Thermodynamics, chemical

Chemical reactions always move towards a dynamic equilibrium. The term dynamic expresses that a reactant A might still react to give a product B, but in an equilibrated system this conversion is exactly compensated by the reverse reaction 

thereby, important to note that the equilibrium composition of a reaction system only determines the maximum conversion and the respective maximum product yield(s), if we are not limited by the reaction time to reach this stage. For example, a piece of graphite in contact with oxygen is thermodynamically not stable with regard to the formation of CO2, but we know from experience that at temperatures below several hundred °C the reaction time needed to convert carbon with oxygen is almost infinite. Similar considerations are (luckily) true for the oxidation of organic substances such as hydrocarbons, which are also kinetically but not thermodynamically stable under moderate conditions, that is, they are metastable. 

In some cases the equilibrium lies close to pure products, for example, the combustion of fuels like methane or crude oil fractions to carbon dioxide and steam. The reaction goes virtually to completion and we consider the reaction as irreversible. Conversely, we may consider a reaction as virtually impossible if the equilibrium lies close to pure reactants. 

Many reactions are in between these two extreme cases. Thermodynamics then give a recipe to calculate the influence of reaction conditions on the tendency of a reaction to run in a particular direction, and to calculate and maximize the product yield(s) by proper choice of temperature and for gas-phase reactions also of the pressure. 

For a basic understanding of chemical thermodynamics, start with perfect gas equilibria (Section 4.2.1), (ideal) liquid-liquid systems (Section 4.2.3 until Example 4.2.5), and (ideal) gas-solid reactions [Section 4.2.4, ignoring Eq. (4.2.60)]. 

In Chapter 14 we learned that the rate of any chemical reaction is controlled largely by a factor related to energy, namely, the activation energy of the reaction, caa (Section 14.5) In general, the lower the activation energy, the faster a reaction proceeds. In Chapter 15 we saw that chemical equihbrium is reached when a given reaction and its reverse reaction occur at the same rate, ooo (Section 15.1) 

Because reaction rates are closely tied to energy, it is logical that equihbrium also depends in some way on energy. In this chapter we explore the connection between 

This overall DNA/protein structure, called the nucleosome, is the basic unit of chromosomes in fhe nuclei of our cells. These sfrucfures are highly ordered, yef also musf be unraveled in order for gene expression fofake place. Both packaging and unpackaging of DNA in the nucleosome involve changes in the energy of the system. 

1 SPONTANEOUS PROCESSES We see that changes that occur in nature have a directional character. They move spontaneously in one direction but not in the reverse direction. 

2 ENTROPY ANDTHE SECOND LAW OF THERMODYNAMICS We discuss entropy, a thermodynamic state function that is important in determining whether a process is spontaneous. The second law of thermodynamics teiis us that in any spontaneous process, the entropy of the universe (system pius surroundings) increases. 

Heat can be released or absorbed during a chemical reaction. This provides a powerful method for studying chemical equilibrium by means of chemical thermodynamics. Thermodynamics is based on a few fundamental postulates, called the first, second, and third laws of thermodynamics. We will discuss these laws first, and then return to the subject of chemical equilibrium. 

Heat balance calculations are usually carried out when developing new rotary kiln chemical processes or when improving old ones. No thermal process would work if too much heat is released or if there is a lack of sufficient thermal energy to drive the process, in other words, to maintain the reaction temperature. Heat balance can only be calculated with given mass balances as the boundary conditions, hence a quantitative description of the chemical processes on the basis of physical or chemical thermodynamics is required. While chemical thermodynamics establishes the feasibility of a particular reaction under certain reactor conditions, chemical kinetics determines the rate at which the reaction will proceed. Before we establish the global rotary kiln mass and energy balance, it is important to examine some fundamental concepts of thermodynamics that provide the pertinent definitions essential for the design of new rotary kiln bed processes. 

Chemical processes are normally carried out at constant pressure (iso-baric). Under isobaric conditions, the total enthalpy, or the total heat content of a chemical species consists of three thermodynamic properties (i) the heat of formation, (ii) sensible heat, and (iii) heat of transformation. When chemical elements react to form a compound. 

