Thermal effects of chemical reactions (1)

Chemical reactions are often accompanied by the release of heat or the absorption of heat.
Thermochemistry is a discipline that uses the theory and methods of thermodynamics to discuss and calculate the heat change of chemical reactions
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1 Thermal effects of chemical reactions

1.
The concept of reaction heat

In systems and reactions without non-volume work, the thermal effect of a chemical reaction (reaction heat for short) can be defined as the heat absorbed or released during the chemical reaction when the temperature of the product and the reactant are the same
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For chemical reactions, the change in thermodynamic energy △rU is (subscript r means reaction)

△ _r U=U _health- U _reverse

The first law of thermodynamics can be expressed as

△ _r U=Q+W

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Constant volume reaction heat

If the reaction is completed under constant volume conditions, △V=0, then W=-p and △V=0
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△rU=QV

It can be seen that in the constant volume reaction, the thermal effect of the system is all used to change the thermodynamic energy of the system
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3.
Constant pressure reaction heat

If the reaction is completed under constant pressure conditions, △p=0, and p is a constant
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Qp=△rU-W

by

W=-p△V

Get

Qp=△rU+p△V

=(U ₂ -U ₁ )+p(V ₂ -V ₁ )

due to

p ₁ = p ₂

therefore

Qp=(U ₂ -U ₁ )+(p ₂ V ₂ -p ₁ V ₁ )

=(U ₂ +p ₂ V ₂ )-(U ₁ +p ₁ V ₁ )

Since U, p, and V are all state functions, (U+pV) is also a state function, and a new state function can be defined accordingly
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make

H=U+pV

Therefore, during constant pressure reaction

Qp=△H

H is an additive state function, called enthalpy or thermal baking
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