Enthalpy of vaporization molar

P8.1 The molar enthalpy of vaporization of liquid mercury is 59.229 kJ-mol l at its normal boiling point of 630.0 K. The heat capacities of the liquid and gaseous phases, valid over the temperature range from 250 to 630 K, are as follows ... 

C14-0051. Table lists molar enthalpies of vaporization of several substances. Calculate the molar entropy of vaporization at its normal boiling point for each of the following (a) molecular oxygen (b) ethane (c) benzene and (d) mercury. 

From Appendix E, the molar enthalpy of vaporization of mercury at the normal boiling point is 58.6 kJ/mol. Using the Clausius-Clapeyron equation to find the vapor pressure of mercury at 25°C, we have... 

Drawing Conclusions Assume that the three alcohols have approximately the same molar enthalpy of vaporization. What can you say about the relative rates of evaporation of the three alcohols ... 

Thinking Critically Molar enthalpies of vaporization for the three alcohols are given in the table on the right. Note that they are not the same. In what way, if any, does this data change your conclusion about intermolecular forces ... 

Because the latent heat A//vap(r, P) is nearly constant over the usual effective range of T and P variations, we assume that it can be replaced by thz fixed molar enthalpy of vaporization AH°wap for standard state conditions (e.g., 25°C, 1 atm) ... 

In making the derivation, you will need to assume that (i) the molar volume of the liquid is negligible when compared to that of the gas (ii) the gas behaves ideally and (iii) the molar enthalpy of vaporization is constant with temperature. 

For liquids that do not have a reported molar enthalpy of vaporization, a convenient method of approximation is Hildebrand s empirical equation, based on the boiling plQjnb Kelvin units ... 

The universal gas constant to be used in this equation is 8.314-Jmor1 KT1 and the molar enthalpy of vaporization is expressed in J moT1. Since the vapor pressure increases exponentially with temperature, the effects of a temperature increase, for example due to an uncontrolled reaction, may be considerable. As a rule of thumb, the vapor pressure doubles for a temperature increase of about 20 K. 

In general, the molar enthalpy of vaporization is obtained from the Clausius-Clapeyron equation, representing the difference per mole of the enthalpy of the vapour and of the liquid at equilibrium with it ... 

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. 

Where U. = AHv l - RT is the molar interaction energy for particles of solute 1 and can be approximated by the molar enthalpy of vaporization AHV j expressed at temperature T. is the molar volume of the pure liquid substance 1. The cohesive energy density is expressed by cn which describes the interaction between the particles of the pure liquid 1. In a two component system it is assumed that the interaction... 

The molar enthalpy of vaporization of water at 373 K is 41.16 kj/mol. What fraction of this energy is used to change the internal energy of the water, and what fraction is used to do work against the atmosphere (Assume that water vapor is an ideal gas.)... 

Define the molar enthalpy of fusion and the molar enthalpy of vaporization, and identify them for a substance by using a heating curve. 

At 373.15 K, the water boils. If the pressure remains constant, so does the temperature as long as the two states (liquid and gas) are present. The energy added is the molar enthalpy of vaporization IsHyap), 40.67 kJ/mol. Mlyap is the difference in enthalpy between liquid and gaseous water at 373.15 K and is defined in the following equation ... 

In other words, the boiling point of a liquid, Tbp, is equal to molar enthalpy of vaporization, AHyap, divided by molar entropy of vaporization, ASyap. 

The molar enthalpy of fusion of a substance is the energy required to melt 1 mol of the substance at the melting point. The molar enthalpy of vaporization of a substance is the energy required to vaporize 1 mol of the substance. 

Why is the molar enthalpy of vaporization of a substance much higher than the molar enthalpy of fusion ... 

What happens in the reverse processes, when water vapor condenses to hq-uid water or liquid water freezes to ice The same amounts of energy are released in these exothermic processes as are absorbed in the endothermic processes of vaporization and melting. Thus, the molar enthalpy (heat) of condensation (A//gojjd) the molar enthalpy of vaporization have the same numerical value but opposite signs. Similarly, the molar enthalpy (heat) of solidification (A/Zg iid) and the molar enthalpy of fusion have the same numerical value but differ in sign. 

Thinking Critically Molar enthalpies of vaporization for the three alcohols are given in the table below. Note that they are not the same. 

Once the methanol has reached its boiling point, it must be vaporized. The molar enthalpy of vaporization for methanol is 38 kj moP. How much heat must be added to vaporize 1.00 kg methanol ... 

Calculate the entropy change when 3.00 mol benzene vaporizes reversibly at its normal boiling point of 80.1°C. The molar enthalpy of vaporization of benzene at this temperature is 30.8 kj moPh... 

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