The names and symbols of the more generally used quantities given here are also recommended by IUPAP [4] and by ISO [5.e, i]. Additional information can be found in [l.d, j and 24] 

Example The partial molar volume of Na2S04 in aqueous solution may be denoted V(Na2S04, aq), in order to distinguish it from the volume of the solution K(Na2S04, aq). 

The equilibrium constant of dissolution of an electrolyte (describing the equilibrium between excess solid phase and solvated ions) is often called a solubility product, denoted Ksol or Ks (or KSoi or K as appropriate). In a similar way the equilibrium constant for an acid dissociation is often written Ka, for base hydrolysis Kb, and for water dissociation Kw. 

A more extensive description of this subject can be found in [24]. 

These symbols should be printed in roman (upright) type, without a full stop (period). 

Since the state of a system of fixed content is completely determined by the values of P and F, the temperature is unambiguously defined by them. Thus r is a function of P and V, written T = T(P, V), Similarly, when P and T are known, it is implied that V has a unique related value, written V = V Py T), Such relationships embody an equation of state in which one variable is a function of the two independent variables or, since the latter specify the state, a function of state. Thus, for a gas, the equation of state is py = a - -bP cP, where a, b,Cy, depend only on the empirical temperature. In terms of absolute temperature a has the value RTy where R is the gas constant for the quantity of gas considered. When a gas behaves ideally the other terms on the right are negligibly small. 

There are many important functions of state. Those of importance in chemistry determine the conditions of equilibrium in chemical systems and hence the equilibrium distribution of different reactants and products among the various phases. 

however, the system is not insulated from its surroundings, the change AU is not generally equal to w. If we write 

The principles introduced so far apply to any kind of system. The facts that there is a function U which depends only on the state, not on how the state is reached, and that conservation of energy can be expressed quite 

For the present purpose the most important heat of reaction refers to the formation of one mole of a chemical compound from its elements, in their most stable forms, at 25° C. This is the standard heat of formation, written AH j. In other instances the standard heat of reaction is denoted by zlH . Although relatively seldom directly measurable, it can be inferred from other reactions. If the heat content of one mole of a substance A is denoted by H and so on for B etc., the heat of reaction for 

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Hicks, C. P. Bibliography of Thermodynaunic Quantities for Binary Fluid Mixtures, "Chemical Thermodynamics", Vol. 2, Chap. 9, edited by M. L. McGlashan, Chemical Society, London, 1978. 

Fundamentais of thermodynamics. Appiications to phase transitions. Primariiy directed at physicists rather than chemists. Reid C E 1990 Chemical Thermodynamics (New York McGraw-Fliii)... 

McGlashan M L (ed) 1973 Specialist Periodical Reports, Chemical Thermodynamics yo 1 (London The Chemical Society)... 

Stull, D. R. Westrum, E. F. Sinke, G. C., 1969. The Chemical Thermodynamics of Organic Compounds. Wiley, New York. 

References D. D. Wagman, et ah, The NBS Tables of Chemical Thermodynamic Properties, in J. Phys. Chem. Ref. Data, 11 2,1982 M. W. Chase, et ah, JANAF Thermochemical Tables, 3rd ed., American Chemical Society and the American Institute of Physics, 1986 (supplements to JANAF appear in J. Phys. Chem. Ref. Data) Thermodynamic Research Center, TRC Thermodynamic Tables, Texas A M University, College Station, Texas I. Barin and O. Knacke, Thermochemical Properties of Inorganic Substances, Springer-Verlag, Berlin, 1973 J. B. Pedley, R. D. Naylor, and S. P. Kirby, Thermochemical Data of Organic Compounds, 2nd ed.. Chapman and Hall, London, 1986 V. Majer and V. Svoboda, Enthalpies of Vaporization of Organic Compounds, International Union of Pure and Applied Chemistry, Chemical Data Series No. 32, Blackwell, Oxford, 1985. 

