--%>

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 : Molecular weight of solute Select right

    Select right answer of the question. A dry air is passed through the solution, containing the 10 gm of solute and 90 gm of water and then it pass through pure water. There is the depression in weight of solution wt by 2.5 gm and in weight of pure solvent by 0.05 gm. C

  • Q : Strength of Nacl in solution To 5.85gm

    To 5.85gm of Nacl one kg of water is added to prepare of solution. What is the strength of Nacl in this solution (mol. wt. of nacl = 58.5)? (a) 0.1 Normal (b) 0.1 Molal (c) 0.1 Molar (d) 0.1 FormalAnswer:

  • Q : Problem based on molarity Select the

    Select the right answer of the question. If 18 gm of glucose (C6H12O6) is present in 1000 gm of an aqueous solution of glucose, it is said to be: (a)1 molal (b)1.1 molal (c)0.5 molal (d)0.1 molal

  • Q : Relative lowering of vapour pressure

    Which of the following solutions will have a lower vapour pressure and why? a) A 5% aqueous solution of cane sugar. b) A 5% aqueous solution of urea.

  • Q : What is ortho effect? Orthosubstituted

    Orthosubstituted anilines are generally weaker bases than aniline irrespective of the electron releasing or electron withdrawing nature of the substituent. This is known as ortho effect and may probably be due to combined electronic and steric factors.The overall basic strength of ort

  • Q : Problem on distribution law The

    The distribution law is exerted for the distribution of basic acid among: (i) Water and ethyl alcohol (ii) Water and amyl alcohol (iii) Water and sulphuric acid (iv) Water and liquor ammonia What is the right answer.

  • Q : Short note on the function of

    Write down a short note on the function of mitochondria?

  • Q : What is schrodinger wave equation? The

    The Schrodinger wave equation generalizes the fitting-in-of-waves procedure.The waves that "fit" into the region to which the particle is contained can be recognized "by inspection" only for a few simple systems. For other problem a mathematical procedure

  • Q : Molar concentration of Iron chloride

    Provide solution of this question. A certain aqueous solution of FeCl3 (formula mass =162) has a density of 1.1g/ml and contains 20.0% Fecl. Molar concentration of this solution is: (a) .028 (b) 0.163 (c) 1.27 (d) 1.47

  • Q : Dipole moment of chloro-octane Describe

    Describe the dipole moment of chloro-octane in brief?