Determination DH Through Experiments
Enthalpy change of the reaction can be determined by using a device called a calorimeter (heat meter). In the calorimeter, a substance that will be reacted reaction put into place. The place is surrounded by water of known mass. Heat of reaction liberated absorbed by water and the water temperature will rise. Changes in water temperature is measured with a thermometer. Calorimeter placed in an insulated container filled with water to prevent the escape of heat.
Figure Calorimeter
Based on this research, to raise the temperature of 1 kg of water at 10C required heat of 4.2 kJ or 1 kcal. Water required to heat 1 gram of 4.2 A or 1 cal. Amount
of heat is called the specific heat of water with the symbol c.Jumlah
heat is absorbed into the water is calculated by multiplying three
factors: the mass of water in the calorimeter (g), changes in water
temperature (0C), and the specific heat of water. The formula is written: q = heat released or absorbed
m = mass of water (g)
c = heat capacity of water (J)
Dt = change in temperature (0C)
b. DH Determination Under DHf0
Based
on the change in the standard enthalpy of formation of substances that
are present in the reaction, the reaction enthalpy change can be
calculated by the formula:
DHr0 = standarPerubahan reaction enthalpy change enthalpy of formation of several substances can be seen in the table below.
Table: Changes in the enthalpy of formation of some substance (t = 250C)
Enthalpy change of reaction sometimes can not be determined directly but must go through the stages of the reaction. For example, to determine the change in enthalpy of formation of CO2 can be done in various ways.
In one way, the reaction is a single stage, while a 2 way and 3 way last two stages. Apparently a number of ways, the same enthalpy change is -394 kJ.
A
scientist, German Hess, had done some research enthalpy change and the
result is that the reaction enthalpy change of a reaction does not
depend on the course of reaction, whether the reaction takes place one
stage or several stages. This discovery is known as Hess's Law, which reads:
According
to Hess's study, the change in enthalpy of a reaction can not be
determined with the calorimeters can be determined by calculation. Here is an example of the determination of the enthalpy change calculations.
c. Determination DH Based Energy Association
A chemical reaction caused by chemical bonds breaking and formation of chemical bonds of the new. At
the time of the formation of chemical bonds of atoms will be the
release of energy, whereas the energy required to break the tie. The amount of energy required to break the tie antaratom in 1 mole of gaseous molecules called bonding energy. The stronger the bond the greater the energy required. Some bond energy prices can be seen in the following table:
Table: Some bond energy prices
Price of the bond energy can be used to determine the H a reaction.
With
this formula can also be determined average bond energy of a molecule
and the energy needed to break a bond or bond dissociation energy of a
molecule. Here is an example calculation using the DH bond energy prices.
