We are constantly confronted with various chemical interactions. The combustion of natural gas, the rusting of iron, the souring of milk are far from all the processes that are studied in detail in a school chemistry course.
Some reactions take fractions of seconds, while some interactions take days or weeks.
Let's try to identify the dependence of the reaction rate on temperature, concentration, and other factors. In the new educational standard, a minimum amount of study time is allocated for this issue. In the 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 experience certain difficulties in finding answers to these questions, so we will analyze this topic in detail.
Relevance of the issue under consideration
Information about the reaction rate is of great practical and scientific importance. For example, in a specific production of substances and products from a giventhe value directly depends on the performance of the equipment, the cost of goods.
Classification of ongoing reactions
There is a direct relationship between the state of aggregation of the initial components and products formed during the chemical process: heterogeneous interactions.
A system is usually understood in chemistry as a substance or a combination of them.
A homogeneous system is one that consists of one phase (the same state of aggregation). As an example, we can mention a mixture of gases, several different liquids.
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 the phase in which the components involved in the analyzed interaction are used.
A homogeneous composition is characterized by the flow of the process throughout the volume, which significantly improves its quality.
If the initial substances are in different phase states, in this case, the maximum interaction is observed at the phase boundary. For example, when an active metal is dissolved in an acid, the formation of a product (s alt) is observed only on the surface of their contact.
Mathematical relationship between process speed and various factors
What does the equation for the rate of a chemical reaction versus temperature look like? For a homogeneous process, the rate is determined by the amounta substance that interacts or is formed during a reaction in the volume of the system per unit of time.
For a heterogeneous process, the rate is determined through the amount of a substance reacting or produced 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 processes. For example, alkali metals form alkalis with water at room temperature, and the process is accompanied by intense evolution of gaseous hydrogen. 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 taken into account in the chemical industry in order to increase the profitability of production.
The relationship between the concentration of reagents and the speed of a chemical reaction has been revealed. The higher it is, the more particles will collide, therefore, the process will proceed faster.
The law of action of masses in mathematical form describes a directly proportional relationship between the concentration of starting substances and the speed of the process.
It was formulated in the middle of the nineteenth century by the Russian chemist N. N. Beketov. For each process, a reaction constant is determined, which is not related to temperature, concentration, or the nature of the reactants.
Toto speed up a reaction involving a solid, you need to grind it to a powder.
In this case, the surface area increases, which has a positive effect on 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 combustion of a mixture of hydrocarbons increases significantly.
Heating
The dependence of the rate of a chemical reaction on temperature is explained by molecular kinetic theory. It allows you to calculate the number of collisions between the molecules of the reagents under certain conditions. Armed with such information, 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 interaction it is necessary to first break the chemical bonds between atoms so that new substances are formed from them. This requires a significant amount of energy. What is the dependence of the reaction rate on temperature? The activation energy determines the possibility of rupture of molecules, it characterizes the reality of the processes. Its units are kJ/mol.
If the energy is insufficient, the collision will be ineffective, so it is not accompanied by the formation of a new molecule.
Graphic representation
The dependence of the rate of a chemical reaction on temperature can be represented graphically. When heated, the number of collisions between particles increases, which contributes to the acceleration of interaction.
What does a reaction rate versus temperature graph look like? The energy of molecules is plotted horizontally, and the number of particles with a high energy reserve is indicated vertically. A graph is a curve that can be used to judge the speed of a particular interaction.
The greater the energy difference from the average, the farther the point of the curve is from the maximum, and a smaller percentage of molecules have such an energy reserve.
Important aspects
Is it possible to write an equation for the dependence of the reaction rate constant on temperature? Its increase is reflected in the increase in the speed of the process. Such a dependence is characterized by a certain value, called the temperature coefficient of the process rate.
For any interaction, the dependence of the reaction rate constant on temperature has been revealed. If it is increased by 10 degrees, the process speed increases by 2-4 times.
Dependence of the rate of homogeneous reactions on temperature can be represented in 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 temperature increase of 100 degrees speeds up the process by almost 50,000 times.
Dependence of reaction rate on temperature can be easily explained by different value of activation energy. It has a minimum value during ionic processes, which are determined only by the interaction of cations and anions. Numerous experiments testify to the instantaneous occurrence of such reactions.
When the activation energy is high, only a small number of collisions between particles will lead to the implementation of the interaction. With an average activation energy, the reactants will interact at an average speed.
Assignments on the dependence of the reaction rate on concentration and temperature are considered only at the senior level of education, often causing serious difficulties for children.
Measuring the speed of a process
Those processes that need a significant activation energy involve an initial break or weakening of bonds between atoms in the original substances. In this case, they pass into a certain intermediate state, called the activated complex. It is an unstable state, rather quickly decays into reaction products, the process is accompanied by the release of additional energy.
In its simplest form, an activated complex is a configuration of atoms with weakened old bonds.
Inhibitors and catalysts
Let's analyze the dependence of the enzymatic reaction rate on the medium temperature. Such substances act as acceleratorsprocess.
They themselves are not participants in the interaction, their number after the completion of the process remains unchanged. If catalysts increase the reaction rate, then 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
Various 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 those 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, to calculate the rate of the process. Among those factors that affect the reaction rate, the change in the temperature index, the percentage concentration of interacting substances, the contact surface area, the presence of a catalyst (inhibitor), and the nature of the interacting components are of particular importance.
