Factors Affecting the Rate of Reaction

Background Information

To be more exact, rate of reaction refers to the rate at which reactants are used up or the rate at which products form in a chemical reaction. 

Ammonia synthesis refers to ammonia synthesized directly by nitrogen and hydrogen in the presence of a catalyst at high temperature and pressure, which is a basic inorganic chemical process. In the modern chemical industry, ammonia is the main raw material of the chemical fertilizer industry and basic organic chemical industry.

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The synthetic ammonia industry came into being in the early 20th century. It began to use ammonia as raw material for the gunpowder and explosive industry to serve the war. After World War I, it turned to serve agriculture and industry. With the development of science and technology, the demand for ammonia is increasing. Since the 1950s, the composition of ammonia raw materials has changed dramatically, and the synthetic ammonia industry has developed rapidly in the past 30 years.

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Thermodynamic calculations show that low temperature and high pressure are beneficial to the synthesis of ammonia, but without catalyst, the activation energy of the reaction is very high and the reaction hardly occurs. When iron catalyst was used, the activation energy of the reaction was reduced and the reaction proceeded at a remarkable rate due to the change of the reaction process. At present, it is believed that one possible mechanism of an ammonia synthesis reaction is chemical adsorption of nitrogen molecule on the surface of the iron catalyst, which weakens the chemical bonds between nitrogen atoms. Then the chemisorbed hydrogen atoms continuously interact with the nitrogen molecules on the surface of the catalyst, and gradually form – NH, – NH2 and NH3 on the catalyst surface. Finally, the ammonia molecules are desorbed on the surface to form gaseous ammonia.

Factors Influence the Rate of Reactions

Temperature Conditions

As long as the temperature is raised, the reactant molecule gains energy, which makes a part of the molecule with lower energy becomes active molecule, increases the percentage of the active molecule, and increases the number of effective collisions, so the reaction rate increases (the main reason). Of course, the higher the temperature, the faster the molecule movement rate, and the higher the number of collisions per unit time, the faster the reaction will be correspondingly (secondary cause).

Catalysts

The use of positive catalysts can reduce the energy required for the reaction, make more reactant molecules become active molecules, greatly increase the percentage of reactant molecules per unit volume, thereby increasing the reactant rate thousands of times. Negative catalysts, on the contrary, increase the reactant rate. Catalysts can only change the rate of chemical reaction, but cannot change the chemical reaction equilibrium.

Conditional concentration

When other conditions are consistent, increasing the concentration of reactants increases the number of activated molecules per unit volume, thus increasing the effective collision and the reaction rate, but the percentage of activated molecules remains unchanged. The process of the chemical reaction is a process in which atoms in the reactant molecules are recombined to form the product molecules. The atoms in the reactant molecule, in order to recombine into the molecule of the product, must first be free, that is, the chemical bonds in the reactant molecule must be broken. The breaking of chemical bonds is achieved by the collision of molecules (or ions). Not every collision can be a breaking of chemical bonds, that is, not every collision can take place chemical reactions, collisions that can take place chemical reactions are very few.

Activated molecules have higher energy than ordinary molecules, so it is possible to break chemical bonds and react. Definitely, the collision of activated molecules is only possible for chemical reactions. It is not necessarily a chemical reaction, but also a suitable orientation. When other conditions remain unchanged, for a reaction, the percentage of active molecules in the reactant is certain, that is, the number of active molecules in unit volume is proportional to the total number of reactant molecules in unit volume, that is, the number of active molecules is proportional to the concentration of reactant.

Therefore, increasing the concentration of reactants can increase the number of activated molecules, increase the number of effective collisions, increase the concentration of reactants, and increase the rate of the chemical reaction.

If there is a chemical reaction between molecules, the collision must occur, that is, the collision between molecules is a necessary condition for the reaction between molecules. But collisions do not necessarily lead to reactions.

Collisions in which molecules react are called effective collisions. Sufficient conditions for chemical reactions occur when effective collisions occur between molecules.

Molecules that collide effectively must have higher energy than ordinary molecules. The molecule that can collide effectively is called activation molecule, and the difference of energy between activation molecules and reactant molecules is called activation energy. As shown in the figure underneath, which is presenting the relationship between the energy of an exothermic reaction and the reaction process, where E is the activation energy.

Diagram: The relationship between energy of an exothermic reaction and the reaction process

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The rate of a chemical reaction depends on the number of activated molecules per unit volume and the percentage of activated molecules. The more active molecules in unit volume, the higher the percentage of active molecules, the higher the effective collision frequency and the higher the chemical reaction rate.

The more active molecules are in the reactants per unit volume, the more effective collisions occur.

The more effective collision times per unit time, the faster the reaction rate.

Thus, The higher the number of active molecules in the reactant per unit volume, the faster the reaction rate.

Behaviours to increase the number of activated molecules per unit volume:

1. Increasing the concentration of reactants increases the number of activated molecules per unit volume. (Concentration)

2. Supply energy to convert energy absorbed by non-activated molecules into activated molecules. (Temperature)

3. Decrease the activation energy and make more non-activated molecules become active molecules by reaching the activation energy. (using catalysts)

Bibliography

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