Conditions of irreversible chemical reaction. Reversible and irreversible chemical reactions
One of the most important characteristics of the chemical reaction is the depth (degree) of the transformation, showing how much the starting materials are converted into reaction products. What it is more, the more economical one can carry out the process. The transformation depth, in addition to other factors, depends on reversibility reversibility.
Reversible Reactions , unlike irreversible, not to the end flow: none of the reacting substances are fully consumed. At the same time, the reaction of the reaction products with the formation of starting materials is underway.
Consider examples:
1) In a closed vessel at a certain temperature, equal volumes of gaseous iodine and hydrogen were introduced. If the collisions of the molecules of these substances occur with the desired orientation and sufficient energy, then chemical ties They can be rebuilt with the formation of an intermediate compound (activated complex, see clause 1.3.1). Further restructuring of bonds can lead to the decay of the intermediate compound into two molecules of iodide hydrogen. Reaction equation:
H 2 + I 2 ® 2HI
But iodide hydrogen molecules will also randomly face hydrogen molecules, iodine and among themselves. In the collision of Hi molecules, nothing hurts to form an intermediate connection, which can then decompose on iodine and hydrogen. This process is expressed by the equation:
2HI ® H 2 + I 2
Thus, in this system, two reactions will be placed at the same time - the formation of iodide hydrogen and its decomposition. They can be expressed by one general equation
H 2 + I 2 "2HI
The reversibility of the process shows the sign ".
Reaction aimed at this case In the direction of the formation of iodide hydrogen, is called direct, and the opposite - reverse.
2) if you mix the two mole of sulfur dioxide with one mole of oxygen, create conditions conducive to the reaction in the system, and after the expiration of the gas mixture, the results will show that the system will be present as SO 3 - the reaction product and the source Substances - SO 2 and O 2. If, in the same conditions as a starting material, placing sulfur oxide (+6), it will be possible to find that part of it will decompose on oxygen and sulfur oxide (+4), and the final ratio between the amounts of all three substances will be the same as In the case when it came from a mixture of sulfur dioxide and oxygen.
Thus, the interaction of sulfur dioxide with oxygen is also one example of a reversible chemical reaction and is expressed by the equation
2SO 2 + O 2 "2SO 3
3) Iron interaction with hydrochloric acid flows according to the equation:
Fe + 2HCl ® FECL 2 + H 2
With a sufficient amount of hydrochloric acid, the reaction will end when
all iron is spent. In addition, if you try to carry out this reaction in the opposite direction - to pass hydrogen through the solution of iron chloride, then the metal iron and hydrochloric acid will not work - this reaction cannot go in the opposite direction. Thus, the interaction of iron with hydrochloric acid is an irreversible reaction.
However, it should be borne in mind that theoretically, any irreversible process can be represented in certain conditions reversible, i.e. In principle, all reactions can be considered reversible. But very often one of the reactions clearly prevails. This happens when the interaction products are removed from the reaction area: a precipitate falls out, gas is released, with ion exchange reactions, almost unfinicing products are formed; Or when due to the explicit excess of the starting materials, the opposite process is practically suppressed. Thus, a natural or artificial elimination of the possibility of reverse reaction leaks allows you to bring the process to almost completely.
Examples of such reactions can be the interaction of sodium chloride with silver nitrate in solution
NaCl + AGNO 3 ® AgCl¯ + Nano 3,
copper bromide with ammonia
CUBR 2 + 4NH 3 ® BR 2,
neutralization of hydrochloric acid solution of caustic satellite
HCl + NaOH ® NaCl + H 2 O.
These are all examples only practically irreversible processesSince silver chloride is somewhat soluble, and the complex cation 2+ is not absolutely stable, and water dissociates, although in an extremely minor degree.
Reversible and irreversible chemical reactions. Chemical equilibrium. Saving equilibrium under the action of various factors
Chemical equilibrium
Chemical reactions flowing in one direction are called irreversible.
Most chemical processes are reversible. This means that with some and the same conditions there are direct, and reverse reaction (especially if we are talking about closed systems).
For example:
a) reaction
$ Caco_3 (→) ↖ (t) Cao + Co_2 $
in open system irreversible;
b) the same reaction
$ Caco_3⇄cao + co_2 $
in the closed system reversible.
