Enthalpy symbol

The right-hand side of (3.48b) has the appearance of a difference in a state function (U + PV) between the two states 1, 2. This state function is identified as the enthalpy (symbolized by H), with fundamental definition... 

The energy of a substance under constant atmospheric pressure (such as in an open test tube or beaker) is called its enthalpy (symbol, H). The enthalpy of elements and compounds cannot be measured or calculated, but differences in enthalpy (enthalpy changes), symbolised AH, are easily measured in the laboratory. If a chemical or physical change is carried out at constant pressure, the amount of heat energy absorbed or produced in the change equals the change in enthalpy that has taken place. 

To make comparisons meaningful, enthalpies of formation are given for the standard states of reactants and products—these are the states found at atmospheric pressure and, usually, room temperature (298.15 K). Thus, the standard state of water is liquid, not gas or solid. The standard state of iron is solid, not a molten liquid. To signify that a value represents measurements on substances in their standard states, a ° sign is added to the enthalpy symbol, giving AiT for the standard enthalpy of a reaction. Adding a subscript/ as in AHp further indicates a standard enthalpy of formation. 

The relations which permit us to express equilibria utilize the Gibbs free energy, to which we will give the symbol G and which will be called simply free energy for the rest of this chapter. This thermodynamic quantity is expressed as a function of enthalpy and entropy. This is not to be confused with the Helmholtz free energy which we will note sF (L" j (j, > )... 

Enthalpy (Section 2.18) The heat content of a substance symbol, H. 

The combination of properties (U + PV) occurs so frequently in thermodynamics that it is given a special symbol, H, and termed the enthalpy or heat content" of the system. Thus Equation 2-107 can be written as... 

We have referred several times to the heat flow for the reaction system, symbolized as reaction- At this point, you may well find this concept a bit nebulous and wonder if it could he made more concrete by relating faction to some property of reactants and products. This can indeed be done the situation is particularly simple for reactions taking place at constant pressure. Under that condition, the heat flow for the reaction system is equal to the difference in enthalpy (H) between products and reactants. That is,... 

The entropy of a substance, unlike its enthalpy, can be evaluated directly. The details of how this is done are beyond the level of this text, but Figure 17.4 shows the results for one substance, ammonia. From such a plot you can read off the standard molar entropy at 1 atm pressure and any given temperature, most often 25°C. This quantity is given the symbol S° and has the units of joules per mole per kelvin (J/mol-K). From Figure 17.4, it appears that... 

Students often ask, What is enthalpy The answer is simple. Enthalpy is a mathematical function defined in terms of fundamental thermodynamic properties as H = U+pV. This combination occurs frequently in thermodynamic equations and it is convenient to write it as a single symbol. We will show later that it does have the useful property that in a constant pressure process in which only pressure-volume work is involved, the change in enthalpy AH is equal to the heat q that flows in or out of a system during a thermodynamic process. This equality is convenient since it provides a way to calculate q. Heat flow is not a state function and is often not easy to calculate. In the next chapter, we will make calculations that demonstrate this path dependence. On the other hand, since H is a function of extensive state variables it must also be an extensive state variable, and dH = 0. As a result, AH is the same regardless of the path or series of steps followed in getting from the initial to final state and... 

The quantity //, - H° is called the relative partial molar enthalpy and given the symbol L,. It is the difference between the partial molar enthalpy in the solution and the partial molar enthalpy in the standard state. That is,... 

Considering the countercurrent flow of water and air in a tower of height z (Figure 13.15), the mass rate of flow of air per unit cross-section G is constant throughout the whole height of the tower and, because only a small proportion of the total supply of water is normally evaporated (1 -5 per cent), the liquid rate per unit area ll can be taken as constant. The temperature, enthalpy, and humidity will be denoted by the symbols 6, H, and Tf respectively, suffixes G, L, 1, 2, and / being used to denote conditions in the gas and liquid, at the bottom and top of the column, and of the air in contact with the water. 

In this expression, the n are the amounts of each substance in the chemical equation and the symbol X (sigma) means a sum. The first sum is the total standard enthalpy of formation of the products. The second sum is the similar total for the reactants. 

