General Chemistry I Dr. PHAN TẠI HUÂN Faculty of Food Science and Technology Nong Lam University Module 4: Chemical Thermodynamics Zeroth Law of Thermodynamics. First Law of Thermodynamics (state quantities: internal energy, enthalpy, heat capacity and work; heat of combustion, bond energies). Second Law of Thermodynamics (state quantities: entropy, free energy; chemical potential, chemical equilibrium). 2 1
What is Thermodynamics? Thermodynamics can be defined as the science of energy. The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which is most descriptive of the early efforts to convert heat into power. In thermodynamics we study the energy changes that accompany physical and chemical processes. 3 Some thermodynamic terms A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The universe is the system plus its surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary. The boundary of a system can be fixed or movable. Systems may be considered to be closed or open. 4 2
A closed system A closed system (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary. But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed. If even energy is not allowed to cross the boundary, that system is called an isolated system. 5 An open system An open system (also known as a control volume) is a properly selected region in space. Both mass and energy can cross the boundary of a control volume. 6 3
Properties of a system Any characteristic of a system is called a property. Some familiar properties are pressure, temperature, volume, and mass, viscosity, thermal conductivity... Properties are considered to be either intensive or extensive. Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density. Extensive properties are those whose values depend on the size (or extent) of the system. Total mass, total volume, and total momentum are some examples of extensive properties. 7 State and equilibrium The word state refers to the condition of a system as described by its properties. Since there are normally relations among the properties of a system, the state often can be specified by providing the values of a subset of the properties. Thermodynamics deals with equilibrium states. A system in equilibrium experiences no changes when it is isolated from its surroundings. 8 4
Process, path and cycle Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a system passes during a process is called the path of the process. To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings. A thermodynamic cycle is a sequence of processes that begins and ends at the same state. 9 State functions The properties of a system, such as P, V, T, are called state functions. The value of a state function depends only on the state of the system and not on the way in which the system came to be in that state. A change in a state function describes a difference between the two states. It is independent of the process or pathway by which the change occurs. The most important use of state functions in thermodynamics is to describe changes. We describe the difference in any quantity, X, as: X = X final - X initial 10 5
The zeroth law of thermodynamics When an object is brought into contact with another object that is at a different temperature, heat is transferred from the object at higher temperature to the one at lower temperature until both objects attain the same temperature. The zeroth law of thermodynamics states that if two objects are in thermal equilibrium with a third object, they are also in thermal equilibrium with each other. 11 The first law of thermodynamics According to the first law of thermodynamics, The total amount of energy in the universe is constant. The Law of Conservation of Energy is just another statement of the First Law of Thermodynamics. Energy is neither created nor destroyed in ordinary chemical reactions and physical. Based on this law, we can say that the energy is just taken from one form and converted to another form. 12 6
Energy Energy is the capacity to do work or to transfer heat. We classify energy into two general types: kinetic and potential. Kinetic energy (KE) is the energy of motion. E kinetic = ½ mv 2 Potential energy (PE) is the energy that a system possesses by virtue of its position or composition. The total internal energy of a system (E) is the sum of its potential and kinetic energies. E total = KE + PE 13 Forms of energy 14 7
Forms of energy If we drop a rock, its potential energy is converted into kinetic energy as it falls, and it could do work on something it hits. 15 Forms of energy The chemical energy in a fuel or food comes from potential energy stored in atoms due to their arrangements in the molecules. This stored chemical energy can be released when compounds undergo chemical changes, such as those that occur in combustion and metabolism. 16 8