Consider in more detail the processes occurring during reversible reactions, for example, for the conditional reaction:
Based on the law of the active masses, the speed of direct reaction
$ (υ) ↖ (→) \u003d k_ (1) · c_ (a) ^ (α) · c_ (b) ^ (β) $
Since with the time of the concentration of substances $ a $ and $ per $ decrease, the speed of direct reaction is also reduced.
The appearance of the reaction products means the possibility of reverse reaction, and with the concentration time of substances $ C $ and $ D $ increases, which means that the rate of reverse reaction increases:
$ (υ) ↖ (→) \u003d k_ (2) · c_ (c) ^ (γ) · c_ (d) ^ (δ) $
Sooner or later, a condition will be achieved in which the speeds of direct and reverse reactions will become equal
${υ}↖{→}={υ}↖{←}$
The state of the system, in which the rate of direct reaction is equal to the rate of reverse reaction, is called chemical equilibrium.
At the same time, the concentrations of reacting substances and reaction products remain unchanged. They are called equilibrium concentrations. At the macro level it seems that in general, nothing changes. But in fact and direct, and reverse processes continue to go, but with equal speed. Therefore, such equilibrium in the system is called movable and dynamic.
Equilibrium constant
Denote the equilibrium concentrations of substances $ [a], [b], [c], [d] $.
Then since $ (υ) ↖ (→) \u003d (υ) ↖ (←), k_ (1) · [a] ^ (α) · [b] ^ (β) \u003d k_ (2) · [c] ^ (Γ) · [d] ^ (Δ) $, from where
$ ([C] ^ (Γ) · [d] ^ (Δ)) / ([a] ^ (α) · [b] ^ (β)) \u003d (k_1) / (k_2) \u003d k_ (equal) $
where $ γ, δ, α, β $ are indicators of degrees equal to coefficients in a reversible reaction; $ K_ (equal.) $ - chemical equilibrium constant.
The resulting expression quantitatively describes the state of equilibrium and is a mathematical expression of the law of active masses for equilibrium systems.
With a constant equilibrium constant temperature, the value is permanent for a reversible reaction. It shows the relationship between the concentrations of the reaction products (numerator) and the starting materials (denominator), which is installed in equilibrium.
The equilibrium constants are calculated from experienced data, determining the equilibrium concentrations of the starting materials and the reaction products at a certain temperature.
The value of the equilibrium constant characterizes the yield of the reaction products, the completeness of its flow. If $ k_ is obtained (equal) \u003e\u003e $ 1, this means that with equilibrium $ [C] ^ (γ) · [d] ^ (Δ) \u003e\u003e [a] ^ (α) · [b] ^ ( β) $, i.e., the concentrations of the reaction products prevail over the concentrations of the source substances, and the yield of the reaction products is large.
With $ k_ (equal)
$ Ch_3coooc_2h_5 + h_2o⇄ch_3cooh + c_2h_5oh $
equilibrium constant
$ K_ (equal) \u003d (·) / (·) $
at $ 20 ° C $ is $ 0.28 $ (i.e. less than $ 1 $). This means that a significant part of the ether is not hydrolyzed.
In the case of heterogeneous reactions to the expression, the equilibrium constant includes concentrations of only those substances that are in the gas or liquid phase. For example, for reaction
the equilibrium constant is expressed as:
$ K_ (equal) \u003d (^ 2) / () $
The value of the equilibrium constant depends on the nature of the reactant substances and the temperature of the tours.
The constant does not depend on the presence of the catalyst, it does not change the activation and direct energy and the reverse reaction to the same value. The catalyst can only accelerate the equilibrium offensive without affecting the value of the equilibrium constant.
Saving equilibrium under the action of various factors
The state of equilibrium is preserved arbitrarily for a long time with constant external conditions: the temperature, the concentration of the starting materials, pressure (if the reaction is involved or formed).
By changing these conditions, you can translate the system from one equilibrium state to another that meets new conditions. Such a transition is called displacement or equilibrium shift.
