The reaction rate is a value that shows the change in the concentration of reacting substances 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 reaction rate between some compounds in the same aggregate form depends on the volume of the substances taken. From a mathematical point of view, one can express the relationship between the speed of a homogeneous process and the change in concentration per unit time.
An example of such an interaction is the oxidation of nitric oxide (2) to nitric oxide (4).
Heterogeneous processes
The reaction rate for the starting materials in different aggregate states is characterized by the number of moles of starting reagents per unit area per unit time.
Heterogeneous interactions are characteristic of systems that have a different state of aggregation.
Summing up, we note that the reaction rate shows a change in the number of moles of the starting reagents (reaction products) over a period of time, per unit of interface or per unit volume.
Concentration
Consider the main factors affecting the reaction rate. Let's start with concentration. A similar dependence is expressed by the law of the masses. Between the product of the concentrations of substances interacting, taken to the extent of their stereochemical coefficients, and the speed of the reaction, there is a direct proportional relationship.
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 coefficient of proportionality, which for each interaction has its own value.
As the main reason for the increase in velocity, an increase in the number of collisions of reacting particles per unit volume can be noted.
Temperature
Consider the effect of temperature on the reaction rate. The processes that occur in homogeneous systems are possible only when particles collide. But not all collisions lead to the formation of reaction products. Only when particles have increased energy. When heating the reagents, 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 indicator and the speed of the process is determined by the Van Goff rule: each increase in temperature by 10 ° C leads to an increase in the speed of the process by 2-4 times.
Catalyst
Considering the factors affecting the reaction rate, we dwell on substances that are able to increase the speed of the process, that is, on catalysts. Depending on the state of aggregation of the catalyst and reacting substances, several types of catalysis are distinguished:
- homogeneous form, in which the reactants and the catalyst have the same state of aggregation;
- heterogeneous appearance when the reacting substances and the catalyst are in the same phase.
Nickel, platinum, rhodium, and palladium can be identified as examples of substances that accelerate interactions.
Inhibitors are substances that slow down the reaction.
Area of contact
What else does the reaction rate depend on? 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 precisely in an aqueous medium.
When grinding solids, measure should be observed. For example, when pyrite (iron sulfite) is converted to dust, sintering of its particles occurs in the kiln, which negatively affects the rate of oxidation 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 enter into the interaction? For example, active metals located in the electrochemical series of Beketov before hydrogen are able to interact with acid solutions, and those that are after H 2 do not have this ability. The reason for this phenomenon is the different chemical activity of metals.
Pressure
How is the reaction rate related to this quantity? Chemistry is a science that is closely related to physics, therefore the dependence is directly proportional, it is regulated by gas laws. There is a direct relationship between the quantities. And in order to understand what 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 highlight the instantaneous and average values. The average rate of chemical interaction is defined as the difference in the concentrations of reacting substances over a time period.
The obtained value has a negative value in the case when a decrease in concentration occurs, a positive value - with an increase in the concentration of interaction products.
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 ].
Chemistry Tasks
Let's look at a few examples of tasks related to determining speed.
Example 1. In a vessel, chlorine and hydrogen are mixed, then the mixture is heated. After 5 seconds, the concentration of hydrogen chloride acquired a value of 0.05 mol / DM 3 . Calculate the average rate of hydrogen chloride formation (mol / dm 3 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 / dm 3 .
We calculate the average rate of formation of hydrogen chloride:
V = 0.05 / 5 = 0.010 mol / dm 3 × s.
Example 2. In a vessel, the volume of which is 3 DM 3 the following process occurs:
C 2 H 2 + 2H 2 = C 2 H 6 .
The initial mass of hydrogen is 1 g. Two seconds after the start of the interaction, the mass of hydrogen acquired a value of 0.4 g. Calculate the average rate of ethane production (mol / dm 3 × s).
The mass of hydrogen that has 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 amount of a mole of hydrogen, it is necessary to divide it by the molar mass of this gas: n = 0.6 / 2 = 0.3 mol. According to the equation, from 2 mol of hydrogen, 1 mol of ethane is formed; therefore, from 0.3 mol of H 2 we obtain 0.15 mol of ethane.
We determine the concentration of the resulting hydrocarbon, we obtain 0.05 mol / DM 3 . Next, you can calculate the average rate of its formation: = 0.025 mol / dm 3 × 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 made the assumption that there is a connection between the masses of the starting reagents and the duration of the process. This hypothesis was confirmed in the law of the masses, established in 1867 by Norwegian chemists: P. Wage and K. Guldberg.
The study of the mechanism and speed of various processes involved in physical chemistry. 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 be able to rely on obtaining the maximum yield of reaction products with minimal energy costs, it is important to take into account the main factors that influence the process.
For example, to accelerate the process of decomposition of water into simple substances, a catalyst is needed, the role of which is played 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.