Equation (5) holds for rate constants of the first order in sec" and of the second order in 1 mol sec". ) Therefore, no distinction will be made between the two pairs of the activation parameters in this paper the computation usually will be carried out in the simpler terms of Arrhenius theory, but all of the results will apply equally well for the activation enthalpy and activation entropy, too. Furthermore, many considerations apply to equilibria as well as to kinetics then the symbols AH, AS, AG will mean AH, AS, AG as well as AH°, AS°, AG°, and k will denote either rate or equilibrium constant. 

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

It is convenient, and useful, to take the terms U and Pv together defining the term enthalpy, usual symbol H, as ... 

To reduce overlap between the symbols, only every tenth experimental point of each titration has been plotted.) The large discontinuity between successive titrations is caused by the fact that the instrument necessarily takes the enthalpy of the initial composition of each titration to be zero. (The treatment of this discontinuity is discussed below.)... 

Figure 4. Measured excess enthalpies for the amphiphilic side of the n-butanol/water diagram at 55.0 °C compared to results from the literature.(14,16) (Symbols same as for Fig. 3.)...

Fig. 9. Energetically favourable cyclodecane conformations with calculated angles (deg inner values) and torsion angles (53) (force field of Ermer and Lifson (19)). The conformations are further characterised by (from top) conformational symbol (11), symmetry, calculated relative potential energy (AV) and relative enthalpy (AH kcal mole-1 T = 298 K reference BCB AH = AV + AHvibr, see Section 3.3.). For BCB experimental angles and torsion angles (46) are also given (in parentheses)...

The enthalpy change of a reaction, AH, is the heat energy change when the reaction is carried out at constant pressure. It is necessary to express these values under standard conditions. For enthalpy changes measured under standard conditions, the symbol AH is used. Thermodynamic standard conditions are ... 

The energy balance for a PFR, as an enthalpy balance, may be developed in a manner similar to that for a CSTR in Section 14.3.1.2, except that the control volume is a differential volume. This is illustrated in Figure 15.3, together with the symbols used. 

In metals the electrons lose their association with individual atoms and the number of valence electrons is often used in rationalization schemes. Estimated enthalpies of formation for equi-atomic alloys, MM, of two elements of the first transition metal series are given as a function of the difference in number of valence electrons in Figure 7.13 [8], Compounds of a given common metal are given a specific symbol. For example, the scandium compounds ScM where M = Ti, V, Cr, Mn, Fe, Co, Ni and Zn, are given by open circles. The metal M of the compound MM is... 

Figure 9.12 (a) Enthalpy of oxidation of Lai xAxB03, 5 as a function of x. Open symbols represents values deduced from non-stoichiometry versus partial pressure isotherms. Closed symbols represent calorimetric values, (b) Comparison of experimental and calculated non-stoichiometry versus partial pressure isotherms [23], Reproduced by permission of the Royal Society of Chemistry. 

This combination of variables occurs so often in physical chemistry, that we give it a name we call it the enthalpy, and give it the symbol H. Accordingly, we rewrite Equation (3.15) as ... 

If the value of AHf is determined within these three constraints of standard T, standard p and standard allotropic form, we call the enthalpy a standard enthalpy, which we indicate using the plimsoll symbol, e as A. 

This enthalpy has a special name we call it the enthalpy of combustion, and define it as the change in enthalpy accompanying the burning of methane, and symbolize it as A77(CombUstion) or just AHc.ln fact, we rarely perform calculations with AHc but with the standard enthalpy of combustion AH, where the plimsoll symbol implies s.t.p. conditions. 

Most authors abbreviate combustion to just c, and symbolize the enthalpy change as AH . Others write... 

Care note the double A symbol in A A H(ads), which represents the change in AH(adSj from its mean value rather than the enthalpy of adsorption itself. 

Entropy, which has the symbol S, is a thermodynamic function that is a measure of the disorder of a system. Entropy, like enthalpy, is a state function. State functions are those quantities whose changed values are determined by their initial and final values. The quantity of entropy of a system depends on the temperature and pressure of the system. The units of entropy are commonly J K1 mole-1. If S has a ° (5°),... 

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