Consider different methods Deviations of equilibrium on the example of the reaction of the interaction of nitrogen and hydrogen with the formation of ammonia:
$ N_2 + 3h_2⇄2hn_3 + q $
$ K_ (equal) \u003d (^ 2) / (· ^ 3) $
The effect of changes in the concentration of substances
When the concentration of these gases is increased to the reaction mixture of nitrogen $ N_2 $ and hydrogen, and the concentration of these gases increases, which means that the rate of direct reaction increases. Equilibrium shifts to the right, towards the product of the reaction, i.e. in the direction of ammonia $ nh_3 $.
The same conclusion can be done by analyzing the expression for the equilibrium constant. With an increase in the concentration of nitrogen and hydrogen, the denominator increases, and since $ k_ (equal) $ is a permanent value, the numerator should increase. Thus, the amount of the $ NH_3 reaction product will increase in the reaction mixture.
An increase in the concentration of the ammonia reaction product of $ NH_3 $ will lead to a displacement of equilibrium to the left, towards the formation of the source substances. This conclusion can be made on the basis of similar reasoning.
Influence of pressure change
The change in pressure is influenced only on those systems, where at least one of the substances is in a gaseous state. With an increase in pressure, the volume of gases is reduced, which means that their concentration increases.
Suppose that the pressure in the closed system has increased, for example, at $ 2 $. This means that the concentrations of all gaseous substances ($ n_2, h_2, NH_3 $) in the reaction under consideration will increase by $ 2 $. In this case, the numerator in the expression for $ k_ (equal) $ will increase 4 times, and the denominator is at $ 16 $ once, i.e. Equilibrium will break. For its recovery, the concentration of ammonia should increase and the concentrations of nitrogen and hydrogen should decrease. Equilibrium will shift to the right. The change in pressure is practically not affected by the volume of liquid and solid tel. Does not change their concentration. Consequently, the condition of the chemical equilibrium of reactions in which gases do not participate, does not depend on pressure.
The effect of temperature change
When increasing the temperature, as you know, the speed of all reactions (exo-endothermic) increase. Moreover, the temperature increases more affects the speed of those reactions that have greater activation energy, and therefore endothermic.
Thus, the rate of reverse reaction (in our example endothermic) increases stronger than the speed of direct. Equilibrium will shift towards the process accompanied by energy absorption.
The direction of displacement of equilibrium can be predicted by using the principle of Le Chateel (1884):
If there is an external effect on the system in equilibrium, the concentration, pressure, temperature) is changed, then the equilibrium is shifted to the other side that weakens this impact.
Make conclusions:
- with an increase in the concentration of reactant substances, the chemical equilibrium of the system shifts towards the formation of reaction products;
- with an increase in the concentration of reaction products, the chemical equilibrium of the system is shifted towards the formation of starting materials;
- with an increase in pressure, the chemical equilibrium of the system shifts towards the reaction in which the volume of the resulting gaseous substances is less;
- with increasing temperature, the chemical equilibrium of the system shifts towards the endothermic reaction;
- with a decrease in temperature - towards the exothermic process.
The principle of Le Chateel is applicable not only to chemical reactions, but also to many other processes: evaporation, condensation, melting, crystallization, etc. In the manufacture of the most important chemical products, the principle of Le Chateel and calculations arising from the law of the active masses make it possible to find such conditions for conducting chemical processes that provide the maximum yield of the desired substance.
The lesson will consider the topic "Reversible and irreversible chemical reactions. Chemical equilibrium"The factors affecting a chemical equilibrium displacement will be considered. You will get acquainted with the principle of Le Chatel. The concept of reversible and irreversible reactions is introduced.
Topic: Reaction classification, thermochemistry, speed
Lesson: reversible and irreversible chemical reactions. Chemical equilibrium and ways to displace
Consider some abstract reaction that we write in the form:
A + B → AB, direct reaction. But many chemical reactions can go in the opposite direction.
AUA + in; Reverse reaction.
For brevity, this reaction is recorded using two arrows, one-forward, other - back.
A + B.AU
With an increase in temperature, the speed of most chemical reactions increases. But it turns out that in the case of some reactions, the reaction product at a temperature when it comes with good speed, it is already beginning to decompose. In particular, this situation is implemented in the interaction of hydrogen with iodine upon receipt of iodorodor.
