Reaction rate is a value that shows the change in the concentration of reactants over a period of time. In order to estimate its size, it is necessary to change the initial conditions of the process.
Homogeneous interactions
The rate of reaction between some compounds that are in the same aggregate form depends on the volume of the substances taken. From a mathematical point of view, it is possible to express the relationship between the rate of a homogeneous process and the change in concentration per unit of time.
An example of such an interaction is the oxidation of nitric oxide (2) to nitric oxide (4).
Heterogeneous processes
The reaction rate for starting substances in different states of aggregation is characterized by the number of moles of starting reagents per unit area per unit time.
Heterogeneous interactions are characteristic of systems that have different aggregate states.
Summarizing, we note that the reaction rate demonstrates the change in the number of moles of the initial reagents (reaction products) forperiod of time, per unit interface or per unit volume.
Concentration
Let's consider the main factors affecting the reaction rate. Let's start with concentration. Such a dependence is expressed by the law of mass action. There is a directly proportional relationship between the product of the concentrations of substances that interact, taken in terms of their stereochemical coefficients, and the speed of the reaction.
Consider the equation aA + bB=cC + dD, where A, B, C, D are liquids or gases. For the above process, the kinetic equation can be written taking into account the proportionality coefficient, which has its own value for each interaction.
As the main reason for the increase in speed, one can note an increase in the number of collisions of reacting particles per unit volume.
Temperature
Consider the effect of temperature on the reaction rate. Processes that occur in homogeneous systems are possible only when particles collide. But not all collisions lead to the formation of reaction products. Only in the case when the particles have an increased energy. When the reagents are heated, an increase in the kinetic energy of the particles is observed, the number of active molecules increases, therefore, an increase in the reaction rate is observed. The relationship between the temperature index and the process rate is determined by the van't Hoff rule: each increase in temperature by 10 ° C leads to an increase in the process rate by 2-4 times.
Catalyst
Considering the factors affecting the reaction rate, let's focus on substances that can increase the speed of the process, that is, on catalysts. Depending on the state of aggregation of the catalyst and the reactants, several types of catalysis are distinguished:
- homogeneous form, in which the reactants and the catalyst have the same state of aggregation;
- heterogeneous when reactants and catalyst are in the same phase.
Nickel, platinum, rhodium, palladium can be distinguished as examples of substances that accelerate interactions.
Inhibitors are substances that slow down a reaction.
Contact area
What else determines the rate of reaction? Chemistry is divided into several sections, each of which deals with the consideration of certain processes and phenomena. The course of physical chemistry examines the relationship between the area of contact and the speed of the process.
In order to increase the contact area of the reagents, they are crushed to a certain size. The fastest interaction occurs in solutions, which is why many reactions are carried out in an aqueous medium.
When grinding solids, measure must be observed. For example, when pyrite (iron sulfite) is converted into dust, its particles are sintered in a kiln, which negatively affects the rate of the oxidation process of this compound, and the yield of sulfur dioxide decreases.
Reagents
Let's try to understand how to determine the reaction rate depending on which reagents interact? For example, active metals located in the Beketov electrochemical series before hydrogen are able to interact with acid solutions, and those that are after H2 do not have such an ability. The reason for this phenomenon lies in the different chemical activity of metals.
Pressure
How is the reaction rate related to this value? Chemistry is a science that is closely related to physics, so the dependence is directly proportional, it is regulated by gas laws. There is a direct relationship between the quantities. And in order to understand which law determines the rate of a chemical reaction, it is necessary to know the state of aggregation and the concentration of reagents.
Types of speeds in chemistry
It is customary to single out instantaneous and average values. The average rate of chemical interaction is defined as the difference in the concentrations of reactants over a time period.
The value obtained is negative when the concentration decreases, positive when the concentration of interaction products increases.
The true (instantaneous) value is such a ratio in a certain unit of time.
In the SI system, the rate of a chemical process is expressed in [mol×m-3×s-1].
Problems in chemistry
Let's consider some examples of problems related to determining the speed.
Example 1. Inchlorine and hydrogen are mixed in a vessel, then the mixture is heated. After 5 seconds, the concentration of hydrogen chloride acquired a value of 0.05 mol/dm3. Calculate the average rate of formation of hydrogen chloride (mol/dm3 s).
It is necessary to determine the change in the concentration of hydrogen chloride 5 seconds after the interaction, subtracting the initial value from the final concentration:
C(HCl)=c2 - c1=0.05 - 0=0.05 mol/dm3.
Calculate the average rate of formation of hydrogen chloride:
V=0.05/5=0.010 mol/dm3 ×s.
Example 2. In a vessel with a volume of 3 dm3, the following process occurs:
C2H2 + 2H2=C2 H6.
The initial mass of hydrogen is 1 g. Two seconds after the start of the interaction, the mass of hydrogen has acquired a value of 0.4 g. Calculate the average rate of ethane production (mol/dm3×s).
The mass of hydrogen that reacted is defined as the difference between the initial value and the final number. It is 1 - 0.4=0.6 (g). To determine the number of moles of hydrogen, it is necessary to divide it by the molar mass of a given gas: n \u003d 0.6/2 \u003d 0.3 mol. According to the equation, 1 mole of ethane is formed from 2 moles of hydrogen, therefore, from 0.3 moles of H2 we get 0.15 moles of ethane.
Determine the concentration of the resulting hydrocarbon, we get 0.05 mol/dm3. Then you can calculate the average rate of its formation:=0.025 mol/dm3 ×s.
Conclusion
Various factors influence the rate of chemical interaction: the nature of the reacting substances (activation energy), their concentration, the presence of a catalyst, the degree of grinding, pressure, type of radiation.
In the second half of the nineteenth century, Professor N. N. Beketov suggested that there is a connection between the masses of the initial reagents and the duration of the process. This hypothesis was confirmed in the law of mass action, established in 1867 by Norwegian chemists: P. Wage and K. Guldberg.
Physical chemistry studies the mechanism and speed of various processes. The simplest processes occurring in one stage are called monomolecular processes. Complex interactions involve several elementary sequential interactions, so each stage is considered separately.
In order to obtain the maximum yield of reaction products with minimal energy costs, it is important to take into account the main factors that affect the course of the process.
For example, to accelerate the process of decomposition of water into simple substances, a catalyst is needed, the role of which is performed by manganese oxide (4).
All the nuances associated with the choice of reagents, the selection of the optimal pressure and temperature, the concentration of reagents are considered in chemical kinetics.
