--%>
Assignment Help - homework help

What is heat capacity and how to calculate heat capacity

The temperature reliance of internal energy and enthalpy depends on the heat capacities at constant volume and constant pressure.


The internal energy and enthalpy of chemical systems and the energy changes that accompany chemical reactions depend on the temperature. To make full use of the thermodynamic date we developed, we must see how these data are extended to temperatures other than 25°C.

Heat capacities; it is convenient to deal separately constant pressure processes when the temperature is raised and the energy of the system increases. The heat capacity, already introduced and experimentally determined, as the decrease in the energy of the thermal surroundings that provides the energy to increase the temperature of the system by 1°C, under the specified conditions. Thus we define

1966_heat capacity.png 

And 

68_heat capacity1.png 

If you think of an actual measurement, you see that to increase the temperature of the system, i.e. for ΔT to be positive, there will be a decrease in the energy of the thermal surroundings, that is, ΔUtherm will be negative. The definitions are then being seen to make heat capacities positive quantities.

Heat capacities at constant pressure CP will be used more than will heat capacities at constant volume CV. Some values for CP are given for a temperature of 25°C. All these values for liquids and solids come from experimental, calorimetric studies that depend on the defining equation. Some of the values for gases are experimental, and others are based on calculations of the type of physical properties.

Heat capacities can be used to extend the 25°C thermodynamic quantities to other temperatures. To do so, we will need heat capacity values over a range of temperatures. An analytical expression, rather than a table of values, is needed for most of the calculations we will do. The two empirical CP versus T expressions that have been most used are

CP = a' + b't + c'T2 + ....

And, CP = a + bT + cT-2 + ...

The second of these two forms is more satisfactory. The coefficients that have been deduced for this equation are given for a few substances.

Heat capacities and internal energies and enthalpies: heat capacities, defined in terms of energy changes in the thermal surroundings, can be expressed in terms energy changes in the system.

If any ordinary chemical process occurs and the system has a constant volume ΔUmech = 0 and ΔU = -ΔUtherm, we can express CV as

2156_heat capacity2.png 

If the system is maintained at a constant pressure, ΔH = - ΔUtherm. We can express CP as

190_heat capacity3.png 

Heat capacities in J K-1 mol-1 at constant pressure (parameters for the equation C°P = (a + bT + cT-2):

327_heat capacity4.png 

Heat capacities are characteristics of the system. They are directly linked to the way the internal energy and enthalpy change with temperature when the volume or pressure of the system is correctly controlled.

   Related Questions in Chemistry

  • Q : Vapour pressure of water Give me answer

    Give me answer of this question. 5cm3 of acetone is added to 100cm3 of water, the vapour pressure of water over the solution: (a) It will be equal to the vapour pressure of pure water (b) It will be less than the vapour pressure of pure water

    		•
  • Q : Relationship between Pressure and

    The pressure-temperature relation for solid-vapor or liquid vapor equilibrium is expressed by the Clausis-Clapeyron equation.We now obtain an expression for the pressure-temperature dependence of the state of equilibrium between two phases. To be specific,

    		•
  • Q : Define Bond Energies - Bond Charges

    Energy changes in some chemical reactions can be used to deduce the energies of chemical bonds. Our understanding of the molecular basis of thermodynamic properties is extended when we ask why the enthalpy change for a reaction is what it is. We deduce,

    		•
  • Q : Colligative properties give atleast two

    give atleast two application of following colligative properties

    		•
  • Q : Define thermal energy The thermal part

    The thermal part of the internal energy and the enthalpy of an ideal gas can be given a molecular level explanation. All the earlier development of internal energy and enthalpy has been "thermodynamic". We have made no use o

    		•
  • Q : Calculation of molecular weight Provide

    Provide solution of this question. In an experiment, 1 g of a non-volatile solute was dissolved in 100 g of acetone (mol. mass = 58) at 298K. The vapour pressure of the solution was found to be 192.5 mm Hg. The molecular weight of the solute is (vapour pressure of ace

    		•
  • Q : How haloalkanes are prepared from

    This is the common method for preparing haloalkanes in laboratory. Alcohols can be converted to haloalkanes by substitution of - OH group with a halogen atom. Different reagents can be used to get haloa

    		•
  • Q : Molecular Diameters The excluded volume

    The excluded volume b, introduced by vander Wall's as an empirical correction term, can be related to the size gas molecules. To do so, we assume the excluded volume is the result of the pairwise coming together of molecules. This assumption is justified when b values

    		•
  • Q : Question associated to vapour pressure

    Choose the right answer from following. The vapour pressure lowering caused by the addition of 100 g of sucrose(molecular mass = 342) to 1000 g of water if the vapour pressure of pure water at 25degree C is 23.8 mm Hg: (a)1.25 mm Hg (b) 0.125 mm Hg (c) 1.15 mm H

    		•
  • Q : Schrodinger equation with particle in a

    Three dimensional applications of the Schrodinger equation are introduced by the particle-in-a-box problem.So far only a one-dimensional problem has been solved by application of the Schrodinger equation. Now the allowed energies and the probability functi

    		•