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5.3 Nature of Heat

When a change in the state of a system occurs, energy is transferred to or from the surroundings. Energy consumed or lost is commonly studied as heat which is a measure of motions of molecules in a given system. Heat is best understood in terms of rise or fall of temperature of a system.

 The sign convention : According to MKS System dq or q is negative (-ve) if heat is added to the system, dq or q is positive (+ve) if heat is removed from the system where q is the symbol of heat.

Units of heat : The unit of heat, which was used for many years is calorie (Cal). It is defined as the quantity of heat required to raise the temperature of one gram of water at 14.50C by one degree.

SI unit of heat is the joule (J).

1 joule = 0.2390 calories
1 calorie = 4.184 J
1K cal = 1000 calories
1K cal = 4.184 KJ

Heat Capacity : It is the amount of heat required to raise the temperature of the system by one degree.

If the system is one gram of pure substance it is called specific heat, but if one mole of the pure substance is considered, it is called molar heat capacity.

Units of Heat Capacity : The usual units of the molar capacity are calorie per degree per mole (Cal K-1 mol-1 ) or joules per degree per mole ( J K-1mol-1 ) the latter being the SI unit.

The following chart shows the heat capacities of several elements and compounds in calories per degree per gram as well as per mole and their inter conversion.

Table 10

 ElementsCompounds State Mol.wt/atomic wt. Cal deg.-1gram-1 Cal deg.-1mol-1A ´ mol. Wt H2O Liquid 18.015 1.000 18.015 H2O Solid 18.015 0.485 8.737 CaCO3 Solid 100.087 0.205 20.518 CO2 Gas 44.009 0.158 6.9788 O2 Gas 31.999 0.203 6.500 MgO Solid 40.304 0.208 8.383 Al Solid 26.982 0.213 5.747

Based on the above table let’s calculate heat capacity value.

Amount of heat required in calories to heat 100 gm of H2O(S) from 0.2 to 0.80C repesented as follows :

= (0.8 - 0.2) deg ´ 100 grams ´ 0.485 Cal deg-1gram-1

= 0.6 ´ 100 ´ 0.485

= 29.1 calories

Similarly heat released into the surrounding while cooling 100 grams of H2O(S)from 0.8 to 0.20C will be 29% calories.

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Index

5.1 Solids, Liquids and Gases
5.2 Phases
5.3 Nature of Heat

Chapter 6  