By convention, species to the left of the arrows are called reactants, and those on the right side of the arrows are called products. As Berthollet discovered, writing a reaction in this fashion does not guarantee that the reaction of A and B to produce C and D is favorable. Depending on initial conditions, the reaction may move to the left, to the right, or be in a state of equilibrium. Understanding the factors that determine the final position of a reaction is one of the goals of chemical thermodynamics. 

A quantity of great importance in chemical thermodynamics is the Gibbs free energy G. The latter is defined in terms of enthalpy H as... 

F. D. Rossini and co-workers. Selected Values of Chemical Thermodynamic Troperties, U.S. Government Printing Office, Washington D.C., 1952. 

JANAE U.S. Department of Commerce National Institute of Standards and Technology chemical thermodynamic properties of inorganic substances and of organic substances containing only one or two carbon atoms... 

The scientific basis of extractive metallurgy is inorganic physical chemistry, mainly chemical thermodynamics and kinetics (see Thermodynamic properties). Metallurgical engineering reties on basic chemical engineering science, material and energy balances, and heat and mass transport. Metallurgical systems, however, are often complex. Scale-up from the bench to the commercial plant is more difficult than for other chemical processes. 

D. D. Wagman and co-workers. The NBS Tables of Chemical Thermodynamic Properties Selected Valuesfor Inorganic and and C Organic Substances in SI Units, in /. Phys. Chem. Ref. Data, 11, suppl. 2 (1982) M. W. Chase, Jr. and co-sso-rkers, JMNAF Thermochemical Tables, 3rd ed.. Part II, in /. Phys. 

F. L. Getting, M. H. Rand, and R. J. Ackermaim, ia F. L. Oettiag, ed.. The Chemical Thermodynamics of Actinide Elements and Compounds, Part 1, The Actinide Elements, SHlPDBj424j 1, IAEA, Vienna, Austria, 1976. 

CHETAH-The MSTM Chemical Thermodynamic and Energy Release Evaluation Program, ASTM Data Series Pubheation DS 51, American Society for Testing Materials, Philadelphia, 1974, original, updated. 

D. D. Wagman, W. H. Evans, V. B. Parker, 1. Halow, S. M. Bailey, and R. H. Schumm, Selected Values of Chemical Thermodynamic Properties, NBS Technical Note 270-3, National Bureau of Standards, U.S. Dept, of Commerce, Washington, D.C., 1968. 

The NBS Tables of Chemical Thermodynamic Properties," J. Phys. Chem. Ref. Data, ll(suppl. 2) (1982). 

D. R. StuU and co-workers. Chemical Thermodynamics of Hydrocarbon Compounds,]olm Wiley Sons, Inc., New York, 1969, p. 368. 

In an energy-conscious world, SI provides a direct relationship among mechanical, electric, chemical, thermodynamic, molecular, and solar forms of energy. AH power ratings are given in watts. 

I. M. Klot2, Chemical Thermodynamics, Benjamin Press, New York, 1967. 

G. Egloff, G. HuUa, and V. I. Komarewski, Isomerisation of Pure Hydrocarbons, Reinhold Publishing Corp., New York, 1942 D. R. Stuhl, E. E. Westrum, and G. C. Sinke, The Chemical Thermodynamics of Organic Compounds, ]ohn Wiley Sons, Inc., New York, 1969. 

Selected Values of Properties of Chemical Compounds., Manufacturing Chemists Association Research Project Tables, Chemical Thermodynamic Properties Center, Dept, of Chemistry, A M College of Texas, College Station, Tex. 

The values given in the following table for the heats and free energies of formation of inorganic compounds are derived from a) Bichowsky and Rossini, Thermochemistry of the Chemical Substances, Reinhold, New York, 1936 (h) Latimer, Oxidation States of the Elements and Their Potentials in Aqueous Solution, Prentice-Hall, New York, 1938 (c) the tables of the American Petroleum Institute Research Project 44 at the National Bureau of Standards and (d) the tables of Selected Values of Chemical Thermodynamic Properties of the National Bureau of Standards. The reader is referred to the preceding books and tables for additional details as to methods of calculation, standard states, and so on. 

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