Thermal effects of chemical reactions (2)

4.
The relationship between Q _p and Q _v

From the definition of enthalpy H=U+pV, we get

△ _r H=△ _r U+p△V

which is

Q _p =Q _V +p△V

(1) For a reaction without gas participation, the volume change before and after the reaction is small, and the volume work can be ignored, then

△ _r H≈△ _r U

Q _p ≈Q _V

In the reaction without gas, the heat of reaction at constant pressure is basically the same as the heat of constant volume reaction
.

(2) For reactions involving gas, Q _p and Q _{V of the} same reaction are not necessarily equal
.

due to

p△V=△(pV)=△nRT

therefore

Qp=QV+△nRT

or

△ _r H=△ _r U+△nRT

In the formula, Δn is the difference in the amount of gas before and after the reaction
.

Obviously, when the amount of matter in the reactant and product gas is equal (△n=0), and the volume work is 0, then

Q _p ≈Q _V

(3) For the same reaction, whether it is completed by a constant pressure approach or a constant volume approach, the enthalpy change of the reaction is the same, and the change of the thermodynamic energy is also the same
.

5.
Molar heat of reaction

The constant volume reaction heat is divided by the reaction progress to obtain the molar constant volume reaction heat △ _r U _m , namely

The constant pressure reaction heat divided by the reaction progress gives the molar constant pressure reaction heat △ _r H _m , namely

Divide both sides of △ _r H=△ _r U+△nRT by the reaction progress ξ, we get

Obviously, so there is

△ _r H _m =△ _r U _m +△vRT

In the formula, Δv is the difference in the stoichiometric number of gas phase substances in the reaction formula; the units on both sides of the equal sign are both J·mol ^-1 or kJ·mol ^-1
.

[Example 2-3] The reaction N ₂ (g)+3H ₂ (g)=2NH ₃ (g) is carried out in a constant-volume calorimeter.

The solution reaction is carried out under constant volume conditions, then the reaction progress is

Substituting Qv=-82.

Related Links: Thermal Effects of Chemical Reactions (1)