Forms of energy CH 4 (g) + 2O 2 (g) => CO 2 (g) + 2H 2 O(l) + 890 kj 17 Enthalpy and enthalpy changes Enthalpy (H) is defined as: H = E + PV Enthalpy is also a state function. It is not necessary to know the absolute enthalpy (heat content) of a system. However, we are interested in the change in enthalpy. The quantity of heat transferred into or out of a system as it undergoes a chemical or physical change at constant pressure, q p, is defined as the enthalpy change, H, of the process. H = H final -H initial H = H substances produced H substances consumed 18 9
Thermochemical equations We always interpret H as the enthalpy change for the reaction as written; that is, as (enthalpy change)/(mole of reaction), where the denominator means for the number of moles of each substance shown in the balanced equation. C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) + 1367 kj C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) H= -1367 kj/mol rxn (mole of reaction = abbreviate mol rxn. ) The negative sign indicates that this is an exothermic reaction (i.e., it gives off heat). 19 Calorimetry The energy change associated with a chemical or physical process can be determined by using an experimental technique called calorimetry. This technique is based on observing the temperature change when a system absorbs or releases energy in the form of heat. The experiment is carried out in a device called a calorimeter, in which the temperature change of a known amount of substance (often water) of known specific heat is measured. This heat capacity of a calorimeter is sometimes called its calorimeter constant. 20 10
Calorimeter Bomb calorimeter Coffee-cup calorimeter 21 Use electric heater to add 3.358 kj of heat to a calorimeter that contains 50g of water. The temperature of the water and the calorimeter, originally at 22.34 C, increases to 36.74 C. Calculate the heat capacity of the calorimeter in J/ C. The specific heat of water is 4.184 J/g C. Ans: Exercise 22 11
Exercise A 50 ml sample of 0.4 M copper(ii) sulfate solution at 23.35 C is mixed with 50 ml of 0.6 M sodium hydroxide solution, also at 23.35 C, in the coffee-cup calorimeter (previous exercise). After the reaction occurs, the temperature of the resulting mixture is measured to be 25.23 C. The density of the final solution is 1.02 g/ml. Calculate the amount of heat evolved. Assume that the specific heat of the solution is the same as that of pure water, 4.184 J/g C. CuSO 4 (aq) + 2NaOH(aq) Cu(OH) 2 (s) + Na 2 SO 4 (aq) Ans: 23 The thermodynamic standard state The thermodynamic standard state of a substance is its most stable pure form under standard pressure (1 atm) and at specific temperature (25 C or 298 K). Keep in mind the following conventions for thermochemical standard states. 1. For a pure substance in the liquid or solid phase, the standard state is the pure liquid or solid. 2. For a gas, the standard state is the gas at a pressure of one atmosphere; in a mixture of gases, its partial pressure must be one atmosphere. 3. For a substance in solution, the standard state refers to onemolar concentration. 24 12
Standard enthalpy changes The standard enthalpy change, H 0 axn, for reaction reactants products refers to the H when the specified number of moles of reactants, all at standard states, are converted completely to the specified number of moles of products, all at standard states. C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) H 0 rxn = - 1367 kj/mol rxn 25 Standard molar enthalpies of formation, H O f The standard molar enthalpy of formation, H f0, of a substance is the enthalpy change for the reaction in which one mole of the substance in a specified state is formed from its elements in their standard states. By convention, the H f0 value for any element in its standard state is zero. H 2 (g) + 1/2O 2 (g) H 2 O(l) H f0 = - 285.8 kj/mol rxn 26 13
Selected Standard Molar Enthalpies of Formation at 298 K 27 Exercise The standard molar enthalpy of formation of ethanol, C 2 H 5 OH(l), is - 277.7 kj/mol. Write the thermochemical equation for the reaction for which H 0 rxn = - 277.7 kj/mol rxn. Ans: 28 14
H 0 axn vs H f 0 The enthalpy change for a balanced equation that gives a compound from its elements does not necessarily give a molar enthalpy of formation for the compound. 29 Hess s law Many reactions are difficult to calculate with the standard enthalpies of formation or calorimetry. Enthalpy is a state function that means that the calculation is independent of the pathway or calculation. Hess s Law states that The enthalpy change for a reaction is the same whether it occurs by one step or by any series of steps. It means that an enthalpy change for a reaction can be calculated from the sum of two or more reactions. H 0 rxn = H 0 a+ H 0 b + H 0 c +... 30 15
