Chemistry 130 Gibbs Free Energy Dr. John F. C. Turner 409 Buehler Hall

Slides:



Advertisements
Similar presentations
Energy and Chemical Change
Advertisements

Thermodynamics versus Statistical Mechanics
Dr. Baljeet Kaur Lecturer Chemistry Government Polytechnic College for Girls Patiala.
Thermodynamic relations for dielectrics in an electric field Section 10.
1. 2 Ludwig Boltzmann (1844 – 1906) who spent much of his life studying statistical mechanics died by his own hand. Paul Ehrenfest (1880 – 1933), carrying.
Chapter 19 Chemical Thermodynamics
The entropy, S, of a system quantifies the degree of disorder or randomness in the system; larger the number of arrangements available to the system, larger.
Copyright 1999, PRENTICE HALLChapter 191 Chemical Thermodynamics Chapter 19 David P. White University of North Carolina, Wilmington.
Quantum Mechanics Brighton College 3 rd March Everything you always wanted to know about Quantum Mechanics but were too embarrassed to ask John.
Chemical Thermodynamics © 2009, Prentice-Hall, Inc. Chapter 19 Chemical Thermodynamics Chemistry, The Central Science, 11th edition Theodore L. Brown;
Chapter 19 Chemical Thermodynamics
Energy Relationships in Chemical Reactions Chapter 6 Dr. Ramy Y. Morjan.
Heat Capacity Amount of energy required to raise the temperature of a substance by 1C (extensive property) For 1 mol of substance: molar heat capacity.
PTT 201/4 THERMODYNAMIC SEM 1 (2012/2013). Objectives Apply the second law of thermodynamics to processes. Define a new property called entropy to quantify.
Chapter 19 Chemical Thermodynamics Lecture Presentation John D. Bookstaver St. Charles Community College Cottleville, MO © 2012 Pearson Education, Inc.
Chapter 19 Chemical Thermodynamics John D. Bookstaver St. Charles Community College St. Peters, MO 2006, Prentice Hall, Inc. Modified by S.A. Green, 2006.
Spontaneity of Chemical and Physical Processes: The Second and Third Laws of Thermodynamics 1.
Thermodynamics Chapter 19 Brown-LeMay. I. Review of Concepts Thermodynamics – area dealing with energy and relationships First Law of Thermo – law of.
Spontaneity, Entropy, and Free Energy
Ch. 19: Chemical Thermodynamics (Thermochemistry II) Chemical thermodynamics is concerned with energy relationships in chemical reactions. - We consider.
Thermochemistry Study of energy transformations and transfers that accompany chemical and physical changes. Terminology System Surroundings Heat (q) transfer.
Chapter 20: Thermodynamics
Prentice Hall © 2003Chapter 19 Chapter 19 Chemical Thermodynamics CHEMISTRY The Central Science 9th Edition David P. White.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 AP Chem h/w , 17, 19, 23, 24, 26, 28, 30, 31.
Thermodynamics Chapter 19. First Law of Thermodynamics You will recall from Chapter 5 that energy cannot be created or destroyed. Therefore, the total.
A.P. Chemistry Spontaneity, Entropy, and Free Energy.
Energy Many ways to describe energy changes in thermodynamics Originally developed to describe changes in heat and ‘work’ (think a steam engine piston)
1 Entropy & Gibbs Free Energy Chapter The heat tax No matter what the process, heat always lost to surroundings No matter what the process, heat.
Chapter 17 Lecture © 2014 Pearson Education, Inc. Sherril Soman Grand Valley State University Lecture Presentation Chapter 17 Free Energy and Thermodynamics.
First Law of Thermodynamics – Basically the law of conservation of energy energy can be neither created nor destroyed i.e., the energy of the universe.
