Posted 14 November 2009on:
When the change in enthalpy or heat involved in a chemical reaction can be determined by using mathematical calculation, in some chemical reaction, the amount of the heat of a reaction can be measured thought an experiment in a laboratory. The measurement of the heat can be conducted by using an instrument called calorimeter. The figure on the bottom shows a type of calorimeter widely used in a chemical laboratory.
A calorimeter is an apparatus used to measure the amount of heat given out or taken in during a certain process. A simple calorimeter consists of an insulated container, a stirring device, and thermometer. Consider the following figure!
Generally calorimeter container is wrapped by using a kind of heat insulator materials, such as polystyrene and Styrofoam. Those materials can be use to reduce the heat exchange between system (substances in the calorimeter) and the environment, so the pressure in the calorimeter is relatively constant. It is because the measurement of heat by using a calorimeter must be conducted at a constant pressure. To comprehend this method, pay attention to the following explanation.
To measure heat using a calorimeter, heat source is placed in the calorimeter and the water is stirred until equilibrium is reached, then the rise of temperature is noted by reading the thermometer. In this case, the amount of heat released by system in the calorimeter can be calculated by the following steps.
The pressure in the calorimeter is relatively constent, the the change in heat of system is equal to the change in enthalpy. It is represented by using the following equation.
ΔH = Ǫ
Because the calorimeter container is wrapped using an insulator materials, it is considered that there is no heat absorbed and released by the system from and into the environment, so the heat of the system is equal to zero.
Ǫreaction + Ǫcalorimeter + Ǫsolution = Ǫsystem
Ǫreaction + Ǫcalorimeter + Ǫsolution = 0
Ǫreaction = – [Ǫcalorimeter + Ǫsolution]
The heat of calorimeter is influenced by its heat capacity (C). If the heat capacity is relatively small (the heat of calorimeter can be neglected), the equation above can be written as follows.
Ǫreaction = – Ǫsolution
Ǫsolution = mcΔT
Ǫ = heat (joule)
m = solution mass (g)
c = the specific heat of solution (J/g0C)
ΔT = Tfinal – Tinitial = the change in temperature (0C or K)
Note : Specific heat is the amount of heat needed to raise the temperature of one gram of a substance by one degree celcius at a constant pressure.
The calculation above is used if the heat capacity of calorimeter is neglected, but if the quantity is considered, the heat of the calorimeter must be involved in the calculation. In this case, amount of the heat of the calorimeter can be determined using the equation as follows.
Ǫcalorimeter = CKΔT
CK = the heat capacity of calorimeter (J/0C or J/K)
Ǫreaction = – [Ǫcalorimeter + Ǫsolution]
Actually, the amount of heat absorbed by a calorimeter is relatively smaller than that absorbed by the solution, so in some types of calorimeter, the value of CKΔT can be neglected.
The heat measurement using a calorimeter as it has been discussed above, generally it is done for reactions involving a system in the form of solution. However, for chemical reactions involving a combustion or a reaction to determine the amount of energy contained in foods the more accurate calorimeter is used, that is bomb calorimeter. The figure on the bottom is an example of a bomb calorimeter.
Basically, a bomb calorimeter consist of a chamber (bomb) where the substance is burned, a water filled outer chamber, a stirrer, a thermometer, and wires for ignition.
The combustion reaction happening inside the bomb will produce heat absorbed by water and the bomb at the same temperature, so the water temperature increases. Because there is no heat transfer between the system (calorimeter) and the environment during the reaction, then:
Ǫreaction = – [Ǫcalorimeter + Ǫwater]
Ǫreaction = – [CbombΔT + mcwaterΔT]
Cbomb = the heat capacity of the bomb calorimeter (J/0C or K)