It is well known that nearly all chemical reactions are accompanied by energy changes. These changes appear ordinarily in the form of evolution or absorption of heat.
The energy given out during a chemical change appears in the form of heat. While that which is absorbed may be in the form of thermal electrical or photo energy. the amount of energy evolved or absorbed during a chemical change always remains same for the same quantities of reacting substances. All thermochemical reactions are governed by two laws, which are given below.
A.L.Lavoisier and P.S.Laplace gave this law in 1780 which states that "the enthalpy of a reaction is exactly equal but opposite in sign for the reverse reaction."For example, if DH is the enthalpy change in going from A to B then the enthalpy change for the process B to A would be -DH. Thus, the enthalpy of formation of a compound is numerically equal but opposite in sign to the enthalpy of decomposition of the compound.
Whenever a thermochemical equation is reversed the sign of DH also gets reversed.
G.H.Hess proposed a law regarding the heat or enthalpies of reaction in 1840 called the Hess's law. This law states that "the heat change in a particular reaction is the same whether it takes place in one step or several steps."For example, a reactant 'A' changes to a product 'B' in one step and the heat change during this process is DH. If the reaction is carried out in two steps where 'A' first changes to 'C' an intermediate stage and then 'C' changes to 'B' in the following step then let the heat change during the formation of 'A' to 'C' be DH1 and that from 'C' to 'B' be DH2. From Hess's law the heat change for the reaction is given as
DH = DH1 + DH2
The formation of carbon dioxide from carbon and oxygen can be illustrated as follows. Carbon can be converted into carbon dioxide in two ways. Firstly solid carbon combines with sufficient amount of oxygen to form CO2. The same reaction when carried out in the presence of lesser amount of oxygen, gives carbon monoxide which then gets converted to CO2 in step two, in the presence of oxygen.
Thus, one can conclude that thermochemical equations can be added, subtracted or multiplied like algebraic equations to obtain the desired equation.→ Read More
Hess's law has been useful in determining the heat changes of reactions, which cannot be measured directly with a calorimeter. Some of its applications are
Compounds whose heats of formation cannot be measured directly using colorimetric methods because they cannot be synthesized from their elements easily e.g. methane, carbon monoxide, benzene etc are determined using Hess's Law.For example, the heat of formation of carbon monoxide can be calculated from the heat of combustion data for carbon and carbon monoxide as shown above.
The heats of transition of allotropic modification of compounds such as diamond to graphite, rhombic sulphur to monoclinic sulfur, yellow phosphorous to red phosphorous etc. can be determined using Hess's Law.For example, the heat of transition of diamond to graphite can be calculated from the heat of combustion data for diamond and graphite, which is -395.4 kJ and -393.5 kJ respectively.
The thermochemical equations showing the combustion reaction of diamond and graphite are
This can be obtained by subtracting the second equation from the first one.
The heat of hydration of substances is calculated using Hess's law.For example the heat of hydration of copper sulphate can be calculated from the heat of solution of anhydrous and hydrated salts of copper. The heat of solution of CuSO4 and CuSO4.5H2O are -66.5 and -11.7 kJ mol-1. The corresponding thermochemical equations are:
The process of hydration can be expressed as
According to Hesss law, DH1 = DH + DH2DH = DH1 - DH2
= - 66.5 11.7 = - 78.2 kJ/mol
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