Enthalpy change at constant pressure

We need to use three definitions the infinitesimal versions of the free energy change at constant temperature (dG = dH — T dS), the enthalpy change at constant pressure (dH = dU + PdV), and the internal energy change (dU = dw + dq). We substitute the second expression into the first and obtain... 

Can you find the enthalpy change at constant pressure of substance such as CO2... 

Therefore, the quantity of heat absorbed or released by the system to the surroundings at constant volume is equal to the change in the internal energy of the system. To adjust the experimental energy change determined in a combustion calorimetric experiment for a given reaction, Qy, to the enthalpy change at constant pressure, we have to use the relation between Qy, and Qp,... 

This enthalpy change is also equivalent to the enthalpy of combustion of hydrogen. The enthalpy of combustion is the enthalpy change (at constant pressure) when one mole of a pure substance undergoes complete combustion under standard thermodynamic conditions. 

Enthalpy change at constant pressure, no non-expansion work... 

This chapter introduces the first law of thermodynamics and its applications in three main parts. The first part introduces the basic concepts of thermodynamics and the experimental basis of the first law. The second part introduces enthalpy as a measure of the energy transferred as heat during physical changes at constant pressure. The third part shows how the concept of enthalpy is applied to a variety of chemical changes, an important aspect of bioenergetics, the use of energy in biological systems. 

The state function that allows us to keep track of energy changes at constant pressure is called the enthalpy, H ... 

The enthalpy change of a process is equivalent to its heat change at constant pressure. 

To avoid the use of the ambiguous term "heat" in connection with "heat content," it is customary to use the term enthalpy. At a given temperature and pressure, every substance possesses a characteristic amount of enthalpy (H), and the heat changes associated with chemical and physical changes at constant pressure are called changes in enthalpy (AH) AHT is the enthalpy of transition. Two common enthalpies of transition are AHf = 1435 cal/mole for the enthalpy of fusion (melting) of ice at 0°C, and AH, = 9713 cal/mole for the enthalpy of vaporization of water at 100°C. 

The enthalpy change associated with a chemical reaction or phase change at constant pressure and temperature can be calculated from the enthalpy of each species involved in the process. When species A undergoes the phase transformation from a to p,... 

The last term of Equation 6-14 represents the enthalpy change at constant temperature and pressure due to a change in composition or the degree of conversion. Consider an irreversible reaction of the form aA + bB —> cC + dD. The change in enthalpy AHR at fixed temperature and pressure is... 

Reaction enthalpy (arH) At constant pressure, the change in enthalpy observed when reactants are converted into products. 

Gibbs Helmholtz equation relates the free energy change to the enthalpy change and the rate of change of free energy with temperature. Thus equation may be applied to any change at constant pressure. 

Remember, the enthalpy change is just the heat change at constant pressure.) Substituting and multiplying through by -T we get Equation 11-4 ... 

The oxygen-steam ratios required to maintain zero net enthalpy change are given in Table 9.3 for several temperatures and pressures (Parent and Katz, 1948). With increased pressure, the ratio necessary to preserve a zero net enthalpy change diminishes. The decrease is most pronounced at low pressures. The effect of temperature change at constant pressure is also shown in Table 9.3. At lower temperatures, the oxygen-steam ratio doubles for each temperature... 

The enthalpy change, dH = T dS + V dp, can be described as dH = dq - -V dp, and for a constant-pressure process, c/p = 0, we have dH = dqp. For a finite state change at constant pressure, qp = AH, that is, the heat transferred is equal to the enthalpy change of the system. This relation is the basis of constant pressure calorimetry, the constant-pressure heat capacity being Cp = dqldT)p. The relationship qp = AH is valid only in the absence of external work, w. When the system does external work, the first law must include dw. Then, the heat transferred to the system under constant-pressure conditions is qp = AH -f w. Thus, if a given chemical reaction has an enthalpy change of -50 kJ mol and does 100 kJ mol" of electrical work, the heat transferred to the system is —50 + 100 = 50 kJ mol". ... 

The quantity of heat transferred into or out of a system as it undergoes a chemical or physical change at constant pressure, q, is defined as the enthalpy change, Aff, of the process. 

Enthalpy (H) An extensive property of a substance that can be used to obtain the heat absorbed or released by a chemical reaction or physical change at constant pressure. It is defined as the sum of the internal energy (U) and the product of the pressure and the volume of the system (PV) H = U + PV. 

In the vast majority of cases, dissolving a solute in a solvent produces measurable heat change. At constant pressure, the heat change is equal to the enthalpy change. The heat of solution, or enthalpy of solution, AHg i , is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent. The quantity represents the difference between the enthalpy of the final solution and the enthalpies of its original components (that is, solute and solvent) before they are mixed. Thus... 

Relating the enthalpy change to the internal energy change at constant pressure (185) ... 

G = H-TS, where T is the absolute temperature, H (the enthalpy) measures the energy change at constant pressure, and S (the entropy) measures the randomness of the system. At constant temperature and pressure,... 

We have seen that the heat supplied to a system at constant pressure is equal to the enthalpy increase. For many chemical reactions it is possible to make a direct determination of the heat change at constant pressure for other reactions indirect methods (to be discussed later) can be used. In these ways it has proved possible to build a considerable body of data on the enthalpy changes of chemical processes of all kinds and these data have been of great value in an understanding of molecular structure and in various other ways, The study of enthalpy changes in chemical systems is known as thermochemistry. 

Sensible Heat and Latent Heat. Since the heat inflow caused by volume change at constant pressure is equal to the enthalpy change, AH, the specific heat Cp is given by the equation ... 

Most physical and chemical changes occur at nearly constant atmospheric pressure—a reaction in an open flask, the freezing of a lake, a biochemical process in an organism. In this section, we discuss enthalpy, a thermodynamic variable that relates directly to energy changes at constant pressure. 

Thus, the change in enthalpy equals the heat absorbed or released at constant pressure. For most changes at constant pressure, is more relevant than Aft and easier to obtain to find AH, measure qp. We discuss the laboratory method in Section 6.3. 

---