H 2 +I. 2 (1)
The rate of chemical reaction increases with an increase in the concentration of the starting materials and, accordingly, decreases with a decrease in the concentration of the starting materials. It turns out that, as the reactions pass, the rate of direct reaction will decrease, since the initial substances will be spent. And the rate of reverse reaction will increase, because the concentration of the substance AV source for the reverse reaction will gradually increase. How long will the speed of direct reaction be reduced, and return to increase? It will be until the moment when the rates of direct and reverse reaction will become equal. Chemical equilibrium will come. Fig. one.
Fig. one
Chemical equilibrium is the state of the reaction system, in which the speeds of direct and reverse reaction are equal.
Equilibrium concentration of substances - It is the concentration of substances in the reaction mixture in a state of chemical equilibrium. Equilibrium concentration is designated chemical formula Substances concluded in square brackets.
For example, the following entry denotes that the equilibrium concentration of hydrogen in the equilibrium system is 1 mol / l.
Fig. 2.
Chemical equilibrium(Fig. 2) is different from the concept of "equilibrium" familiar to us. Chemical equilibrium is dynamic. In a system in a state of chemical equilibrium, straight and reverse reaction occur, but their speeds are equal, and therefore the concentration of participating substances do not change. Chemical equilibrium is characterized by a constant equilibrium, equal to attitude direct and reverse speeds reactions.
The rate constants of the direct and reverse reaction are the velocities of this reaction at concentrations of the initial substances for each of them in equal units. Also, equilibrium constant is equal to the ratio of equilibrium concentrations of direct reaction products in the degrees of stoichiometric coefficients to the product of equilibrium concentrations of reagents.
If a 
, then in the system there are more source substances. If a 
The system has more reaction products.
If equilibrium constant is significantly more than 1, such a reaction is called irreversible.
Irreversible are the chemical reactions that occur only in one direction until one of the reagents is full.
For example, this is a reaction:
4P + 5O 2 \u003d 2P 2 O 5(2)
Reversible are chemical reactions that are carried out in mutually opposite directions With the same conditions.
If you change the external conditions, the condition of chemical equilibrium will violate. Displacement of equilibrium depending on the change in external conditions in general Determined
· The principle of Le Chateel: If the system, which is in equilibrium, is exposed to externally by changing any of the conditions determining the equilibrium position, then it shifts in the direction of the process that weakens the effect of the impact.
Thus, the temperature increase causes an equilibrium displacement towards that of the processes, the flow of which is accompanied by the absorption of heat, and the temperature decrease in the opposite direction.
Equilibrium is shifted to the right if the equilibrium concentrations of direct reaction products increased. If the equilibrium concentrations of the source substances of the direct reaction increase, the equilibrium is shifted to the left. What factors can be changed to displays the balance? it
· Temperature
· Pressure
· Concentration of substances
· Adding catalyst
· Changes in the area of \u200b\u200bthe reaction surface of heterogeneous reactions
Adding a catalyst and a change in the area of \u200b\u200bthe reaction surface of heterogeneous reactionsdo not affect the displacement of chemical equilibrium.
The remaining factors are considered in more detail.
Temperature
Ammonia synthesis reaction (Fig. 3)
refers to exothermicreactions. When the direct response of the heat is allocated, and when the reverse pass is absorbed. If you increase the temperature, then, according to the rule of Le Chatel, the balance will shift in this direction to reduce this impact. In this case left Since the warmth is absorbed. Ammonia synthesis reaction is carried out at a temperature of about 500
If the reaction endothermic, then the temperature rise will lead to equilibrium displacement right.
Changes in substance concentration
With an increase in the concentration of any of the substances involved in an equilibrium reaction, the reaction equilibrium will shift towards its spending, and, accordingly, with a decrease in the concentration of any of the substances - towards the reaction of its formation. For example, with an increase in nitrogen concentration in the ammonia synthesis reaction, the equilibrium will shift to the right, that is, in the direction of nitrogen spending. If in this reaction is removed from the reaction mixture of ammonia, then the balance will shift towards its formation. This can be done, for example, when dissolving ammonia in water.
Change pressure
Changing pressure can affect only reactions involving gaseous substances. If in the reaction of the synthesis of ammonia increase the pressure, the equilibrium will shift towards the decrease in the number of mol of gas. If the number of mole gas is greater than on the right, the equilibrium will shift towards the form of ammonia.
