid state at a particular temperature
SubstanceS°,
(J/mol∙K)
H2O (l) 70.0
H2O (g) 188.8
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Relative Standard EntropiesMolar Mass
Relative Standard EntropiesMolar Mass
the larger the molar mass, the larger the entropy
available energy states more closely spaced, allowing more dispersal of energy through the states
the larger the molar mass, the larger the entropy
available energy states more closely spaced, allowing more dispersal of energy through the states
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Relative Standard EntropiesAllotropes
Relative Standard EntropiesAllotropes
the less constrained the structure of an allotrope is, the larger its entropy
the less constrained the structure of an allotrope is, the larger its entropy
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Relative Standard EntropiesMolecular Complexity
Relative Standard EntropiesMolecular Complexity
larger, more complex molecules generally have larger entropy
more available energy states, allowing more dispersal of energy through the states
larger, more complex molecules generally have larger entropy
more available energy states, allowing more dispersal of energy through the states
SubstanceMolarMass
S°, (J/mol∙K)
Ar (g) 39.948 154.8
NO (g) 30.006 210.8
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Relative Standard EntropiesDissolution
Relative Standard EntropiesDissolution
dissolved solids generally have larger entropy
distributing particles throughout the mixture
dissolved solids generally have larger entropy
distributing particles throughout the mixture
SubstanceS°,
(J/mol∙K)
KClO3(s) 143.1
KClO3(aq) 265.7
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Calculate ΔS for the reaction4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(l)
Calculate ΔS for the reaction4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(l)
ΔS is +, as you would expect for a reaction with more gas product molecules than reactant molecules
standard entropies look up in appendix to textbook or googleΔS, J/K
Check:
Solution:
Concept Plan:
Relationships:
Given:Find:
ΔSSoNH3, So
O2, SoNO, So
H2O,
Substance S, J/mol/K
NH3(g) 192.8
O2(g) 205.2
NO(g) 210.8
H2O(g) 188.8
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Calculating ΔGoCalculating ΔGo
at 25oC:ΔGo
reaction = ΣnGof(products) - ΣnGo
f(reactants)
at temperatures other than 25oC:
assuming the change in ΔHoreaction and ΔSo
reaction is negligible
ΔGoreaction = ΔHo
reaction – TΔSoreaction
at 25oC:ΔGo
reaction = ΣnGof(products) - ΣnGo
f(reactants)
at temperatures other than 25oC:
assuming the change in ΔHoreaction and ΔSo
reaction is negligible
ΔGoreaction = ΔHo
reaction – TΔSoreaction
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Calculate ΔGo at 25oC for the reactionCH4(g) + 8 O2(g) CO2(g) + 2 H2O(g) + 4
O3(g)
Calculate ΔGo at 25oC for the reactionCH4(g) + 8 O2(g) CO2(g) + 2 H2O(g) + 4
O3(g)
standard free energies of formation from Appendix of textbook or google ΔGo, kJ
Solution:
Concept Plan:
Relationships:
Given:Find:
ΔGoΔGof of prod & react
Substance ΔGof, kJ/mol
CH4(g) -50.5
O2(g) 0.0
CO2(g) -394.4
H2O(g) -228.6
O3(g) 163.2
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The reaction SO2(g) + ½ O2(g) SO3(g) has ΔHo = -98.9 kJ and ΔSo = -94.0 J/K at 25°C.
Calculate ΔGo at 125oC and determine if it is spontaneous.
The reaction SO2(g) + ½ O2(g) SO3(g) has ΔHo = -98.9 kJ and ΔSo = -94.0 J/K at 25°C.
Calculate ΔGo at 125oC and determine if it is spontaneous.
