Support the Monkey! Tell All your Friends and Teachers

Help / FAQ



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

Elements
Compounds

State

Mol.wt/
atomic wt.

Cal deg.-1
gram-1

Cal deg.-1
mol-1
A ´ 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.

**********

[next chapter]

Index

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

Chapter 6





All Contents Copyright © All rights reserved.
Further Distribution Is Strictly Prohibited.


Search:
Keywords:
In Association with Amazon.com