Hydrogen molar enthalpy

The molar enthalpy of combustion increases with molar mass as might be expected, because the number of moles of CO, and H,0 formed will increase as the number of carbon and hydrogen atoms in the compounds increases. The heat released per gram of these hydrocarbons is essentially the same because the H to C ratio is similar in the three hydrocarbons. 

In Investigation 5-B, you used the reaction of oxygen with hydrogen to form water. Reactions like this one are known as formation reactions. In a formation reaction, a substance is formed from elements in their standard states. The enthalpy change of a formation reaction is called the standard molar enthalpy of formation, AH°f. The standard molar enthalpy of formation is the quantity of energy that is absorbed or released when one mole of a compound is formed directly from its elements in their standard states. 

P-C-T Determinations Low Pressure Studies. Absorption isotherms obtained for the reaction of hydrogen with TiMo are shown in Figure 3 for 590°-392°C. These temperatures are above the decomposition temperature of /J-TiMo (see Figure 2) consequently, decomposition of the solid solution plays no role here. These data follow Sieverts Law only in the very dilute region—to hydrogen-to-metal ratios (H/M) of about 0.02. Thereafter, deviations in the direction of decreased solubility are observed. Data in the region of Sieverts Law can be used to determine the relative partial molar enthalpy and entropy at infinite dilution (47). From Sieverts Law (Equation 1), where Ks is a tempi/2 = Ksn (1)... 

On the basis of dissociative solution, i.e., hydrogen is absorbed as H atoms rather than H2 molecules, the relative partial molar free energy of absorption is related to the equilibrium constant via the equilibrium hydrogen pressure (Equation 3). Values of the relative partial molar enthalpy as a function of hy-... 

Figure 4. Relative partial molar enthalpy of hydrogen absorption exhibited by TiMo as a function of hydrogen content. Data are presented as — AHh (kcalfg-atom hydrogen) vs. hydrogen-to-metal atom ratio.

In electrochemistry we make it a rule that the standard chemical potential ju. of hydrogen ions is set zero as the level of reference for the chemical potentials of all other hydrated ions. The standard chemical potentials of various hydrated ions tabulated in electrochemical handbooks are thus relative to the standard chemical potential of hydrogen ions at unit activity in aqueous solutions. Table 9.3 shows the numerical values of the standard chemical potential, the standard partial molar enthalpy h°, and the standard partial molar entropy. 5 ,° for a few of hydrated ions. 

Now let turn to an examination of the obtained calorimetric data presented in Figs. 3-6 and in sequence to analyze the changes of differential partial molar enthalpy depending on hydrogen concentration in the IMC and experimental temperature. 

As seen from Fig. 6 at 305°C the values of partial molar enthalpy for different runs of hydrogen desorption have a large deviation probably because of a proximity to critical temperature, thus it is difficult to determine the phase boundaries. The critical temperature for the existence of ZrMn2 hydride phase estimated by different authors is 277-327 °C [8] and 318°C [15], The plot of the AHdes. -C could be divided into three parts the hydrogen a-solid solution region (0pi transition (0.6p2 transition (1.0[Pg.353]

It has been established that in the ZrMn2-H2 system the partial molar enthalpy of hydrogen reaction with ZrMn2 changed with the temperature of experiment and hydrogen concentration in this system. The obtained data allow us to... 

ASvapjn is equal to ratio between the molar enthalpy of vaporisation and the boiling temperature, which leads to Eq. (18.10). The molar enthalpy and entropy of vaporisation and the boiling temperature of some simple liquids are presented in Table 18.2 Some liquids deviate sharply from the rule. This is often because these liquids have structure and so a greater amount of disorder is introduced when they evaporate. Examples are water and methanol due to hydrogen bonds between the molecules. (Atkins and De Paula, 2006)... 

The relative partial molar enthalpy changes for hydrogen desorption were... 

This work is a continuation of our earlier study [1] of the hydrogen interaction with intermetallic compound (IMC) AB2-type Tio.9Zro.1Mn . 3V0.5. The measurements were carried out in twin-cell differential heat-conducting Tian-Calvet type calorimeter connected with the apparatus for gas dose feeding, that permitted us to measure the dependencies of differential molar enthalpy of desorption (AHdes.) and equilibrium hydrogen pressure (P) on hydrogen concentration x (x=[H]/[AB2]) at different temperatures simultaneously. The measurements were carried out at 150°C, 170°C and 190°C and hydrogen pressure up to 60 atm. 

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