--%>
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 : Explain preparation and properties of

    It may be prepared by the action of phosphorus on thionyl chloride.P4 + 8SOCl2    4

    		•
  • Q : Henry law question Answer the following

    Answer the following qustion. The definition “The mass of a gas dissolved in a particular mass of a solvent at any temperature is proportional to the pressure of gas over the solvent” is: (i) Dalton’s Law of Parti

    		•
  • Q : Problem on solutions The 2N aqueous

    The 2N aqueous solution of H2S04 contains: (a) 49 gm of H2S04 per litre of solution (b) 4.9 gm of H2S04 per litre of solution (c) 98 gm of H2S04

    		•
  • Q : Explain Rotational Vibrational Spectra

    The infrared spectrum of gas samples shows the effect of rotational-energy changes along with the vibrational energy change.As we know from the interpretations given to thermodynamic properties of gases, gas molecules are simultaneously rotating and vibrating. It follows that an absor

    		•
  • Q : Mole fraction of urea Choose the right

    Choose the right answer from following. When 6gm urea dissolve in180gm H2O . The mole fraction of urea is : (a)10/ 10.1 (b)10.1/10 (c)10.1/ 0.1 (d) 0.1/ 10.1

    		•
  • Q : Means of molality Give me answer of

    Give me answer of this question. The number of moles of solute per kg of a solvent is called its: (a) Molarity (b) Normality (c) Molar fraction (d) Molality

    		•
  • 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 : Linde liquefaction process Liquefied

    Liquefied natural gas (LNG) is produced using a Linde liquefaction process from pure methane gas at 3 bar and 280 K (conditions at point 1 in figure below). A three-stage compressor with interceding is used to compress the methane to 100 bar (point 2). The first stage

    		•
  • Q : Electron Spin The total angular

    The total angular momentum of an atom includes an electron spin component as well as an orbital component.The orbital motion of each electron of an atom contributes to the angular momentum of the atom, as described earlier. An additional

    		•
  • Q : Molarity of Nacl solution When 5.85 g

    When 5.85 g of NaCl (having molecular weight 58.5) is dissolved in water and the solution is prepared to 0.5 litres, the molarity of the solution is: (i) 0.2 (ii) 0.4 (iii) 1.0 (iv) 0.1

    		•