Exercise Use the thermochemical equations shown here to determine H 0 axn at 25 C for the following reaction. Ans 31 Hess s law Another interpretation of Hess s Law lets us use tables of H f0 values to calculate the enthalpy change for a reaction. H 0 axn = n H 0 f products - n H 0 f reactants 32 16
Exercise Ans: 33 Bond energies The bond energy (B.E.) is the amount of energy necessary to break one mole of bonds in a gaseous covalent substance to form products in the gaseous state at constant temperature and pressure. 34 17
Bond energy and Hess s law A special case of Hess s Law involves the use of bond energies to estimate heats of reaction (the enthalpy change for a reaction). H 0 axn = B.E. reactants - B.E. products (only in gas phase) 35 Single bond energies (kj/mol of bonds) 36 18
Single and multiple bond energies (kj/mol of bonds) 37 Exercise Estimate the heat of reaction at 298 K for the following reaction: N 2 (g) + 3H 2 (g) 2NH 3 (g) Ans: 38 19
Change in internal energy, E The internal energy of a system can be changed by flow of work, heat or both. E = E final E initial = E products E reactants = q + w The terms q and w represent heat and work, respectively. Based on the above equation, these conventions are followed: q is positive: Heat is absorbed by the system from the surroundings. q is negative: Heat is released by the system to the surroundings. w is positive: Work is done on the system by the surroundings. w is negative: Work is done by the system on the surroundings. 39 Change in internal energy, E The only type of work involved in most chemical and physical changes is pressure volume work. From dimensional analysis we can see that the product of pressure and volume is work. 40 20
Change in internal energy, E When energy is released by a reacting system, E is negative; energy can be written as a product in the equation for the reaction. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) + 887 kj CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) E= - 887 kj When the system absorbs energy from the surroundings, E is positive; energy can be written as a reactant in the equation. CO 2 (g) + 2H 2 O(l) + 887 kj CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) CH 4 (g) + 2O 2 (g) E= + 887 kj 41 Change in internal energy, E E = E final E initial = E products E reactants = q + w When a gas is produced against constant external pressure, such as in an open vessel at atmospheric pressure, the gas does work as it expands against the pressure of the atmosphere. If no heat is absorbed during the expansion, the result is a decrease in the internal energy of the system. On the other hand, when a gas is consumed in a process, the atmosphere does work on the reacting system. The work done on or by a system depends on the external pressure and the volume. 42 21
Change in internal energy, E When the external pressure is constant during a change, the amount of work done is equal to this pressure times the change in volume. The work done on a system equals -P V or -P(V 2 -V 1 ). Compression (volume decreases) Work is done by the surroundings on the system, so the sign of w is positive. V 2 is less than V 1, so V = (V 2 -V 1 ) is negative. W = -P V is positive Can be due to a decrease in number of moles of gas ( n is negative). 43 Change in internal energy, E Expansion (volume increases) Work is done by the system on the surroundings, so the sign of w is negative V 2 is greater than V 1, so V = (V 2 -V 1 ) is positive. W = -P V is negative Can be due to an increase in number of moles of gas ( n is positive). In constant-volume reactions, no P V work is done. The change in internal energy of the system is just the amount of heat absorbed or released at constant volume, q v. E = q v 44 22
Ans: Change in internal energy, E Predict the sign of w and tell whether work is done on or by the system. a) 2NH 4 NO 3 (s) 2N 2 (g) + 4H 2 O(g) + O 2 (g) b) H 2 (g) + Cl 2 (g) 2HCl(g) c) 2SO 2 (g) + O 2 (g) 2SO 3 (g) 45 Exercise A 1 gram sample of ethanol was burned in a bomb calorimeter whose heat capacity had been determined to be 2.71 kj/ C. The temperature of 3000 grams of water rose from 24.284 C to 26.225 C. Determine E for the reaction of ethanol. The specific heat of water is 4.184 J/g C. The combustion reaction is C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) Ans: 46 23
