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From a thermodynamic point of view, many reactions can proceed spontaneously at room temperature, but the reaction rate is too slow to obtain products, and even some reactions are extremely slow at high temperatures
.
E.
For this kind of thermodynamically possible and important application value reaction, it is an important subject for chemists to find the best catalyst through kinetic research so that the desired product can be obtained more easily
.
1.
Catalyst and catalytic reaction
Studies have shown that heating and pressurizing the mixed gas of N 2 and H 2 and adding iron powder will greatly increase the reaction rate of generating NH 3 ; heating the mixed gas of SO 2 and O 2 and adding V 2 O 5 will rapidly generate SO 3
.
For a thermodynamically spontaneous but slow reaction, if a substance is added to the reaction system to greatly accelerate the reaction rate, then this substance is called a catalyst
.
A catalyst is a substance that can change the rate of a chemical reaction, but its quality and chemical composition do not change before and after the reaction
.
Generally speaking, the catalyst is a positive catalyst, which can accelerate the reaction rate
.
For example, iron powder is a catalyst for the reaction of ammonia synthesis, and V 2 O 5 is a catalyst for the reaction of SO 2 and O 2
The reaction in which the catalyst participates is called the catalytic reaction, and the effect of the catalyst to change the reaction rate is called catalysis
.
2.
Catalysis principle and reaction characteristics
Catalytic reactions are generally divided into homogeneous catalytic reactions and heterogeneous catalytic reactions
.
Homogeneous catalytic reaction refers to the reaction in which the catalyst and the reactants are in the same phase
.
For example, the reaction of NO 2 catalyzing the oxidation of SO 2 to form SO 3 and the reaction of Mn 2+ catalyzing MnO 4 - oxidation of H 2 C 2 O 4 are all homogeneous catalyzed reactions
Heterogeneous catalytic reaction refers to a reaction in which the catalyst and the reaction system are not in the same phase, which is also called heterogeneous catalytic reaction or heterogeneous catalytic reaction
.
For example, the reaction of Fe catalyzed synthesis of ammonia and the reaction of V 2 O 5 catalyzed by the oxidation of SO 2 to SO 3 are all heterogeneous catalyzed reactions
If a certain product of the reaction has a catalytic effect on the reaction without adding a catalyst, this kind of reaction is called an autocatalytic reaction
.
Autocatalytic reactions are often homogeneous catalytic reactions
2MnO .
4 - + 6H + + 5H 2 C 2 O .
4 = 10CO 2 + 8H 2 O + 2Mn 2+
The reason why the catalyst can accelerate the reaction rate is that the catalyst changes the course of the reaction and reduces the activation energy of the reaction
.
For example, the mechanism of NO 2 catalytic oxidation of SO 2 is
SO 2 +NO 2 =SO 3 +NO E a (1)
NO+1/2O 2 =NO 2 E a (2)
The total response is
SO 2 +1/2O 2 =SO 3 E a
Since the activation energy Ea(1) and Ea(2) are both smaller than the activation energy Ea of the total reaction, NO 2 can greatly accelerate the SO 2 oxidation reaction rate
.
For chemical reactions
A+B=AB E a
Adding a catalyst (expressed as cat) to the reaction system, the course of the reaction can be expressed as
A+B+cat=Acat+B Ea (1)
Acat+B=AB+cat Ea (2)
Figure 3-5 shows the reaction process-potential energy diagram of the same reaction without catalyst and with catalyst
.
Curve I is the reaction process-potential energy diagram without catalyst
For reactions involving catalysts, the course of the reaction changes, which changes the activation energy of the reaction and reduces the activation energy of the reaction
.
According to Arrhenius's law, the reduction of activation energy greatly accelerates the reaction rate
It can be seen from Figures 3-5 that although the catalyst reduces the activation energy of the forward and reverse reactions at the same time and accelerates the rate of the forward and reverse reactions at the same time, the catalyst does not change the initial and final states of the reaction
.
The catalyst does not change the difference between the activation energy of the forward and reverse reactions, that is, the reaction heat Δ r H m Θ does not change
.
Therefore, the catalyst only changes the reaction rate and does not change the reaction equilibrium conversion rate (please refer to the chapter "Chemical Equilibrium")
.
Figure 3-5 Schematic diagram of catalyst change reaction process