Thermodynamics Brown, LeMay Ch 19 AP Chemistry Monta Vista High School To properly view this presentation on the web, use the navigation arrows below and.
Chapter 19: Chemical Thermodynamics
Chapter 19 Chemical Thermodynamics Lecture Presentation John D. Bookstaver St. Charles Community College Cottleville, MO © 2012 Pearson Education, Inc.
Chemical Thermodynamics Chapter 17 Chemical Thermodynamics.
Prentice Hall © 2003Chapter 19 Chapter 19 Chemical Thermodynamics CHEMISTRY The Central Science 9th Edition.
AP Chapter 19.  Energy can not be created nor destroyed, only transferred between a system and the surroundings.  The energy in the universe is constant.
Thermodynamics Thermodynamics Thermodynamics Way to calculate if a reaction will occur Way to calculate if a reaction will occur Kinetics Kinetics Way.
Thermodynamics. study of energy changes that accompany physical and chemical processes. Thermochemistry is one component of thermodynamics which focuses.
Second law of thermodynamics. It is known from everyday life that nature does the most probable thing when nothing prevents that For example it rains.
CHE 116 No. 1 Chapter Nineteen Copyright © Tyna L. Meeks All Rights Reserved.
Chapter 19 Lecture presentation
 State Function (°)  Property with a specific value only influenced by a system’s present condition  Only dependent on the initial and final states,
Thermodynamics: Spontaneity, Entropy and Free Energy.
Chemical Thermodynamics © 2009, Prentice-Hall, Inc. Chapter 19 Chemical Thermodynamics Chemistry, The Central Science, 11th edition Theodore L. Brown;
The study of energy and the changes it undergoes.
Prentice Hall © 2003Chapter 19 Chapter 19 Chemical Thermodynamics CHEMISTRY The Central Science 9th Edition David P. White.
Chemical Thermodynamics  2009, Prentice-Hall, Inc. First Law of Thermodynamics You will recall that energy cannot be created nor destroyed. Therefore,
THEME: Theoretic bases of bioenergetics. LECTURE 6 ass. prof. Yeugenia B. Dmukhalska.
Chemical Thermodynamics Lecture 1. Chemical Thermodynamics Prepared by PhD Halina Falfushynska.
Chemical Thermodynamics First Law of Thermodynamics You will recall from earlier this year that energy cannot be created nor destroyed. Therefore, the.
Energy Changes in Chemical Reactions -- Chapter First Law of Thermodynamics (Conservation of energy)  E = q + w where, q = heat absorbed by system.
Spontaneous and Nonspontaneous Processes - Entropy.
Define internal energy, work, and heat. internal energy: Kinetic energy + potential energy Heat: energy that moves into or out of the system because of.
Chapter 6 Thermochemistry: pp The Nature of Energy Energy – Capacity to do work or produce heat. – 1 st Law of Thermodynamics: Energy can.
Chapter 19 Spontaneity, entropy and free energy (rev. 11/09/08)
Thermodynamics Thermodynamics Thermodynamics Way to calculate if a reaction will occur Way to calculate if a reaction will occur Kinetics Kinetics Way.
Chemical Thermodynamics Chapter 19 Chemical Thermodynamics 19.1 Spontaneous Processes 19.2 Entropy and the Second Law of Thermodynamics 19.3 The Molecular.
Chemistry 130 Chemical Equilibrium Dr. John F. C. Turner 409 Buehler Hall
Chapter 19 Chemical Thermodynamics
Chapter 19 Chemical Thermodynamics
Chapter 19 Chemical Thermodynamics
Chapter 19 Chemical Thermodynamics
Chapter 19 Chemical Thermodynamics
Chapter 19 Chemical Thermodynamics
Chapter 19 Chemical Thermodynamics
Chemistry: The Central Science
Presentation transcript:

