Chemical equilibrium with increasing temperature. Chemistry. What we learned

Chemical reactions can be reversible or irreversible.

Irreversible reactions These are reactions that go in only one (direct →) direction:

those. if some reaction A + B = C + D is irreversible, this means that the reverse reaction C + D = A + B does not occur.

Reversible reactions - these are reactions that occur in both forward and reverse directions (⇄):

i.e., for example, if a certain reaction A + B = C + D is reversible, this means that both the reaction A + B → C + D (direct) and the reaction C + D → A + B (reverse) occur simultaneously ).

Essentially, because Both direct and reverse reactions occur; in the case of reversible reactions, both the substances on the left side of the equation and the substances on the right side of the equation can be called reagents (starting substances). The same goes for products.

For any reversible reaction, a situation is possible when the rates of the forward and reverse reactions are equal. This condition is called state of balance.

At equilibrium, the concentrations of both all reactants and all products are constant. The concentrations of products and reactants at equilibrium are called equilibrium concentrations.

Shift in chemical equilibrium under the influence of various factors

Due to external influences on the system, such as changes in temperature, pressure or concentration of starting substances or products, the equilibrium of the system may be disrupted. However, after the cessation of this external influence, the system will, after some time, move to a new state of equilibrium. Such a transition of a system from one equilibrium state to another equilibrium state is called displacement (shift) of chemical equilibrium .

In order to be able to determine how the chemical equilibrium shifts under a particular type of influence, it is convenient to use Le Chatelier’s principle:

If any external influence is exerted on a system in a state of equilibrium, then the direction of the shift in chemical equilibrium will coincide with the direction of the reaction that weakens the effect of the influence.

The influence of temperature on the state of equilibrium

When temperature changes, the equilibrium of any chemical reaction shifts. This is due to the fact that any reaction has a thermal effect. Moreover, the thermal effects of the forward and reverse reactions are always directly opposite. Those. if the forward reaction is exothermic and proceeds with a thermal effect equal to +Q, then the reverse reaction is always endothermic and has a thermal effect equal to –Q.

Thus, in accordance with Le Chatelier’s principle, if we increase the temperature of some system that is in a state of equilibrium, then the equilibrium will shift towards the reaction during which the temperature decreases, i.e. towards an endothermic reaction. And similarly, if we lower the temperature of the system in a state of equilibrium, the equilibrium will shift towards the reaction, as a result of which the temperature will increase, i.e. towards an exothermic reaction.

For example, consider the following reversible reaction and indicate where its equilibrium will shift as the temperature decreases:

As can be seen from the equation above, the forward reaction is exothermic, i.e. As a result of its occurrence, heat is released. Consequently, the reverse reaction will be endothermic, that is, it occurs with the absorption of heat. According to the condition, the temperature is reduced, therefore, the equilibrium will shift to the right, i.e. towards direct reaction.

Effect of concentration on chemical equilibrium

An increase in the concentration of reagents in accordance with Le Chatelier’s principle should lead to a shift in equilibrium towards the reaction as a result of which the reagents are consumed, i.e. towards direct reaction.

And vice versa, if the concentration of the reactants is reduced, then the equilibrium will shift towards the reaction as a result of which the reactants are formed, i.e. side of the reverse reaction (←).

A change in the concentration of reaction products also has a similar effect. If the concentration of products is increased, the equilibrium will shift towards the reaction as a result of which the products are consumed, i.e. towards the reverse reaction (←). If, on the contrary, the concentration of products is reduced, then the equilibrium will shift towards the direct reaction (→), so that the concentration of products increases.

Effect of pressure on chemical equilibrium

Unlike temperature and concentration, changes in pressure do not affect the equilibrium state of every reaction. In order for a change in pressure to lead to a shift in chemical equilibrium, the sums of the coefficients for gaseous substances on the left and right sides of the equation must be different.

Those. of two reactions:

a change in pressure can affect the equilibrium state only in the case of the second reaction. Since the sum of the coefficients in front of the formulas of gaseous substances in the case of the first equation on the left and right is the same (equal to 2), and in the case of the second equation it is different (4 on the left and 2 on the right).

From here, in particular, it follows that if there are no gaseous substances among both the reactants and products, then a change in pressure will not in any way affect the current state of equilibrium. For example, pressure will not affect the equilibrium state of the reaction:

If, on the left and right, the amount of gaseous substances differs, then an increase in pressure will lead to a shift in equilibrium towards the reaction during which the volume of gases decreases, and a decrease in pressure will lead to a shift in the equilibrium, as a result of which the volume of gases increases.

Effect of a catalyst on chemical equilibrium

Since a catalyst equally accelerates both forward and reverse reactions, its presence or absence has no effect to a state of equilibrium.

The only thing a catalyst can affect is the rate of transition of the system from a nonequilibrium state to an equilibrium one.

