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The effect of reaction heat is a common phenomenon in chemical synthesis reactions
1.
The energy required for a molecule in a chemical reaction to change from a normal state (non-activated molecule) to an active state (activated molecule) that is prone to chemical reactions is called activation energy, that is, activation energy is the average energy (E) of the activated molecule and the reactant molecule The difference (E) of the average energy (E), in kilojoules per mole (kJ/mol)
Second, the heat of reaction
When a chemical reaction is carried out at a certain temperature, the heat released or absorbed by the reaction is the thermal effect of the reaction at this temperature, referred to as reaction heat
The heat of reaction mainly includes heat of generation, heat of combustion and heat of dissolution
The energy of a substance can be described by "enthalpy", which is a physical quantity related to the internal energy of a substance, represented by "H"
If it is under constant pressure, the reaction heat of most chemical reactions is equal to the enthalpy change, that is: △H=Qp
From the perspective of enthalpy change, △H=H(reaction product)-H(reactant); from the perspective of reaction heat, △H=energy of reaction product-energy of reactant
Exothermic reaction: △H<0 (such as the reaction of hydrogen combustion to produce water: △H=242kJ/mol), as shown in Figure 24-1
Figure 24-1 Energy change of exothermic reaction
Endothermic reaction: △H>0 (such as the reaction of water splitting to generate hydrogen and oxygen: △H=+242kJ/mol), as shown in Figure 24-2
Figure 24-2 Energy change of endothermic reaction
Most organic synthesis reactions involve a process in which old bonds are broken and new bonds are formed
Figure 24-3 The change of thermal effect in the reaction process
Related Links: The operation of the catalyst in the catalytic hydrogenation reaction