We are constantly confronted with various chemical interactions. Combustion of natural gas, rusting of iron, souring of milk are far from all the processes that are studied in detail in the school chemistry course.
For the reactions to take place, fractions of seconds are required, and for some interactions, days and weeks are needed.
Let us try to identify the dependence of the reaction rate on temperature, concentration, and other factors. In the new educational standard, the minimum amount of study time is allocated to this issue. In tests of the unified state exam, there are tasks on the dependence of the reaction rate on temperature, concentration, and even calculation tasks are offered. Many high school students have some difficulty finding answers to these questions, so we will analyze this topic in detail.
The relevance of the issue
Information on the reaction rate is of great practical and scientific importance. For example, in a specific production of substances and products, the productivity of equipment and the cost of goods directly depend on a given quantity.
Classification of ongoing reactions
There is a direct relationship between the state of aggregation of the starting components and products formed during the chemical process: heterogeneous interactions.
Under the system, it is customary to mean in chemistry a substance or their combination.
Homogeneous consider such a system, which consists of one phase (the same state of aggregation). As an example, a mixture of gases, several different liquids can be mentioned.
Heterogeneous is a system in which reactants are in the form of gases and liquids, solids and gases.
There is not only a dependence of the reaction rate on temperature, but also on which phase the components that enter into the analyzed interaction are used.
A homogeneous composition is characterized by the course of the process throughout the volume, which significantly improves its quality.
If the starting materials are in different phase states, then the maximum interaction is observed at the phase boundary. For example, when the active metal is dissolved in acid, the formation of a product (salt) is observed only on the surface of their contact.
The mathematical relationship between process speed and various factors
What is the equation of the dependence of the rate of a chemical reaction on temperature? For a homogeneous process, the rate is determined by the amount of substance that interacts or is formed during the reaction in the volume of the system per unit time.
For a heterogeneous process, the speed is determined through the amount of substance that reacts or is obtained in the process per unit area for a minimum period of time.
Factors affecting the rate of a chemical reaction
The nature of the reacting substances is one of the reasons for the different rates of the processes. For example, alkali metals form alkalis with water at room temperature, and the process is accompanied by intensive evolution of hydrogen gas. Noble metals (gold, platinum, silver) are not capable of such processes either at room temperature or when heated.
The nature of the reactants is a factor that is considered in the chemical industry in order to increase the profitability of production.
The relationship between the concentration of reagents and the speed of the chemical reaction is revealed. The higher it is, the more particles will collide, therefore, the process will proceed faster.
The law of masses in mathematical form describes a directly proportional relationship between the concentration of the starting materials and the speed of the process.
It was formulated in the mid-nineteenth century by the Russian chemist N. N. Beketov. For each process, a reaction constant is determined, which is not related either to temperature, or to concentration, or to the nature of the reacting substances.
In order to accelerate the reaction in which the solid is involved, it is necessary to grind it to a powder state.
In this case, an increase in surface area occurs, which positively affects the speed of the process. For diesel fuel, a special injection system is used, due to which, when it comes into contact with air, the rate of the combustion process of a mixture of hydrocarbons increases significantly.
Heating
The temperature dependence of the rate of a chemical reaction is explained by molecular-kinetic theory. It allows you to calculate the number of collisions between reagent molecules under certain conditions. If armed with such information, then under normal conditions all processes should proceed instantly.
But if we consider a specific example of the dependence of the reaction rate on temperature, it turns out that for the interaction it is necessary to first break the chemical bonds between the atoms so that new substances form from them. This requires a significant amount of energy. What is the temperature dependence of the reaction rate? The activation energy determines the possibility of breaking molecules; it is precisely this that characterizes the reality of processes. Its units are kJ / mol.
With insufficient energy, the collision will be ineffective, therefore, it is not accompanied by the formation of a new molecule.
Graphical representation
The temperature dependence of the rate of a chemical reaction can be represented graphically. When heated, the number of collisions between particles increases, which helps to accelerate the interaction.
What does the graph of the reaction rate versus temperature look like? The energy of molecules is deposited horizontally, and the number of particles having a high energy reserve is indicated vertically. A graph is a curve by which one can judge the rate of a specific interaction.
The greater the difference in energy from the average, the farther the point of the curve is located from the maximum, and a smaller percentage of molecules has such an energy reserve.
Important aspects
Is it possible to write the equation of the dependence of the reaction rate constant on temperature? Its increase is reflected in an increase in the speed of the process. This dependence is characterized by a certain value, called the temperature coefficient of the speed of the process.
For any interaction, the dependence of the reaction rate constant on temperature was revealed. In case of its increase by 10 degrees, the process speed increases by 2-4 times.
The temperature dependence of homogeneous reactions can be represented in a mathematical form.
For most interactions at room temperature, the coefficient is in the range from 2 to 4. For example, with a temperature coefficient of 2.9, a 100-degree increase in temperature speeds up the process by almost 50,000 times.
The dependence of the reaction rate on temperature can easily be explained by different values of the activation energy. It has a minimum value during ionic processes, which are determined only by the interaction of cations and anions. Numerous experiments indicate an instantaneous course of such reactions.
With a high value of activation energy, only a small number of collisions between particles will lead to interaction. With an average value of activation energy, the reagents will interact with an average speed.
Tasks for the dependence of the reaction rate on concentration and temperature are considered only at the senior level of training, often cause serious difficulties for children.
Process Speed Measurement
Those processes that require substantial activation energy suggest an initial break or weakening of the bonds between atoms in the starting materials. In this case, they transition to a certain intermediate state called the activated complex. It is an unstable state, it decomposes quite quickly into reaction products, the process is accompanied by the release of additional energy.
In the simplest case, the activated complex is a configuration of atoms with weakened old bonds.
Inhibitors and Catalysts
Let us analyze the dependence of the rate of the enzymatic reaction on the temperature of the medium. Such substances act as process accelerators.
They themselves are not participants in the interaction, their number after the process is completed remains unchanged. If the catalysts contribute to an increase in the reaction rate, then the inhibitors, on the contrary, slow down this process.
The essence of this is the formation of intermediate compounds, as a result of which a change in the speed of the process is observed.
Conclusion
A variety of chemical interactions occur every minute in the world. How to establish the dependence of the reaction rate on temperature? The Arrhenius equation is a mathematical explanation of the relationship between the rate constant and temperature. It gives an idea of the values of activation energy at which the destruction or weakening of bonds between atoms in molecules, the distribution of particles into new chemicals is possible.
Thanks to the molecular-kinetic theory, it is possible to predict the probability of interactions between the initial components, and to calculate the rate of the process. Among the factors that affect the reaction rate, the change in temperature, the percentage concentration of interacting substances, the surface area of contact, the presence of a catalyst (inhibitor), and the nature of the interacting components are of particular importance.