Enthalpy, liquid water

FIG. 2-11 Enthalpy-concentration diagram for aqueous hydrogen chloride at 1 atm. Reference states enthalpy of liquid water at 0 C is zero enthalpy of pure saturated HCl vapor at 1 atm (—85.03 C) is 8000 kcal/moL NOTE It should he observed that the weight basis includes the vapor, which is particularly important in the two-phase region. Saturation values may be read at the ends of the tie lines. [Van Nuys, Trans. Am. Inst. Chem. Eng., 39, 663 (1943).]... 

The diagram in Fig. 11-101 presents enthalpy data for LiBr-water solutions. It is needed for the thermal calculation of the cycle. Enthalpies for water and water vapor can be determined from the table or properties of water. The data in Fig. 11-101 are apphcable to saturated or subcooled solutions and are based on a zero enthalpy of liquid water at 0°C and a zero enthalpy of solid LiBr at 25°C. Since... 

For dry gas and liquid water at 25 C, the following enthalpies are computed for the inlet- and exit-gas streams (basis, 100 kmol of gas entering) ... 

Energy balances differ from mass balances in that the total mass is known but the total energy of a component is difficult to express. Consequently, the heat energy of a material is usually expressed relative to its standard state at a given temperature. For example, the heat content, or enthalpy, of steam is expressed relative to liquid water at 273 K (0°C) at a pressure equal to its own vapor pressure. 

High-pressure condensate forms at the same temperature as the high-pressure steam from which it condenses, as the enthalpy of evaporation (latent heat) is transferred from it. When this condensate is discharged through a steam trap to a lower pressure the energy it contains is greater than it can hold while remaining as liquid water. The excess... 

Liquid water has a higher enthalpy than ice, so heat must be transferred from the surroundings to melt the ice. 

The enthalpy of a substance, like its volume, is a state property. A sample of one gram of liquid water at 25.00°C and 1 atm has a fixed enthalpy, H. In practice, no attempt is made to determine absolute values of enthalpy. Instead, scientists deal with changes in enthalpy, which are readily determined. For the process... 

E6.2 The fugacity of liquid water at 298.15 K is approximately 3,17 kPa. Take the ideal enthalpy of vaporization of water as 43.720 TmoD1, and calculate the fugacity of liquid water at 300 K. 

In the first reaction, the water is produced as a vapor in the second, it is produced as a liquid. The heat generated is different in each case. We have already seen that the enthalpy of water vapor is 44 kj-mol 1 higher than that of liquid water at 25°C (see Table 6.3). As a result, an additional 88 kj (for 2 mol H20) remains stored in the system if water vapor is formed (Fig. 6.28). If the 2 mol H20(g) subsequently condenses, an additional 88 kj is given off as heat. 

An enthalpy of reaction also depends on the conditions (such as the pressure). All the tables in this book list data for reactions in which each reactant and product is in its standard state, its pure form at exactly 1 bar. The standard state of liquid water is pure water at 1 bar. The standard state of ice is pure ice at 1 bar. A solute in a liquid solution is in its standard state when its concentration is 1 mol-L". The standard value of a property X (that is, the value of X for the standard state of the substance) is denoted X°. 

FIGURE 6.29 The standard reaction enthalpy is the difference in enthalpy between the pure products and the pure reactants, each at I bar and the specified temperature (which is commonly but not necessarily 298 K). The scheme here is for the combustion of methane gas to carbon dioxide gas and liquid water. 

Seif-TeS I 6.13A Gasoline, which contains octane, may burn to carbon monoxide if the air supply is restricted. Determine the standard reaction enthalpy for the incomplete combustion of liquid octane in air to carbon monoxide gas and liquid water from the standard reaction enthalpies for the combustions of octane and carbon monoxide ... 

Amino acids are the building blocks of proteins, which have long chainlike molecules. They are oxidized in the body to urea, carbon dioxide, and liquid water. Is this reaction a source of heat for the body Use the information in Appendix 2A to predict the standard enthalpy of reaction for the oxidation of the simplest amino acid, glycine (NH2CH2COOH), a solid, to solid urea (H2NCONH2), carbon dioxide gas, and liquid water ... 

He placed two 150.-g samples of water at 0.00°C (one ice and one liquid) in a room kept at a constant temperature of 5.00°C. He then observed how long it took for each sample to warm to its final temperature. The liquid sample reached 5.00°C after 30.0 min. However, the ice took 10.5 h to reach 5.00°C. He concluded that the difference in time that the two samples required to reach the same final temperature represented the difference in heat required to raise the temperatures of the samples. Use Black s data to calculate the enthalpy of fusion of ice in kj-mol-1. Use the known heat capacity of liquid water. 

