Water standard enthalpy

This value is the standard enthalpy of formation of glycerol dissolved in water in a hypothetical m = 1 solution that obeys Henry s law. 

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 ... 

Self-Test 8.2A The vapor pressure of water at 25°C is 23.76 Torr and its standard enthalpy of vaporization at that temperature is 44.0 kj-mol-1. Estimate the vapor pressure of water at 35°C. 

When ammonium nitrate, NH jNOj, dissolves in water, it absorbs heat. Consequently, its standard enthalpy of solution must be positive. This means that the entropy change caused by ammonium nitrate going from solid to solution must increase for the process to proceed spontaneously. This is exactly what one would expect based on the concept of entropy as a measure of randomness or disorder. 

There is considerable variation in the heat of reaction data employed in different articles in the literature that deals with this reaction. Cited values differ by more than an order of magnitude. If we utilize heat of combustion data for naphthalene and phthalic anhydride and correct for the fact that water will be a gas instead of a liquid at the conditions of interest, we find that for the first reaction (equation 13.2.3) the standard enthalpy change will be approximately — 429 kcal/g mole for the second reaction it will be approximately — 760 kcal/g mole. These values will be used as appropriate for the temperature range of interest. Any variation of these parameters with temperature may be neglected. 

The standard enthalpy of reaction for the dissolution of ammonium nitrate in water can be calculated using the enthalpies of formation for reactants and products ... 

The enthalpy change, AH, can be calculated for a steady-state process, using H°f, which is the enthalpy of formation of the various output and input components. Under the assumption that the inputs and outputs are at ambient conditions, the enthalpy of the components corresponds to the standard enthalpy of formation of each component. The kinetic and potential energy terms are neglected from the energy balance. It is also assumed that water enters the process as a liquid and hydrocarbon products leave the process as a liquid. All other components are in the gas phase. 

In all cases studied the standard enthalpy change accompanying the replacement of two water molecules by the chelating ligand is... 

Standard enthalpy of neutralisation, AHneut, is the enthalpy change, under standard conditions, when one mole of water is produced as a result of the reaction between an acid and an alkali. 

The standard enthalpy of formation of monomeric HF is a hypothetical state that must be related to that of the real associated liquid, gas, or aqueous solution met in calorimetiy. Considerable difficulty has been encountered in allowing for the heat of association, which varies with temperature and pressure. For example, the presence of traces of water can affect the polymerization by entering into the hydrogen bonding (30) the treatment of results will depend on the association model adopted. The magnitude of corrections for gas imperfections has... 

In the foregoing calculation of Asin//(1) and Asin//(3), we have used the tabulated values for the standard enthalpies of formation of ethanol and acetic acid aqueous solutions. This looks sensible (after the definitions given in section 2.3), because the standard states of ethanol and acetic acid solutions in water correspond to 1 mol of C2H5OH or CH3COOH in about... 

Each hydration enthalpy is simply the difference between the standard enthalpies of formation of the substance in water and in the gas phase. Because Af//°(Li+, g) and A 7/°(C1 , g) are both experimentally known, the unknown quantities are A //°(Li+, ao) and Af//°(Cr, ao). In other words, the central issue is the determination of standard enthalpies of formation of aqueous ions. The usual approach is simple to describe it is agreed that the standard enthalpy of formation of aqueous H+ is arbitrarily zero, Af//°(H+, ao) = 0. 

Equation 2.67 indicates that the standard enthalpy and entropy of reaction 2.64 derived from Kc data may be close to the values obtained with molality equilibrium constants. Because Ar// is calculated from the slope of In AT versus l/T, it will be similar to the value derived with Km data provided that the density of the solution remains approximately constant in the experimental temperature range. On the other hand, the error in ArSj calculated with Kc data can be roughly estimated as R In p (from equations 2.57 and 2.67). In the case of water, this is about zero for most solvents, which have p in the range of 0.7-2 kg dm-3, the corrections are smaller (from —3 to 6 J K-1 mol-1) than the usual experimental uncertainties associated with the statistical analysis of the data. 

The standard enthalpy change for the ionization of water is +55.83 kJ mol", which means that the reverse reaction, which occurs when acids are neutralized by bases, is exothermic, i.e. ArH = —55.83 kJ mol-1. The corresponding change in standard Gibbs energy is —79.9 kJ mol - . The reaction ... 

Q Given that the standard enthalpy change for dissolving potassium chloride in water is - 17.0 k.l inol 1 and the standard enthalpy of formation of KCI is —436.5 kJ mol1, use the value for the enthalpy of formation of the aqueous chloride ion given in Table 2.2 to calculate a value for the enthalpy of formation of aqueous potassium ions. 

The most stable state of nitrogen in acidic solution is the ammonium ion, NH4(aq), which is isoelectronic with CH4 and H30+. It is a tetrahedral ion with strong N-H bonds. The mean N-H bond enthalpy in NH4(aq) is 506 kJ mol 1 (that of the O-H bonds in H30 + is 539 kJ mol" ). The enthalpy of hydration of the ammonium ion is — 345 kJ mol V This value placed into the Born equation (3.32) gives an estimate of the radius of the ammonium ion of 135 pm, a value insignificantly different from its thermochemical radius of 136 pm. The value is comparable to that estimated for the smaller H30+ ion (99 pm) from its more negative enthalpy of hydration (— 420 kJ mol -see Section 2.6.1). The proton affinity of the ammonia molecule is of interest in a comparison of its properties with those of the water molecule. The proton affinity is defined as the standard enthalpy change for the reaction ... 

Standard heat capacities of transfer can be derived from the temperature dependence of standard enthalpies of solution (8). While this technique can give general trends in the transfer functions from water to mixed solvents (9), it is not always sufficiently precise to detect the differences between similar cosolvents, and the technique is rather laborious. Direct measurements of the difference between heat capacities per unit volume of a solution and of the solvent a — gq can be obtained with a flow microcalorimeter (10) to 7 X 10 5 JK 1 cm-3 on samples of the order of 10 cm3. A commercial version of this instrument (Picker dynamic flow calorimeter, Techneurop Inc.) has a sensitivity improved by a factor oi about two. 

Table I. Standard Enthalpies of Solution in kj mol 1 of Tetra -n-butylammonium Bromide in Mixtures of Af,iV-dimethylformamide and Water at 5°, 25°, and 55°C as a Function of the Mole Fraction of Water, XHp...

We shall discuss now the variation of the three main thermodynamic functions with solvent composition for the case of n-Bu4NBr-water-acetone system and shall extend this discussion to the n-Bu4NBr-water-THF system. Figure 4 and Table IV present the results obtained. The figure was constructed as follows first the standard enthalpy of transfer AH°t, obtained by Ahluwalia and co-workers (12) from pure water to Z2 = 0.30, was used in order to get the standard entropy of transfer function from the relation ... 

As indicated by Eq. 6-9, the standard enthalpy of transfer of a compound i from water to air is given by ... 

Table 6.3 Experimental Standard Enthalpies of Vaporization (AvapH,) and Standard Enthalpies of Transfer from Water to Air (Aaw//,) of Selected Organic Compounds at 25 °C ... 

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