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.
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