Chemistry 130 Gibbs Free Energy Dr. John F. C. Turner 409 Buehler Hall

Chemistry 130 Equilibrium and energy So far in chemistry 130, and in Chemistry 120, we have described chemical reactions thermodynamically by using - the change in internal energy, U, which involves heat transferring in or out of the system only or - the change in enthalpy, H, which involves heat transfers in and out of the system as well as changes in work. U applies at constant volume, where as H applies at constant pressure.

Chemistry 130 Equilibrium and energy When chemical systems change, either physically through melting, evaporation, freezing or some other physical process variables (V, P, T) or chemically by reaction variables (n i ) they move to a point of equilibrium by either exothermic or endothermic processes. Characterizing the change as exothermic or endothermic does not tell us whether the change is spontaneous or not. Both endothermic and exothermic processes are seen to occur spontaneously.

Chemistry 130 Equilibrium and energy Our descriptions of reactions and other chemical changes are on the basis of exothermicity or endothermicity – whether is negative or positive is negative – exothermic is positive – endothermic As a description of changes in heat content and work, these are adequate but they do not describe whether a process is spontaneous or not. There are endothermic processes that are spontaneous – evaporation of water, the dissolution of ammonium chloride in water, the melting of ice and so on. We need a thermodynamic description of spontaneous processes in order to fully describe a chemical system

Chemistry 130 Equilibrium and energy A spontaneous process is one that takes place without any influence external to the system. The opposite of a spontaneous change is a non-spontaneous change – one where there must be an external influence to force the change. For any observed, spontaneous change, the reverse process is non- spontaneous.

Chemistry 130 Equilibrium and energy If we use energy as the sole criterion of spontaneous change, we are using effectively a mechanical analogy – systems move to the local minimum in energy as the point of equilibrium. In the example of a ball falling, we have one variable, position, in a field, the gravitational field of the earth, and there is no endothermic path. Potential energy is converted into kinetic energy in flight and then into heat and sound (q and w) at impact.

Chemistry 130 Equilibrium and energy In this case, minimization of the potential energy and conversion ultimately into heat defines the point of equilibrium and appears to be linked to the direction of spontaneous change. Early theories of chemical thermodynamics rested on the evolution of heat as the 'driving force' for a reaction, which fails.

Chemistry 130 Equilibrium and energy It is certainly true that many reactions that are spontaneous are accompanied by the evolution of heat: But some are not and are endothermic and yet are still spontaneous: So, in the 'mechanical' view of chemical change, in the case of ammonium chloride, we have a system that spontaneously moves to a higher state of energy. Physical changes such as melting and boiling are inherently endothermic and the same problem occurs.

Chemistry 130 Equilibrium and energy Both of these examples have an associated enthalpy change: Some processes do not result in a net change of energy in the system and are still spontaneous: In an insulated system that cannot exchange heat with the surroundings, heat will spontaneously move to equalize a temperature gradient between two bodies.

Chemistry 130 Equilibrium and energy Similarly, there are no forces between a particles of a perfect gas and the internal energy of the perfect gas is independent of volume. Yet a perfect gas will always expand to fill the volume available, with no net change in energy.

Chemistry 130 Equilibrium and energy So spontaneous changes can take place endothermically, exothermically or with no exchange of energy

Chemistry 130 Equilibrium and energy We need a description of spontaneous change that includes the direction of the change, a description of the point of equilibrium quantitatively Energy is not a good description – chemical changes are not mechanical.

Chemistry 130 Reversibility and irreversibility Thermodynamically, we define a reversible change as one that takes place within an infinitesimal step from the point of equilibrium If the point of equilibrium is defined by then irreversible changes can occur via Reversible changes occur infinitely slowly and maximize the amount of work that is possible from a system. They also do not occur in practice but are the theoretical limit for developing our description of spontaneous change and the point of equilibrium.

Chemistry 130 Reversibility and irreversibility In practice, the maximum amount of work is never achievable. Real changes are irreversible and the amount of work that can be extracted is always less than the maximum amount of work.

Chemistry 130 Equilibrium, energy and entropy The magnitude and sign of the change in enthalpy associated with a chemical or physical change does not reflect the spontaneity of the process. It is not a good measure. However, any description of a molecular system such as a mole of a perfect gas is inherently statistical: A mole contains particles and the number of ways that we can arrange a mole of particles is going to be of the order of There are therefore many (!) ways of describing a chemical system while conserving the observed macroscopic properties – internal energy, pressure, temperature etc.

