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1.
Measure the heat of reaction through experiments
The constant volume reaction heat QV (△ r U m Θ ) of the reaction can be measured by the bomb calorimeter, and the constant pressure reaction heat Q p (△ r H m Θ ) of the reaction can be measured by the cup calorimeter .
According to △ r The relationship between U m Θ and △ r H m Θ can be calculated from one kind of reaction heat
△ r H m Θ =△ r U m Θ +△vRT
2.
Calculate the reaction heat from Hess's law
The reaction can be designed as a thermodynamic cycle process, and then the heat effect of the reaction can be calculated indirectly from Hess's law; or the reaction equation of the known heat effect can be obtained by adding and subtracting the reaction of the heat effect, and then the heat effect of the reaction can be calculated
.
[Example 2-6] Known
Calculate the thermal effect △ r H m Θ of the decomposition of ZnSO 4 into ZnO and SO 3 .
The solution is to design a thermodynamic cycle based on the desired reaction
Hess Law
3.
Calculate the reaction heat from the heat of formation
Look up the table to obtain the heat of formation of various substances, and further use the heat of formation to calculate the heat of reaction
.
4.
Calculate the reaction heat from the combustion heat
If the reactants and products are substances or combustion products that can undergo combustion reactions, the heat of reaction can be calculated from the heat of combustion, especially for the calculation of the heat of organic reaction
.
5.
Estimate the reaction heat from the bond energy E
The essence of a chemical reaction is the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products
.
Breaking a chemical bond requires endothermic heat, and forming a chemical bond requires exothermic heat.
By analyzing the breakage and formation of chemical bonds during the reaction process and using bond energy data, the heat of reaction of the chemical reaction can be estimated.
The relationship is
[Example 2-7] Use bond energy to calculate the reaction heat of the following reaction
.
C 2 H 4 (g)+H 2 (g)=C 2 H 6 (g)
Solve the lookup table to get the data of the relevant key energy E
It can be seen from the reaction equation that the bonds broken during the reaction are
4 CH bonds, 1 C=C bond, 1 HH bond
The bonds formed during the reaction are
6 CH keys, 1 CC key
Therefore, the thermal effect of the reaction is
The general error in calculating the reaction heat from the bond energy is relatively large, which is related to two factors
.
In one aspect, the same type of chemically different compounds can not be the same key as ethane of Ec-c and chlorinated ethane of Ec-c is not the same
.
When the heat of reaction cannot be accurately measured, it is of practical value to estimate the heat of reaction from the bond energy
Related Links: Standard Molar Heat of Combustion