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  Calorimetry, Biology tutorial

Introduction:

Calorimetry deals, with measurement of heat energy. Term calorimetry, as it were, is composed of two key words, calorie which, by definition, is amount of heat needed to increase temperature of 1g of water by 10C and metry that is, measurement (from word meter that is device utilized for making measurements. The SI unit of heat is known as joule J, some scientists, like biologists and nutritionists, still estimate heat energy in calories cal. Heat is form of energy which flows from the part of system or body to another consequently of temperature gradient between them. Heat flows from high to low temperature parts of system.

Heat and temperature:

Concepts of heat and temperature are so intricately associated that there is requirement to illustrate distinction between them. Heat is form of energy which flows from high to low temperature parts of system. As the form of energy, SI unit of heat is joule (J). Though, there are other units of heat in common use. One of them is calorie(call). One calorie was initially defined as heat needed in raising temperature of one gram of water by one degree Celsus.

1 cal-4.184j exactly

Calorie utilized by nutritionists is really kilocalorie and is denoted Calorie (call). Temperature is degree or measure of hotness or coolness of the body. Standard SI unit of temperature is Kelvin (K). Other older scales such Fahrenheit (F) is also available. Inter-conversions among the scales are simply worked out.

Temperature is property of body which finds out direction of heat flow in it. Heat naturally flows hot to cold parts of the body.

Specific heat capacity and latent heats:

One of the effects of heat on the body is that when heat flows in the body, its temperature rises and vice versa. One of the measurements in calorimetry is skill or capacity of the material to hold or store heat flows into it. Experimental observation tells that capacity of material to store heat relies on its mass and arrangement of atoms or molecules and bonding forces which hold them together. Different substances need different amounts of heat to increase temperatures by 10C. Amount of heat needed to raise temperature of the substance by 10C is proportional to its mass and change in temperature. We define specific capacity as the amount of heat required to increase temperature of one kilogram of substance by one degree celsius.

We can measure it as follows:

Specific heat capacity= (amount of heat absorbed or given out by a body/ mass of substances x change in temperature) in J/kg0C

We must note that for a given mass of a substance, we need to measure to determine specific heat capacity is change in temperature that we may get by calorimetry. Effect of heat on substance is change of phase or state. During the change of phase, heat which flows in or out of substance goes to break or make bonds and atoms or molecules are rearranged to give rise to different state of matter. Until this process is finished, temperature of substance doesn't change. Though, amount of heat that body takes in or gives out is assessable in terms of amount of substance converted in different phase. Amount of heat needed to change phase of substance at stable temperature is proportional to its mass. State specific latent heat as amount of heat required to change/transform one kilogram of substance from one phase to another at constant temperature. We can measure this as follows:

Specific latent heat = (amount of heat absorbed or given out/ mass of substances) in J/kg

Experimental methods of Calorimetry:

1. Method of Mixtures and Heat Exchanges:

Methods depends on fact that when the hot object and cold object are in thermal contact, heat flows from hot to cold object until thermal equilibrium is set up, that is, objects achieve same temperature. In mathematical data principle can sum up as follows:

Heat lost by the hot object = Heat gained by the cold object

The method supposes that there is no heat exchange between mixture and surrounding. In practice, mixture will always be held in the container. Therefore, part of heat lost by hot object is utilized to heat up container is thermally insulted or lagged from surrounding by the poor conductor like felt or wool. The calorimeter is generally composed of copper, a good conductor so that it reaches same temperature as its content as soon as possible. It is also generally polished inside and outside so as to decrease loss of heat by radiation.

2. Electrical Methods:

Source of heat is electric heater. Power is rate of which energy is generated or expended. Therefore, power rating of heater and heat generated by it in given time interval is stated as

Heat absorbed or given out = power rating of heater x time in watts (W)

Electrical power is stated as product of voltage and current. Therefore, for given voltage, quantity of heat produced in given time by electric heater when current flows in it in the certain time interval is Heat absorbed or given out = voltage x current x time in joules (J)

Applications of Calorimetry:

Method of Calorimetry can be utilized to find out temperature of furnace by placing metal of known specific heat capacity in furnace and later removing it into calorimeter to find out change in temperature by method of mixtures. Another application of method of Calorimetry is to find out energy values of food substances.

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