Chemistry 130 Equilibrium, energy and entropy If we have a large number of ways of describing the inside of the system so that the outside stays the same, then we have a large number of ways of distributing the energy of the system amongst these different configurations. There are many equivalent ways of distributing the thermal energy of a system given a certain macroscopic energy. A change is spontaneous when the number of ways of distributing the energy increases. The point of equilibrium is when this number of ways is maximized.

Chemistry 130 Entropy The measure of the number of ways of distributing energy that we use to describe this is the entropy of the system. We need entropy to be a state function and we cannot use heat, q, to do this because heat is not a state function. Instead, we define the entropy, S, for a reversible change as where q rev is the reversible heat transferred and T is the thermodynamic temperature.

Chemistry 130 Gibbs Free Energy Dr. John F. C. Turner 409 Buehler Hall

Chemistry 130 Entropy Summary 1. A spontaneous change is one that takes place without any external action on the system and the reverse of a spontaneous change is non- spontaneous 3. Energy and the First Law of Thermodynamics does not predict the direction of spontaneous change 4. Spontaneous change occurs when the number of ways of distributing the energy associated with the change increases 5. We quantify this increase in the distribution of energy associated with a spontaneous change by the entropy of a system 6. Entropy is a state function and is defined by where q rev is the reversible heat change. 7. A reversible change is one that takes place infinitesimally close to the point of equilibrium

Chemistry 130 The Gibbs function In order to predict the direction of spontaneous change, we need to consider the total entropy change in the universe. We write this as from our definition of entropy. We know that the heat change in the system is equivalent to the opposite of the heat change in the surroundings: and we know, that for a system that can do work, q System =H

Chemistry 130 The Gibbs function Now we can write the change in the universe solely in terms of changes in the system. This is important because the system is the part of the universe that we know enough about for an accurate description in principle. The entropy then becomes We define where G is the Gibbs function.

Chemistry 130 The Gibbs function The Gibbs function is a disguised form of entropy and has the units of energy; it is not an energy term in the First Law sense (H, U etc) but is a measure of the change in entropy of the universe. For a spontaneous change, i.e. the change in the Gibbs function must be zero or less than zero for a spontaneous change. The Gibbs function allows us to define quantitatively the direction of spontaneous change in the universe – it allows us to determine what will take place amongst all the energetically possible changes allowed by the First Law

Chemistry 130 All conceivable chemical changes The Gibbs function The Gibbs function allows us to draw a 'map' of chemical change for the universe: All possible chemical changes allowed by the First Law All observed, spontaneous chemical changes allowed by the Second Law You are here

Chemistry 130 The Gibbs function We manipulate the Gibbs function and the entropy in the same way as we manipulate any other state function such as The reference state that we use is the same as the standard state for - the standard state of the pure material at 1 atm pressure. Under these conditions, we write where the delta is the usual 'Products – Reactants'. The units of the Gibbs function and the entropy are

Chemistry 130 The Gibbs function There are three criteria for the Gibbs function in terms of sign: The equilibrium position is defined by Note that this is the standard change in the Gibbs function

Chemistry 130 The Gibbs function This relationship, allows us to determine - the standard change in Gibbs function, given K eq or - the equilibrium constant, given the standard change in Gibbs function It also explains why the equilibrium constant is sensitive to temperature, pressure and the number of moles of species present as

Chemistry 130 The Gibbs function We can also calculate the variation of the equilibrium constant with temperature. As Then and at two different temperatures, Subtracting the two gives

Chemistry 130 The Gibbs function The van't Hoff equation shows the reason why the equilibrium constant is sensitive to temperature – it is based on the entropy of the system and is not simply a bookkeeping device. Le Chatelier's principle therefore reveals the thermodynamic nature of chemical equilibrium.