Relationship between H and E The fundamental definition of enthalpy, H, is H = E + PV For a process at constant temperature and pressure: H = E + P V H = q + w + P V H = q + (-P V) + P V H = q p (constant T and P) For a reaction studied at constant pressure, the flow of heat is a measure of the change in enthhalpy for the system. Therefore, the terms heat of reaction and change in enthalpy are used interchangeably for reaction studied at constant pressure. 47 Relationship between H and E H = E + P V (1) The difference between E and H is the amount of expansion work (P V work) that the system can do. Unless there is a change in the number of moles of gas present, this difference is extremely small and can usually be neglected. For an ideal gas, At constant temperature and constant pressure, P V = ( n)rt. H = E + ( n)rt (2) The form (1) is very useful for physical changes that involve volume changes, such as expansion or compression of a gas. When a chemical reaction occurs and causes a change in the number of moles of gas, it is more convenient to use the relationship in the form (2). 48 24
Spontaneity of physical and chemical changes Reactants Products Which would be more stable at the given conditions the reactants or the products? A change for which the collection of products is thermodynamically more stable than the collection of reactants under the given conditions is said to be productfavored, or spontaneous, under those conditions. A change for which the products are thermodynamically less stable than the reactants under the given conditions is described as reactant-favored, or nonspontaneous, under those conditions. A spontaneous chemical reaction or physical change is one that can happen without any continuing outside influence. 49 Entropy, S The energy needed to cause a small temperature change in a sample of matter is a measure of the dispersal of matter, or disorder in that sample. This measure is a thermodynamic function called entropy, S. S = k B ln Ω where k B = Boltzmann s constant, the gas constant per molecule (R/NA) Ω = the number of microstates corresponding to a given state (including both position and energy). S = S final -S initial The greater the disorder of a system, the higher is its entropy. 50 25
Entropy, S 51 Entropy The entropy of a substance at any condition is its absolute entropy, also called standard molar entropy (often at 1atm, 298 K - S 0 298 or S 0 ). Standard molar entropy values are usually expressed in units of J/K.mol. The entropy of a pure, perfect crystalline substance (perfectly ordered) is zero at absolute zero (0 K) (The third law of thermodynamics). At 298 K, any substance is more disordered than if it were in a perfect crystalline state at absolute zero, so S 0 298 values for compounds and elements are always positive (unlike H f0 for element is zero). 52 26
Standard entropy change The standard entropy change, S 0, of a reaction can be determined from the absolute entropies of reactants and products. The relationship is analogous to Hess s Law. S 0 rxn = n S 0 products - n S 0 reactants Absolute entropies at 298 K for a few common substances 53 The second law of thermaldynamics In spontaneous changes, the universe tends toward a state of greater disorder (If the entropy of a system increases during a process, the spontaneity of the process is favored but not required.) The Second Law of Thermodynamics says that the entropy of the universe (not the system) increases during a spontaneous process. S universe = S system + S surroundings > 0 (spontaneous process) 54 27
Example Because the ideal gas samples (below) do not interact with the surroundings, so the entropy of the surroundings does not change. In this case S universe = S system. unmixed gases mixed gases S universe = S system > 0. mixed gases unmixed gases S universe = S system < 0. 55 Example 56 28
Exercise Calculate the entropy change at 25 C and 1 atm pressure for the reaction of hydrazine with hydrogen peroxide. Do you think the reaction is spontaneous? Ans: 57 Attention S universe is positive for all spontaneous (product-favored) processes. Unfortunately, it is not possible to make direct measurements of S universe. Consequently, entropy changes accompanying physical and chemical changes are reported in terms of S system. The symbol S refers to the change in enthalpy of the reacting system. Because changes in the thermodynamic quantity entropy may be understood in terms of changes in molecular disorder, we can often predict the sign of S system. 58 29
