Constant-pressure conditions enthalpy

Many of the reactions which chemists study are reactions that occur at constant pressure. Because this constant pressure situation is so common in chemistry, scientists use a special thermodynamic term to describe this energy, enthalpy. The enthalpy change, AH, is equal to the heat gained or lost by the system during constant pressure conditions. The following sign conventions apply ... 

The enthalpy change, AH, is equal to the heat lost or gained by the system under constant pressure conditions. 

A similar expression is given for the heat of explosion under constant pressure conditions as shown in Equation 5.5, where AHet represents the corresponding standard enthalpies of formation ... 

The molar enthalpy of fusion (AEP J is the heat necessary to convert one mole of a solid into a liquid at its normal melting point. The molar enthalpy of vaporization (AH°vap) is the heat required to convert one mole of a liquid to a gas at its normal boiling point. When melting or vaporization occurs at constant pressure, it is acceptable to use heat instead of enthalpy. This is because heat and change in enthalpy are equal to each other under constant pressure conditions. The interested student should consult any physical chemistry textbook for more details. Both AHfm and AHyap are inherently endothermic, and represent an amount of energy that must be added to the sample in order for the phase transition to occur. The heat of fusion represents the amount of energy necessary to overcome the intermolecular forces to the point that the molecules can start to move around each other. The heat of vaporization represents the amount of energy necessary to overcome all intermolecular forces so that the molecules can escape into the gas phase. 

For an isomerization reaction such as this one, the change in volume A V 0. In a more general reaction done under constant-pressure conditions, we would have to add the work done on the surroundings (PAV discussed in Section 3.2) to the energy difference between the reactants and products, and we would replace AE with the enthalpy difference A H = AH+PAV. Now take the natural log ofboth sides of Equation 4.47, and convert Q into the entropy using Equation 4.29 ... 

Thus, Internal Energy is related to the state of the molecules or atoms - all the energy contained within them - including kinetic energy (vibrations in case of solids and velocity of movements in case of fluids). For real processes - processes realised in practice - most of the changes take place at atmospheric or nearly constant pressure. At constant pressure conditions, it is the Enthalpy or Heat Content which is more relevant. It is the sum total of the Internal Energy and the work it has already performed on the surroundings. 

The stationary-state heat release rate may also be interpreted from the measured temperature excess in well-stirred flow systems. The energy conservation equation for a well-stirred flow system is similar to equation (6.13) but an additional term is required to represent heat transport via the outflowing gases (a-Cp(T- Tafltres) as shown in equation (4.4). The inflowing gases are assumed to be pre-heated to the vessel temperature, Ta- Under constant pressure conditions, normally applicable to flow reactors, Cp replaces C, and A.H replaces AU in equation (6.13). The heat release is obtained from a summation of the product of individual reaction rates and their enthalpy change (-AH)jRj) in equation (5.4)). 

Henceforth we concentrate on the use of Eqs. (l.lS.lf), (1.13.2f), (1.13.3f), (1.13.4e) as the fundamental building blocks (as applied to equilibrium processes) for all subsequent thermodynamic operations. The enormous advantage accruing to their use is that by the First Law all of these functions depend solely on the difference between the initial and the final equilibrium state. We no longer rely on the use of quantities such as heat and work that are individually path dependent. As will be shown shortly and in much of what is to follow, these functions of state may be manipulated to obtain useful information for characterizing experimental observations. One should note that the choice of the functions E, H, A, or G depends on the experimental conditions. For example, in processes where temperature and pressure are under experimental control one would select the Gibbs free energy as the appropriate function of state. Processes carried out under adiabatic and constant pressure conditions are best characterized by the enthalpy state function. 

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". ... 

As with many state functions, we are interested in the change in enthalpy and not its absolute value. For the MCAT, we will be interested in the change in enthalpy under constant pressure conditions only ... 

During most experimental calorimetric measurements, the external pressure is kept constant (generally under approximately 1 bar) rather than the volume. For such constant pressure conditions, a new term, enthalpy, H, is defined as a new thermodynamic function,... 

Note that because reactions in a bomb calorimeter occur under constant-volume rather than constant-pressure conditions, the heat changes do not correspond to the enthalpy change A// (see Section 6.3). It is possible to correct the measured heat changes so that they correspond to A// values, but the corrections usually are quite small, so we will not concern ourselves with the details of the correction procedure. Finally, it is interesting to note that the energy contents of food and fuel (usually expressed in calories where 1 cal = 4.184 J) are measured with constant-volume calorimeters (see Chemistry in Action essay on p. 215.) Example 6.3 illustrates the determination of the heat of combustion of an organic compound. 

Because the experiments discussed were performed under constant pressure conditions, it is appropriate to use the thermodynamic entity H, the enthalpy. H is defined as... 

We see from Equation 5.9 that the right side of Equation 5.15 is the enthalpy change under constant-pressure conditions. Thus, AH = qp, as we saw in Equation 5.10. 

But what if we do the experiment at constant pressure instead It would be useful to have a function that would be equal to the heat flow under constant pressure conditions. This function, known as enthalpy, is defined as... 

Now we have two ways to define heat flow into a system, under two different sets of conditions. For a process at constant volume, the measurable heat flow is equal to AE, the change in internal energy. For a process at constant pressure, the measurable heat flow is equal to the change in enthalpy, AH. In many ways, enthalpy is the more useful term because constant pressure conditions are more common. A reaction carried out in a beaker in the chemistry laboratory, for instance, occurs under constant pressure conditions (or very nearly so). Thus, when we refer to the heat of a process, we are typically referring to a change in enthalpy, AH. As in previous definitions, AH refers to Fffinai -ffinraai-... 

So far we have considered enthalpy changes for simple physical processes such as temperature changes and phase transitions. But the importance of chemistry to the energy economy arises from the fact that there are enthalpy changes in chemical reactions as well. This enthalpy change is commonly referred to as the heat of reaction. Because many reactions are carried out under constant pressure conditions, this term is sensible, even if slightly imprecise. 

We then become acquainted with a new term for energy, called enthalpy, whose change applies to processes carried out under constant-pressure conditions. (6.4)... 

Enthalpy (H) n. A thermodynamic quantity, which is useful for describing heat exchanges taking place under constant-pressure conditions. The enthalpy of a system is defined as the sum of its energy, E, and its pressure-volume product H = E + pv... 

Because most reactions occur under constant-pressure conditions, we can equate the heat change in these cases with the change in enthalpy. For any reaction of the type... 

If the bond-breaking process occurs under constant-pressure conditions, however, then the energy required for bond breaking is better described by the bond enthalpy, rather than the bond energy. The bond enthalpy AH is the enthalpy change, per mole of gaseous molecules, required to break a particular bond in a molecule. 

Now, under constant pressure condition, AE in equation (8) refers to the enthalpy change in the dissociation process ... 

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