Mechanism of chemical interaction
For a chemical reaction to occur between substances, their particles must meet in space. However, far from every such contact is productive. In chemical kinetics, the concept of an active collision is introduced—this is a collision of reacting particles that is guaranteed to end in their chemical transformation.
For a collision to become active, molecules must possess a certain excess of energy. This necessary excess is called the activation energy. If the kinetic energy of the colliding molecules is less than $E_{act}$, they will simply bounce off each other, and the reaction will not occur. Thus, the activation energy acts as a kind of filter, allowing only the most «energetic» molecules into the reaction.
Effect of temperature on process rate
Temperature is one of the key factors determining how fast a chemical interaction will proceed. Its effect is directly related to the energy state of particles in the system:
- At low temperature: There are extremely few particles in the reaction mixture whose kinetic energy reaches or exceeds the activation energy value. Due to the deficit of such active molecules, most collisions are fruitless, and the chemical reaction proceeds very slowly.
- Upon temperature increase: Energy redistribution occurs in the system. The proportion of «fast» particles possessing energy greater than or equal to $E_{act}$ increases significantly. The increase in the number of active molecules leads to a sharp rise in the proportion of active collisions, as a result of which the rate of the chemical reaction increases rapidly.
Role of reacting substance concentration
The second universal factor that strictly determines the rate of a chemical reaction is the concentration of the interaction participants. This parameter affects the frequency of molecule encounters in a given space.
The mechanism of concentration influence can be described by the following logical chain:
- The higher the initial concentration of substances, the more densely particles are packed in the available volume.
- High particle density naturally leads to them colliding with each other more frequently in the process of random thermal motion.
- Increasing the total number of collisions proportionally raises the number of active collisions.
- As a consequence, the chemical reaction proceeds significantly faster.
The main regularity of this process is that the total collision frequency in the system is directly proportional to the product of the concentrations of the reacting substances.
Briefly: answers to topical questions
How do biological catalysts (enzymes) affect the activation energy of biochemical processes?
Biological catalysts (enzymes) lower the activation energy (energy barrier) required for the reaction to start. Due to this, the chemical transformation proceeds at a high rate with lower energy expenditure of the starting substances. The main factor of such a decrease is the stabilization of the transition state in the active center of the enzyme. At the same time, enzymes do not alter reaction thermodynamics, affecting only its kinetics by increasing the proportion of reactive molecules.
What is an activated complex (transition state) at the moment of reacting particle collision?
An activated complex (transition state) is an intermediate state of colliding particles formed during an active collision. It arises when the energy of interacting molecules is sufficient to overcome the energy barrier (activation energy). After the formation of this complex, a chemical transformation occurs with the release or absorption of energy, culminating in the formation of final products. If the particle energy is insufficient, they collide and fly apart unchanged without forming a transition state.
How is the empirical van 't Hoff rule on the effect of temperature on chemical reaction rate formulated?
The empirical van 't Hoff rule is formulated as follows: with every 10 °C temperature increase, the rate of most chemical reactions increases by a factor of 2–4. This dependence is approximate, as with a significant temperature difference, the inconstancy of the temperature coefficient manifests itself. The main conclusion from the rule is that the reaction rate increases exponentially with temperature rise, since the temperature variable is in the exponent.
How does the Maxwell-Boltzmann distribution explain the increase in the proportion of active molecules upon system heating?
The Maxwell-Boltzmann distribution law shows that the energy potential of molecules fluctuates around the average value, and only a certain proportion of particles possesses energy sufficient for chemical interaction. When the temperature in the system increases, the proportion of fast particles with kinetic energy equal to or exceeding the activation energy increases. Collisions of such energetic molecules become active and culminate in a chemical transformation, due to which the reaction rate increases.
Imagine a high jump: the bar is the activation energy. To jump over it (enter the reaction), you need a run-up (kinetic energy). Temperature is the force of the athlete's push, while concentration is the total number of people storming the bar at the stadium.
Class and exam questions
What is called an active collision in chemistry?
This is a collision between particles of reacting substances that successfully ends in their chemical transformation into new products.
Why do chemical reactions slow down upon cooling?
At low temperatures, too few particles remain in the system possessing sufficient kinetic energy (equal to or exceeding the activation energy) to carry out the chemical transformation.
How exactly does concentration affect collision frequency?
The frequency of particle collisions in solution or gas is directly proportional to the product of the concentrations of the reacting substances. The more densely particles are packed in the volume, the more frequently they encounter each other.
What else to study on this topic
- Regularities of chemical reaction courses
- Relationship between chemical reactions and time
- Mathematical description of concentration effect on rate
- Potential and kinetic energy of molecules in reactions
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