Since ΔG is -, the reaction is spontaneous at this temperature, though less so than at 25oC
ΔHo = -98.9 kJ, ΔSo = -94.0 J/K, T = 398 K
ΔGo, kJ
Answer:
Solution:
Concept Plan:
Relationships:
Given:
Find:
ΔGoT, ΔHo, ΔSo
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ΔG RelationshipsΔG Relationships
if a reaction can be expressed as a series of reactions, the sum of the ΔG values of the individual reaction is the ΔG of the total reaction ΔG is a state function
if a reaction is reversed, the sign of its ΔG value reverses
if the amounts of materials is multiplied by a factor, the value of the ΔG is multiplied by the same factor the value of ΔG of a reaction is extensive
if a reaction can be expressed as a series of reactions, the sum of the ΔG values of the individual reaction is the ΔG of the total reaction ΔG is a state function
if a reaction is reversed, the sign of its ΔG value reverses
if the amounts of materials is multiplied by a factor, the value of the ΔG is multiplied by the same factor the value of ΔG of a reaction is extensive
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Free Energy and Reversible ReactionsFree Energy and Reversible Reactions
the change in free energy is a theoretical limit as to the amount of work that can be done
if the reaction achieves its theoretical limit, it is a reversible reaction
the change in free energy is a theoretical limit as to the amount of work that can be done
if the reaction achieves its theoretical limit, it is a reversible reaction
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Real ReactionsReal Reactions
in a real reaction, some of the free energy is “lost” as heat if not most
therefore, real reactions are irreversible
in a real reaction, some of the free energy is “lost” as heat if not most
therefore, real reactions are irreversible
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ΔG under Nonstandard ConditionsΔG under Nonstandard Conditions
ΔG = ΔGo only when the reactants and products are in their standard states
• there normal state at that temperature
• partial pressure of gas = 1 atm
• concentration = 1 M
under nonstandard conditions, ΔG = ΔGo + RTlnQ
• Q is the reaction quotient
at equilibrium ΔG = 0
• ΔGo = ─RTlnK
ΔG = ΔGo only when the reactants and products are in their standard states
• there normal state at that temperature
• partial pressure of gas = 1 atm
• concentration = 1 M
under nonstandard conditions, ΔG = ΔGo + RTlnQ
• Q is the reaction quotient
at equilibrium ΔG = 0
• ΔGo = ─RTlnK
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Example - ΔG
Calculate ΔG at 427°C for the reaction below if the PN2 = 33.0 atm, PH2= 99.0 atm, and PNH3= 2.0 atm
N2(g) + 3 H2(g) 2 NH3(g)
Q = PNH3
2
PN21 x PH2
3
(2.0 atm)2
(33.0 atm)1 (99.0)3= = 1.2 x 10-7
ΔG = ΔG° + RTlnQ
ΔG = +46400 J + (8.314 J/K)(700 K)(ln 1.2 x 10-7)
ΔG = -46300 J = -46 kJ
ΔH° = [ 2(-46.19)] - [0 +3( 0)] = -92.38 kJ = -92380 J
ΔS° = [2 (192.5)] - [(191.50) + 3(130.58)] = -198.2 J/K
ΔG° = -92380 J - (700 K)(-198.2 J/K)
ΔG° = +46400 J
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Example - K
Estimate the equilibrium constant and position of equilibrium for the following reaction at 427°C
N2(g) + 3 H2(g) 2 NH3(g)
ΔG° = -RT lnK
+46400 J = -(8.314 J/K)(700 K) lnK
lnK = -7.97
K = e-7.97 = 3.45 x 10-4
since K is << 1, the position of equilibrium favors reactantssince K is << 1, the position of equilibrium favors reactants
ΔH° = [ 2(-46.19)] - [0 +3( 0)] = -92.38 kJ = -92380 J
ΔS° = [2 (192.5)] - [(191.50) + 3(130.58)] = -198.2 J/K
ΔG° = -92380 J - (700 K)(-198.2 J/K)
ΔG° = +46400 J
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Temperature Dependence of KTemperature Dependence of K
for an exothermic reaction, increasing the temperature decreases the value of the equilibrium constant
for an endothermic reaction, increasing the temperature increases the value of the equilibrium constant
for an exothermic reaction, increasing the temperature decreases the value of the equilibrium constant
for an endothermic reaction, increasing the temperature increases the value of the equilibrium constant