Heat Transfers at Constant Pressure

Similarly, because heat transferred at constant pressure can be identified with the change in enthalpy, AH, we can define the heat capacity at constant pressure, Cr, as... 

We can use Eq. 1 to calculate the entropy change of the surroundings, provided that we assume that the surroundings are so large that their temperature and pressure remain constant. If the enthalpy change of the system is AH, then, for heat transfers at constant pressure, gSLlrr = —AH. We can now use Eq. 1 to write... 

J 8 Estimate the change in entropy of the surroundings due to heat transfer at constant pressure and temperature (Example 7.10). 

Alternatively, a reaction might be carried out in an open flask or other apparatus that keeps the pressure constant and allows the volume of the system to change freely. In such a case, AU 0 and the energy change in the system might be due to both heat transfer and PV work. We indicate the heat transfer at constant pressure by the symbol qp ... 

Most chemical reactions are done under constant (atmospheric) pressure conditions rather than at constant volume, so it is desirable to relate the heat transferred at constant pressure, q, to some state property analogous to the internal energy, U. 

Combustion reactions are studied in a bomb calorimeter of constant volume. Heat transfer at constant pressure, qp (notation not used in the text)... 

Heat Transfers at Constant Pressure Definition of enthalpy, H... 

Enthalpy (FT) Measured in joules, H = E + pV the amount of energy in a system capable of doing work the sum of the internal energy of the system plus the product of its volume times the pressure on it. The change in enthalpy (AH) is equal to the heat transferred at constant pressure. 

However, an increase in the heat transfer at constant pressure drop, which can be expressed as the ratio of the Nusselt number and the Poiseuille number Po, is not achieved for most channel geometries. Po is the ratio of the friction factor / according to Eq. (7.2) to Re. Drese and Hardt showed that diamond-shaped fins have the potential to increasing the Nu/Po ratio by a factor of more than 2, if the ratio of fin length to fin width is increased by a factor of 4, because recirculation zones are suppressed by the longer fins [9]. 

The special relationship between AS and the heat transferred in a reversible process is similar to the special relationship between AH and the heat transferred at constant pressure. (Section 5.3)... 

Enthalpy (H) is also a state function and is related to E. The change in enthalpy (AH) equals the heat transferred at constant pressure, qp. Most laboratory, environmental, and biological changes occur at constant P, so AH is more relevant than AE and easier to measure. (Section 6.2)... 

For a given chemical change, the heat transferred at constant pressure (qp) is equal to the heat transferred at constant volume (qy) only if there is no net consumption or production of gas (i.e., when AWgas = 0). 

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