We could evaluate the enthalpy for this reaction step by step, first decomposing methane into its elemental constituents and then recombining the elements into carbon dioxide and liquid water. There is no need to do this, however, because Equation summarizes the step-by-step processes. The products are one mole of CO2 and two moles of H2 O, and the reactants are one mole of CH4 and two moles of O2 ... 

C14-0133. The enthalpy of sublimation of Ice at 273.15 K Is not the simple sum of the enthalpies of fusion and vaporization of water, but it can be calculated using Hess law and an appropriate path that Includes fusion and vaporization. Devise such a path, show it on a phase diagram for water, and carry out the calculation, making reasonable assumptions If necessary (C(liquid water) = 75.3 7 mol K , and C(water vapor) = 33.6 K ). 

The equation shows the change in enthalpy when one mole of liquid water vaporizes into water vapor. This is called the molar heat of vaporization. Given this information, which of these is the proper value for the molar heat of condensation ... 

Figure 3.3. Enthalpy-concentration diagram for aqueous ammonia. Reference states enthalpies of liquid water at 0°C and liquid ammonia at —77°C are zero. (Bosniakovic, Technische Thermodynamik, T. Steinkopff,... 

Tunon et al.194 studied the water molecule in liquid water. The sample of conformations by the microscopic environment (water in this case) was obtained using Monte Carlo technique. The energy was calculated as in the approach of Stanton et al.189 i.e., using Eqs. 4.25 and 4.26. The solvent induced increase of the dipole moment amounted to 0.61 Debye in line with the results by Wei and Salahub and close to the experimental value of 0.75 Debye. The solvation enthalpy amounted —12.6 kcal/mol, while the value calculated by Salahub and Wei and the experimental ones were —10.4 kcal/mol and —9.9 kcal/mol, respectively. 

The standard molar enthalpy of formation, A// , is the amount of heat absorbed when 1 mole of the substance is produced from its elements in their standard states. At 25°C, A// of liquid water is -285.8 kJ/mol and A// of water vapor is -241.8 kJ/mol. This means that more heat is released when liquid water is formed from its elements, then when gaseous water is formed from its elements. So, the formation reaction of liquid water is... 

To find the mass ratio of vapor to liquid produced, we note the discharge enthalpy //tot and sampling pressure Ps from Table 23.2. From the steam tables (Keenan et al., 1969), we find the sampling temperature 7) corresponding to the boiling point at Ps, and the enthalpies //uq and Hyap of liquid water and steam at this... 

The AH is dependent upon the state of matter. The enthalpy change would be different for the formation of liquid water instead of gaseous water. 

Figure 7.10 shows the flame combustion calorimeter used by Rossini in 1931 to determine the enthalpy of formation of liquid water, from the direct reaction of hydrogen with oxygen [54,99] ... 

Johnson et al. [143] also studied the dehydrated form of mordenite by reaction-solution calorimetry. Their results and the foregoing enthalpy of formation data lead to Af//°(Cao.289Nao.36iAlo.94oSi5.06oOi2.ooo, cr) = —5661.7 4.8 kJ mol-1. From the enthalpies of formation of both forms of mordenite and the enthalpy of formation of liquid water already quoted (—285.830 0.040 kJ mol-1), it is possible to conclude that at 298.15 K, the enthalpy of dehydration of mordenite, which corresponds to the reaction... 

Fig. 15. The enthalpy of liquid water calculated from the Weres-Rice model (from Ref. 64>) Fig. 16. The entropy of liquid water calculated from the Weres-Rice model (from Ref. 84>) Fig. 17. The specific heat of liquid water calculated from the Weres-Rice model (from Ref. 64>)...

Applying Concepts When ice is added to water at room temperature, the water provides the energy for two processes. The first process is the melting of the ice. The energy required to melt ice is the enthalpy of fusion (AHfus). The second process is raising the temperature of the melted ice from its initial temperature of 0.0°C to the final temperature of the liquid water. In this experiment, you will collect data to calculate the enthalpy of fusion for ice. 

There are three different ways to represent the enthalpy change of an exothermic reaction. The simplest way is to use a thermochemical equation a balanced chemical equation that indicates the amount of heat that is absorbed or released by the reaction it represents. For example, consider the exothermic reaction of one mole of hydrogen gas with half a mole of oxygen gas to produce liquid water. For each mole of hydrogen gas that reacts, 285.8 kj of heat is produced. Notice that the heat term is included with the products because heat is produced. 

The sensible heat required to heat the adsorbed water on the molecular sieve again over the same temperature range. The properhes of the adsorbed phase may safely be assumed to be those of liquid water and the calculahons of the enthalpy change can be made from available data. 

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