If the number of mol of gas is equally left and right, for example, in the reaction of nitrogen oxide (II),
N. 2 + O. 2 (3)
the change in pressure will not affect the position of chemical equilibrium in such reactions. The study of chemical equilibrium has great importance, as for theoretical studiesand to solve practical tasks. Determining the equilibrium position for various temperatures and pressures, you can choose the most favorable conditions for the chemical process. The final choice of conditions requires accounting for their influence and the speed of the process.
Summing up lesson
At the lesson, the topic "Chemical equilibrium" was studied, the conditions of equilibrium displacement in the event of reversible reactions were considered.
Bibliography
1. Rudzitis G.E. Chemistry. Basics general Chemistry. Grade 11: Textbook for general education institutions: a basic level of / G. Rudzitis, F.G. Feldman. - 14th ed. - M.: Enlightenment, 2012.
2. Popel P.P. Chemistry: 8 cl.: Textbook for general education educational institutions / P.P. Popel, hp skill. - K.: IC "Academy", 2008. - 240 s.: Il.
3. Gabrielyan O.S. Chemistry. Grade 11. A basic level of. 2nd ed., Ched. - M.: Drop, 2007. - 220 s.
1. Electricity. The difference between polar covalent and ionic connections ().
3. Electric monitance of atoms by Pauling ().
Homework
1. №№7-9 (p. 63) Rudzitis G.E. Chemistry. Basics of general chemistry. Grade 11: Tutorial for general education institutions: Basic level / G.E. Rudzitis, F.G. Feldman. - 14th ed. - M.: Enlightenment, 2012.
2. What factors do not affect chemical equilibrium displacement?
3. Name the conditions for chemical equilibrium.
\u003e\u003e Chemistry: reversible and irreversible reactions
CO2 + H2O \u003d H2CO3
Let us leave the resulting acid solution to stand in a tripod. After some time, we will see that the solution became violet again, as the acid was decomposed on the starting materials.
This process can be carried out much faster if a third solution coalic acid. Consequently, the reaction of the production of coalic acid proceeds as in the straight, so n in the opposite direction, that is, is reversible. The reversibility of the reaction is denoted by two opposite directed arrows:
Among the reversible reactions underlying the acquisition of the most important chemical products, the synthesis (compound) of sulfur oxide (VI) from sulfur oxide (IV) and oxygen is called as an example.
1. Reversible and irreversible reactions.
2. Barroll rule.
Record the combustion reaction equations referred to in the text of the paragraph, gaping that as a result of these reactions, oxides of those elements were formed from which the starting materials were constructed.
Give the characteristic of the three recent reactions, carried out by the end of the paragraph, according to plan: a) the nature and number of reagents and products; b) aggregate state; c) direction: d) the presence of a catalyst; e) selection or heat absorption
What is the inaccuracy of the limestone firing equation proposed in the text of the paragraph?
How valid is the assertion that the connection reactions will, as a rule, are zkzotermic reactions? Justify your point of view, using the facts in the text textbook.
Design of lesson Abstract lesson reference frame presentation lesson accelerative methods interactive technologies Practice Tasks and exercises self-test Workshop, trainings, cases, quests Home tasks Discussion issues rhetorical questions from students Illustrations Audio, video clips and multimedia Photos, pictures, tables, Schemes of humor, jokes, jokes, Comics Proverbs, sayings, crosswords, quotes Supplements Abstracts Articles Chips for Curious Cheat Sheets Textbooks Basic and Additional Globes Other Terms Improving textbooks and lessons Fixing errors in the textbook Updating fragment in the textbook. Innovation elements in the lesson replacing outdated knowledge new Only for teachers Perfect lessons Calendar Plan for a year guidelines Discussion programs Integrated lessonsAll chemical reactions can be divided into two groups: irreversible and reversible reactions. Reasonable reactions proceed to the end - until the complete consolidation of one of the reactants. Reversible reactions proceed not to the end: with a reversible reaction, none of the reacting substances spent completely. This difference is due to the fact that the irreversible reaction can only flow in one direction. The reversible reaction may flow both in direct and in the opposite directions.
Consider two examples.
