Combustion ( bomb ) Calorimeter , Specific heat and calculation of the quantity of heat
Heat and temperature
Heat is a form of energy , The flow of heat from one position to another depends on the difference in temperature between them , The atoms or molecules of substances are in a continuous motion ( vibration ) , but they differ in their speed in the same substance .
Temperature is a measurement of the average kinetic energy of matter molecules and it is an indication of hotness and coldness of an object .
When the system absorbs the heat energy , the average speed of its molecules increases , This expresses the kinetic energy of the molecules and leads to the rise of temperature of the system and vice versa ( direct relationship ) .
Kinetic energy is expressed by the average speed of molecules not by speed of molecules because of the difference in speed of molecules in one substance .
There are two measuring units of quantity of heat lost or gained by the system :
Calorie is the quantity of heat needed to raise the temperature of 1 g of water by 1 ° C ( 15° C : 16° C ) , Joule is the quantity of heat needed to raise the temperature of 1 g of water by ( 1 ⁄ 4.184 ) ° C .
The Calorie is used when calculating quantity of heat gained from the food , The level of your Calorie consumption depends on the level of your activity .
For example , if you spend a day working in the library , You consume approximately 800 Calories , while a marathon runner consumes approximately 1800 Calories to complete the race .
1 Calorie = 1 kilocalorie = 1000 calorie .
Specific Heat ( Cs )
Specific heat : The quantity of heat required to raise the temperature of one gram of the substance by one degree Celsius ( 1 ° C ) , The unit used in measuring specific heat is J/g . °C .
The specific heat of water is higher than the specific heat of any other substance , The substance that has a large specific heat needs a long time to decrease or increase its temperature – like water – in contrary of the substance that has lower specific heat .
Example : The of heat required to raise the temperature of 1 g of iron by 1° C equals 0.448 J , While the quantity of heat requires to raise the temperature of 1 g of water by 1° C equals 4.18 J .
Specific heat differs according to the type of substance and its physical state , when the specific heat of copper is 0.385 J/g.°C , this means that the quantity of heat required to raise temperature of 1 g of copper by 1°C equals 0.385 J .
Specific heat is a characteristic property for the substance because specific heat is a constant value for the substance , but it differs from one substance to another and also it depends on the physical state of the substance .
Specific heat of water is higher than that of the other substances because the quantity of heat required to raise temperature of 1 g of water by 1° C is higher than that of other substance .
Water causes a moderate climate in coastal areas in both winter and summer because the specific heat of water is higher than that of the other substances , So , Water can absorb a high quantity of heat in summer and release a high quantity of heat in winter .
When two equal masses of copper and aluminum gain the same amount of heat , the temperature of copper ( specific heat = 0.385 J/g.°C ) rises much greater than the temperature of aluminum (specific heat = 0.9 J/g.°C ) because specific heat of copper is less than specific heat of aluminum .
Calculation of the quantity of heat
The quantity of heat ( absorbed or released ) is directly proportional with the difference in the temperature , The quantity of heat needed to raise or decrease the temperature of a system can be calculated by the following relation :
q = m × c × Δ T
q is the quantity of heat measured at constant pressure , m is the mass , c is the specific heat , Δ T is the change in temperature ( Δ T = T2 – T1 ) .
It is an isolated system used to determine the change in the temperature of chemical reactions Δ T by knowing each of the initial temperature T1 and the final temperature T2 , It prevents losing or gaining of any quantity of heat or substance with its surrounding .
The Calorimeter is consisted of Isolated container , Stirrer , The reactants ( represent an isolated system ) and Thermometer , There are types of Calorimeters , such as the combustion Calorimeter .
The combustion ( bomb ) Calorimeter
Usage : It is used to measure the heat of combustion of some substances .
A known amount of substance is burned in an excess amount of oxygen under constant atmospheric pressure .
It occurs in an isolated steel container called the steel bomb which is surrounded by an identified amount of the heat exchange liquid ( almost water ) .
The substance is ignited by using an electric wire , The combustion temperature is determined by measuring the change in temperature of the exchange liquid ( water ) .
Water is used in the calorimeter as a heat exchange liquid due to its high specific heat which allows it to absorb or lose a large amount of heat energy .