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Calculating the Change of Enthalphy Using Calorimeter



When the changes of enthalpy or heat involved in the chemical reactions can be determined using a mathematical calculation, the amount of the heat in a chemical reaction can be measured using an experiment in the laboratory. Measurement heat reaction can be carried out using a tool called a calorimeter.

Calorimeter is an instrument used to measure the amount of heat given or taken in a particular process. A simple thermometer consists of a vessel isolation, mixer equipment, and thermometers.
Generally, the calorimeter wrapped by heat insulator material, such as polystyrene or styrofoam. The material can be used to reduce heat exchange between the system and environment, so that the pressure in the calorimeter is relatively fixed. It is because the measurement of heat using calorimeter should be done on the constant pressure. How do I measure heat using calorimeter? To understand how to measure heat using calorimeter, pay attention to the following explanation .
To measure heat using calorimeter, a heat source stored in the calorimeter and the water is stirred until equilibrium is reached, then the increase of temperature noted by reading the thermometer. In this case, the number of heat were released by the system in the calorimeter can be calculated.
The pressure in the calorimeter is relatively fixed, then the change in heat of system is equal with the enthalpy changes. This is expressed with the following equation.
ΔH = Q
Because the calorimeter vessel wrapped using the insulator material, they have not considered the heat absorbed and released by the system to and from the environment, so that heat of system is equal to zero.

Qreaction + Qcalorimeter + Qsolution = Qsystem
Qreaction + Qcalorimeter + Qsolution = 0
Qreaction = - (Qcalorimeter + Qsolution)

The heat of calorimeter is influenced by the heat capacity (C). If the heat capacity is relatively small (the heat pf calorimeter can be ignored), then the above equation can be written as follows.

Qreaction =-Qsolution
Qsolution = m c ΔT

with:
Q = heat (Joule)
m = solution mass (g)
c = the specific heat of solution (J/goC)
ΔT = Tfinal - Tinitial = the change in temperature (o or K)
Note:
Specific heat is the amount needed to raise the temperature of one gram of substance by one degree Celcius at a constant pressure.
The calculation above is used if the heat capacity of calorimeter is ignored, but if the quantity is considered, the heat fo the calorimeter must be involved in the calculated. In this case, the amount of the heat calorimeter can be determined using the following equation.

Qcalorimeter = Ck ΔT

With:
Ck = capacity of calorimeter (J/goC)

So
Qreaction = - (Qcalorimeter + Qsolution)

In fact, the number of heat absorbed by calorimeter is relatively small than the heat absorbed ny the solution, so that in some types of calorimeter, the price of CkΔT can be ignored.

The heat measurement using a calorimeter as discussed above are generally made for the reactions involving the system in the form of solution .However, for chemical reactions involving a combustion or reaction to determine the amount of energy contained in food the more accurate calorimeter is used, that is bomb calorimeter.
Basically, a bomb calorimeter consisting of a chamber where the substance is burned, the water filled into the chamber, a stirrer, a thermometer, and the contact wire.
The combustion reactions that occur inside the bomb will produced heat absorbed by the water and the bomb at the same temperature, so that the water temperature increases. Because there is no heat transfer between systems and environment during the reaction, then:
Qreaction = - (Qcalorimeter + Qwater)
= - (ΔT Cbomb + m Cwater ΔT)
where:
Cbomb = heat capacity of the bomb calorimeter (J/goC)

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