Example 1. Interaction between zinc and concentrated nitric acid flows according to the equation:
With a sufficient amount of nitric acid, the reaction is terminated only when the entire zinc is dissolved. In addition, if you try to carry out this reaction in the opposite direction - to pass nitrogen dioxide through the zinc nitrate solution, then the metal zinc and nitric acid will not work - this reaction cannot flow in the opposite direction. Thus, zinc interaction with nitric acid is an irreversible reaction.
Example 2. Ammonia synthesis flows according to the equation:
If one mole of nitrogen with three moles of hydrogen is mixed, to carry out the conditions conducive to the reaction flow in the system, and after a sufficient time is expired, the analysis of the gas mixture is expired, the results of the analysis will show that the system will present not only the reaction product (ammonia), but also the source Substances (nitrogen and hydrogen). If now in the same conditions as a starting material, it is not a nitrogen-hydrogen mixture as a starting material, and ammonia, then it will be possible to find that part of the ammonia will decompose on nitrogen and hydrogen, and the final ratio between the amounts of all three substances will be the same as in the event When the nitrogen mixture with hydrogen proceeded. Thus, ammonia synthesis is a reversible reaction.
In the equations of reversible reactions, instead of the sign of equality, the arrows can be put; They symbolize the reaction leakage both in direct and reverse directions.
In fig. 68 shows the change in the speeds of direct and reverse reactions over time. Initially, when mixing the starting materials, the rate of direct reaction is large, and the feedback rate is zero, as the reaction occurs, the initial substances are consumed and their concentrations are falling.
Fig. 63. Changing the speed of direct and reverse reaction over time.
As a result, the rate of direct reaction decreases. At the same time, reaction products appear, and their concentration increases. As a result, the reverse reaction begins to go, and its speed is gradually increasing. When the rates of direct and reverse reactions become the same, chemical equilibrium occurs. So, B. the last example An equilibrium is established between nitrogen, hydrogen and ammonia.
Chemical equilibrium is called dynamic equilibrium. This emphasizes that in equilibrium flows and direct, and the reverse reaction, but their speed are the same, as a result of which changes in the system are not noticeable.
The quantitative characteristic of chemical equilibrium is the value called a chemical equilibrium constant. Consider it on the example of the synthesis reaction of iodo hydrogen:
According to the law, the mass, the speed of direct and reverse reactions is expressed by the equations:
When equilibrium, the speed of direct and reverse reactions is equal to each other, from where
The ratio of the strain rate of direct and reverse reactions is also a constant. It is called the equilibrium constant of this reaction (K):
Hence finally
In the left side of this equation there are those concentrations of interacting substances that are established with equilibrium concentrations. The right side of the equation is a permanent (at a constant temperature) value.
It can be shown that in the general case of reversible reaction
the equilibrium constant will be expressed by the equation:
Here large letters Denote formulas substances, and small - coefficients in the reaction equation.
Thus, at a constant temperature of the equilibrium constant, the reversible reaction is a constant value indicating that the relationship between the concentrations of the reaction products (numerator) and the starting materials (denominator), which is set in equilibrium.
The equilibrium constant equation shows that under the conditions of equilibrium, the concentration of all substances involved in the reaction are associated between themselves. Changing the concentration of any of these substances entails changes in the concentrations of all other substances; As a result, new concentrations are established, but the ratio between them again corresponds to the equilibrium constant.
The numerical value of the equilibrium constant in the first approximation characterizes the yield of this reaction. For example, when the reaction is large, because at the same time
i.e., with equilibrium, the concentration of the reaction products has a lot more concentrations of the source substances, and this means that the reaction yield is great. When (by a similar reason), the yield of the reaction is small.
In the case of heterogeneous reactions to the expression of the equilibrium constant, as well as in the expression of the law action of the masses (see § 58), there are concentrations of only those substances that are in the gas or liquid phase. For example, for reaction
the equilibrium constant has the form:
The magnitude of the equilibrium constant depends on the nature of the reactant substances and on temperature. It does not depend on the presence of catalysts. As already mentioned, the equilibrium constant is equal to the ratio of the strain rate of the direct and reverse reaction. Since the catalyst changes the activation and direct energy and reverse reactions to the same value (see § 60), then it does not affect the ratio of the constants of their speed.
Therefore, the catalyst does not affect the value of the equilibrium constant and, therefore, can neither increase nor reduce the yield of the reaction. It can only speed up or slow down an equilibrium.