Chemistry 130 The Gibbs function For an equilibrium between a gas and a pure liquid, we can calculate how the pressure will change with temperature. As the equilibrium constant for is given by then

Chemistry 130 State functions reprise From 120 State functions are extrinsic functions of variables such as temperature and pressure that define the thermodynamic state of a system. The important state functions are: Internal EnergyUEntropy S EnthalpyHGibbs function G An extrinsic quantity is one that depends on the amount of material present, where as an intrinsic variable is one that does not depend on the quantity of material. Intrinsic variables include: density and temperature

Chemistry 130 State functions reprise The energy level spacing and the temperature control the number of particles in the individual state i. The sum simply represents all possible states. Any system above 0 K contains energy and can in principle do work with certain limitations. This energy is stored in modes inside the system; these modes can be translational, rotational, vibrational and electronic. The type of modes, the number of them and the energy separations between individual modes depends critically on the system concerned and the state of matter. The availability of the modes depends on the temperature and is governed by the Boltzmann distribution. E i is the energy of the particular mode and n i is the population that is present in that mode. N is the total number of particles in the system.

Chemistry 130 State functions reprise The energy level spacing and the temperature control the number of particles in the individual state i. The sum simply represents all possible states. E i = 10 E i = 100 E i = 1000 E i = 10000

Chemistry 130 State functions reprise Though we cannot hope to define the precise thermodynamic state of a system explicitly – the magnitude of Avogadro's number prevents this – we can measure straightforwardly changes in the state of the system by observing the heat and work involved in the chemical or physical change. We define, in a system that can do no work and is at constant volume, the internal energy U. U can change by the absorption or emission of heat with respect to the surroundings but in no other way. If we allow the system to do work, i.e. it is at constant pressure, then we must account for the work done as well as heat changes, and in this case Internal energy at constant pressure = q + w We term this 'work-corrected' internal energy H, the enthalpy.

Chemistry 130 State functions reprise As as state function, the manner in which the state is prepared is irrelevant. Only the initial and final states are important. The path between them is not important. Thermodynamics has no bearing on the speed of chemical change – this is the province of chemical kinetics – but only on the magnitude (1 st Law) and direction (2 nd Law) of chemical change. As the path is independent, we are free to choose any path that connects two state functions at will. Using this approach, we are able to calculate several quantities that are hard or difficult to measure, such as entropy or the Gibbs free energy, as well as predict and determine other thermodynamic quantities.

Chemistry 130 State functions reprise Hess' Law of summation reflects this path independence. If we choose two different paths from one thermodynamic state to another, then the sum of any state function along those paths must be equivalent. This allows us to write a Hess cycle for a chemical change that will include the quantity that we are interested in, as well as other values that we know.

Chemistry 130 State functions reprise Example: Determine the heat of reaction for the formation of carbon monoxide from graphite and oxygen: We have two paths to the same material and so we can construct a Hess cycle.

Chemistry 130 State functions reprise Example: Determine the heat of reaction for the formation of carbon monoxide from graphite and oxygen: Following the direction of the arrows and remembering that the direction of the arrow tracks with the sign of we can construct the correct equations for the cycle:

Chemistry 130 State functions reprise This method is identical to the rearrangement of the equations algebraically. The enthalpy of formation is the enthalpy for the formation of the material from the elements – we define the zero-point for enthalpy from the elements. In this case, the heat of reaction is given by the difference between the total heats of formation of all the products less the total heats of formations of the reactants:

Chemistry 130 State functions reprise We use exactly the same approach with the Gibbs function. In the case of the Gibbs function of formation, we write and treat the equations in the same manner. We do the same for any state function. The Gibbs function can be dissected into the entropy change of the system and the enthalpic contribution. Therefore, we can calculate the change in entropy separately and the enthalpy separately, and then combine the two at the temperature of interest.

Chemistry 130 State functions reprise Once we have the standard Gibbs function for a reaction, we can calculate the equilibrium constant directly: (note that this is true only for the standard change in Gibbs function). Thus from the Gibbs function, we can find the equilibrium constant and vice versa. The temperature dependence of the equilibrium constant also allows us to calculate the Gibbs function at T 2 given K 1 at T 1