Heat capacities of liquids

LATENT HEAT OF VAPORIZATION, kJ/kg. SPECIFIC HEAT CAPACITY OF LIQUID, kJ/kg.K SET TEMPERATURE, OC ... 

Heat capacity is an extensive property the larger the sample, the more heat is required to raise its temperature by a given amount and so the greater is its heat capacity (Fig. 6.10). It is therefore common to report either the specific heat capacity (often called just specific heat ), Cs, which is the heat capacity divided by the mass of the sample (Cs = dm), or the molar heat capacity, Cm, the heat capacity divided by the amount (in moles) of the sample (Cm = C/n). For example, the specific heat capacity of liquid water at room temperature is 4.18 J-(°C) -g, or 4.18 J-K 1-g and its molar heat capacity is 75 J-K -mol1. 

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. 

The heat capacity of liquid iodine is 80.7 J-K -mol, and the enthalpy of vaporization of iodine is 41.96 kj-mol 1 at its boiling point (184.3°C). Using these facts and information in Appendix 2A, calculate the enthalpy of fusion of iodine at 25°C. 

Estimate the specific heat capacity of liquid 1,4 pentadiene and aniline at 20 °C. 

The heat capacity of liquid water at 20°C, for example, is 4.2 J g 1K-1 Thus, for every degree above the freezing point of water, one gram of water releases 4.2 J upon cooling one degree. 

Heat capacity of liquid 2.5 kJ/kg K Heat of vaporization 300 kJ/kg Molecular weight 100 Vapor acts as an ideal triatomic gas. 

In contrast to crystalline solids characterized by translational symmetry, the vibrational properties of liquid or amorphous materials are not easily described. There is no firm theoretical interpretation of the heat capacity of liquids and glasses since these non-crystalline states lack a periodic lattice. While this lack of long-range order distinguishes liquids from solids, short-range order, on the other hand, distinguishes a liquid from a gas. Overall, the vibrational density of state of a liquid or a glass is more diffuse, but is still expected to show the main characteristics of the vibrational density of states of a crystalline compound. 

Equations 2.39 and 2.40 lead to Avap//°(C2l I5OH) = 42.4 0.5 kJ mol-1 [40], which agrees with the mean of the calorimetric results for the same liquid, 42.30 0.04 kJ mol-1 [39]. Note that the less sophisticated approach (equation 2.33) apparently underestimates the vaporization enthalpy by 0.6 kJ mol-1. However, this is not true because AvapH = 41.8 kJ mol-1 refers to the mean temperature, 326 K. A temperature correction is possible in this case, because the molar heat capacities of liquid and gaseous ethanol are available as a function of T [40]. That correction can be obtained as ... 

M. Zabransky, V Ruzicka Jr., E. S. Domalski. Heat Capacity of Liquids Critical Review and Recommended Values. Supplement I. J. Phys. Chem. Ref. Data 2002, 30, 1199-1689. 

You can use the specific heat capacity of a substance to calculate the amount of energy that is needed to heat a given mass a certain number of degrees. You can also use the specific heat capacity to determine the amount of heat that is released when the temperature of a given mass decreases. The specific heat capacity of liquid water, as shown in Table 5.2, is 4.184 J/g °C. This relatively large value indicates that a considerable amount of energy is needed to raise or lower the temperature of water. 

No adequate theoretical treatment has been developed that might serve as a guide in interpreting and correlating data on the heat capacities of liquids, but a critical review and recommended values are available for several hquids [18], However, it has been observed that the molar heat capacity of a pure hquid generally is near that of the sohd, so if measurements are not available we may assume that Cvm is 25 J mol K However, the heat capacities of solutions carmot be predicted reliably from the corresponding properties of the components. Empirical methods of treating solutions will be considered in later chapters. 

A powerful argument in favor of Group Additivity is that a start has already been made in the application of Group Additivity to condensed phases. In 1969, groups were derived (Ref 11) for the heat capacities of liquids at 298°K that improved the precision of estimation from + 4 to better than +1.5 cal/(mole-K). 

Determination of Heat Capacity of Liquids with Time-Resolved Thermal Lens Calorimetry 46... 

Heat transfers are measured by using a calibrated calorimeter. The heat capacity of an object is the ratio of the heat supplied to the temperature rise produced. Molar heat capacities of liquids are generally greater than those of the solid phase of the same substance. Molar heat capacities increase as molecular complexity increases. 

If we want to calculate the entropy of a liquid, a gas, or a solid phase other than the most stable phase at T =0, we have to add in the entropy of all phase transitions between T = 0 and the temperature of interest (Fig. 7.11). Those entropies of transition are calculated from Eq. 5 or 6. For instance, if we wanted the entropy of water at 25°C, we would measure the heat capacity of ice from T = 0 (or as close to it as we can get), up to T = 273.15 K, determine the entropy of fusion at that temperature from the enthalpy of fusion, then measure the heat capacity of liquid water from T = 273.15 K up to T = 298.15 K. Table 7.3 gives selected values of the standard molar entropy, 5m°, the molar entropy of the pure substance at 1 bar. Note that all the values in the table refer to 298 K. They are all positive, which is consistent with all substances being more disordered at 298 K than at T = 0. 

Continued addition of heat to liquid water raises the temperature until it reaches 100°C. We can calculate from the molar heat capacity of liquid water [75.4 J/(mol °C)] that 7.54 kj/mol is required ... 

Figure 13.12 Heat capacity of liquid 4He. The lambda transition temperature is 2.172 K.

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