Predictable entropy changes for the system Phase changes: melting vaporization and sublimation take place with large increases in disorder, and hence with increases in entropy. For the reverse processes of freezing, condensation, and deposition, entropy decreases because order increases. Temperature changes: as any sample is warmed, the molecules undergo more (random) motion; hence entropy increases as temperature increases. Volume changes: when the volume of a sample of gas increases, the molecules can occupy more positions, and hence are more randomly arranged than when they are closer together in a smaller volume. 59 Predictable entropy changes for the system. Mixing of substances, even without chemical reaction. Situations in which the molecules are more mixed up are more disordered, and hence are at higher entropy. Increase in the number of particles: any process in which the number of particles increases results in an increase in entropy. Changes in the number of moles of gaseous substances: processes that result in an increase in the number of moles of gaseous substances have S system > 0. 60 30
Free energy change and spontaneity Free energy (G) is a thermodynamic function, related to spontaneity and is especially useful in dealing with the temperature dependence of spontaneity. It is defined as: G = H TS (Gibbs-Helmholtz equation) The Gibbs free energy change, G, at constant temperature and pressure, is G = H - T S (constant T and P) The amount by which the Gibbs free energy decreases is the maximum useful energy obtainable in the form of work from a given process at constant T and P. It is also the indicator of spontaneity of a reaction or physical change at constant T and P. If there is a net decrease of useful energy, G is negative and the process is spontaneous. 61 Free energy change and spontaneity G is positive Reaction is nonspontaneous (reactantfavored). G is zero System is at equilibrium. G is negative Reaction is spontaneous (product-favored). 62 31
The standard state for G 0 The standard state for G 0 is the same as for H 0-1 atm and the specified temperature, usually 25 C (298 K). The standard free energy of formation of a compound, G 0 f, is analogous to the enthalpy of formation, H 0 f. It is defined as the free energy change per mole when a compound is formed from the elements in their stable states at 1 atm. 63 The standard state for G 0 Values of G 0 f can be used to calculate free energy changes for reactions. G 0 rxn = ng 0 f products - ng 0 f reactants The value of G 0 rxn allows us to predict the spontaneity of a very special hypothetical reaction that we call the standard reaction. In the standard reaction, the numbers of moles of reactants shown in the balanced equation, all at standard conditions, are completely converted to the numbers of moles of products shown in the balanced equation, all at standard conditions. 64 32
Exercise Evaluate G 0 for the following reaction at 298 K. Is the standard reaction spontaneous? Ans: 65 Calculation of G 0 at other temperature To a good degree of approximation, the temperature variation of H 0 and S 0 can be neglected in the Gibbs- Helmholtz equation. G 0 = H 0 -T S 0 at temperature other than 25 C, only the value of T needs to be changed. The quantities of H 0 and S 0 can be calculated in the usual way. 66 33
Exercise Make the same determination as in the previous Exercise, using heats of formation and absolute entropies rather than free energies of formation. Ans: 67 Exercise Estimate the normal boiling point of bromine, Br 2. Assume that H and S do not change with temperature. Ans: 68 34
The temperature dependence of spontaneity 69 Examples 70 35
Exercise Estimate the temperature range for which the following standard reaction is product-favored. Ans: 71 Summary After you have finished studying this module 4, you should be able to Understand the terminology of thermodynamics, and the meaning of the signs of changes. Use the concept of state functions. Carry out calculations of calorimetry to determine changes in energy and enthalpy. Use Hess s Law to find the enthalpy change, H, for a reaction by combining thermochemical equations with known H values. Use Hess s Law to find the enthalpy change, H, for a reaction by using tabulated values of standard molar enthalpies of formation. 72 36
Summary Use the First Law of Thermodynamics to relate heat, work, and energy changes. Relate the work done on or by a system to changes in its volume. Use bond energies to estimate heats of reaction for gas phase reactions. Understand what is meant by a product-favored process; by a reactant-favored process. Understand the relationship of entropy to the order or disorder of a system. Understand how the spontaneity of a process is related to entropy changes the Second Law of Thermodynamics. 73 Summary Use tabulated values of absolute entropies to calculate the entropy change, S. Calculate changes in Gibbs free energy, G, (a) from values of H and S and (b) from tabulated values of standard molar free energies of formation. Use G to predict whether a process is product-favored at constant T and P. Understand how changes in temperature can affect the spontaneity of a process. Predict the temperature range of spontaneity of a chemical or physical process. 74 37