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1.
The concept of standard molar heat of combustion
The heat of formation is defined by reference to the zero point of the melting of the elemental substance
.
For substances that can undergo combustion reactions, the melting point of the combustion products can be used as a reference
For example, the combustion reaction of methane
CH 4 (g)+2O 2 (g)=CO 2 (g)+2H 2 O (1)
If the roasting value of the combustion products of methane CO 2 (g) and H 2 O (1) is specified to be 0, the thermal effect of methane combustion reaction is the relative melting value of methane
.
Thermodynamics stipulates that the thermal effect when 1 mol of a substance is completely burnt under standard atmospheric pressure is called the standard molar combustion heat of the substance, referred to as standard combustion heat or combustion heat, and is represented by the symbol △ c H m Θ (where the subscript c is the English combustion Prefix, which means "burning"), the unit is kJ·mol -1
According to the definition of combustion heat, the thermal effect of methane combustion reaction is the combustion heat of methane
.
Thermodynamically stipulates that complete combustion means: carbon→CO 2 (g), hydrogen→H 2 O(1), nitrogen→N 2 (g), sulfur→SO 2 (g), chlorine→HCI(aq)
.
The heat of combustion of these combustion products is zero
2.
Find the reaction heat from the standard molar combustion heat
As shown in Figure 2-8, Pathway I is the combustion process of reactants, Pathway II is the combustion process of products, and Pathway III is the process of converting reactants into products
.
Figure 2-8 The relationship between combustion heat and reaction heat
Derived from Hess's law
△ r H m (I)=△ r H m (Ⅱ)+△ r H m (Ⅲ)
which is
△ r H m (Ⅲ)=△ r H m (I)-△ r H m (Ⅱ)
due to
therefore
The thermal effect of a chemical reaction can be obtained from the standard combustion heat of the reactants and the standard combustion heat of the products
[Example 2-5] Calculate the thermal effect of the following reaction from the standard heat of combustion △ r H m Θ
2HCHO(g)+2H 2 O(l)=2CH 3 OH(l)+O 2 (g)
The heat of combustion of each substance is obtained by solving the look-up table as
The thermal effect (enthalpy change) of the reaction is