The impact of all the above factors on chemical equilibrium is summarized below in a cheat sheet, which you can initially look at when performing equilibrium tasks. However, it will not be possible to use it in the exam, so after analyzing several examples with its help, you should learn it and practice solving equilibrium problems without looking at it:

Designations: T - temperature, p - pressure, With – concentration, – increase, ↓ – decrease

Catalyst

T	T - equilibrium shifts towards the endothermic reaction
T	↓T - equilibrium shifts towards the exothermic reaction
p	p - equilibrium shifts towards the reaction with a smaller sum of coefficients in front of gaseous substances
p	↓p - equilibrium shifts towards the reaction with a larger sum of coefficients in front of gaseous substances
c	c (reagent) – the equilibrium shifts towards the direct reaction (to the right)
	↓c (reagent) – the equilibrium shifts towards the reverse reaction (to the left)
	c (product) – equilibrium shifts towards the reverse reaction (to the left)
	↓c (product) – the equilibrium shifts towards the direct reaction (to the right)
Doesn't affect balance!!!

The state of chemical equilibrium is disrupted by various external influences on the system: heating and cooling, pressure changes, addition and removal of individual substances or solvent. As a result, the equality of the rates of forward and reverse reactions is violated and a certain shift in the state of the system occurs.

A shift in chemical equilibrium is a process that occurs in an equilibrium system as a result of an external influence.

A shift in equilibrium leads to the establishment of a new state of equilibrium in the system, characterized by changed concentrations of substances.

Example 10.6. In what direction will the equilibrium of the reaction shift when oxygen is added?

Solution. When oxygen is added, its concentration increases, and hence the speed in the forward direction. The balance will shift to the right. This increases the proportion of conversion of S0 2 to S0 3.

The displacement of equilibrium under any influence obeys Le Chatelier's principle (1884).

An external influence on a system in a state of equilibrium causes a process leading to a decrease in the result of the influence.

When deciding a specific question about the direction of the equilibrium shift, one should clearly understand the essence of the effect produced and its result. For example, a change in concentration cannot be considered as an effect on the system. Substances can be introduced or removed into the system (ego effects), resulting in a change in concentrations. The application of Le Chatelier's principle to the practically important reaction for the production of ammonia is shown in table. 10.1. The first two columns indicate the impact on the system and the result of the impact. Arrows T and >1 indicate an increase or decrease in the corresponding characteristic. The “System Response” column indicates changes that are opposite to the effect of the impact. These changes are associated with the occurrence of a direct or reverse reaction in the system. Some difficulties arise in understanding the influence of pressure on the state of equilibrium. The pressure of a gas mixture, according to the equation of gas state, depends on temperature and volume for a given amount of substance, but a system as such, having a certain volume and temperature, can respond to changes in pressure only by changing the total amount of substance as a result of the reaction. A corollary follows from Le Chatelier’s principle: with increasing pressure, the equilibrium shifts in the direction of decreasing the sum of stoichiometric coefficients for substances in the gaseous state.

Table 10.1

Application of Le Chatelier's principle using the example of the reaction N2 + 3Н2 2NH3, ArH° =-92 kJ/mol

In reversible heterogeneous reactions, a shift in equilibrium is associated with changes in the concentrations of gaseous and dissolved substances. A change in the mass of a solid does not affect the equilibrium position in the system.

Shifting chemical equilibrium is widely used when carrying out reactions in laboratories and in technological processes. In this case, we are not talking about achieving balance, but shifting it one by one. The process is planned from the very beginning so that the established equilibrium is optimal from the point of view of saving the most valuable reagents. Production costs decrease as product yield increases. It depends on temperature and pressure conditions. Using the example of the reaction for producing ammonia, the principle of the approach to choosing process conditions is shown (the signs “+” and “-” symbolize the desired or undesirable nature of the influence on the final result).

From the data presented it follows that in the production of ammonia it is desirable to use high pressure and find the most active catalysts. Temperature has a positive effect from a technological and economic point of view on the reaction rate and a negative effect on the yield of ammonia. Therefore, it is necessary to choose the optimal temperature, which ultimately ensures the minimum cost of producing the product.

Chemical equilibrium is inherent reversible reactions and is not typical for irreversible chemical reactions.

Often, when carrying out a chemical process, the initial reactants are completely converted into reaction products. For example:

Cu + 4HNO 3 = Cu(NO 3) 2 + 2NO 2 + 2H 2 O

It is impossible to obtain metallic copper by carrying out the reaction in the opposite direction, because given the reaction is irreversible. In such processes, reactants are completely converted into products, i.e. the reaction proceeds to completion.

But the bulk of chemical reactions reversible, i.e. the reaction is likely to occur in parallel in the forward and reverse directions. In other words, the reactants are only partially converted into products and the reaction system will consist of both reactants and products. The system in this case is in the state chemical equilibrium.

In reversible processes, initially the direct reaction has a maximum speed, which gradually decreases due to a decrease in the amount of reagents. The reverse reaction, on the contrary, initially has a minimum speed, which increases as products accumulate. Eventually, a moment comes when the rates of both reactions become equal—the system reaches a state of equilibrium. When a state of equilibrium occurs, the concentrations of the components remain unchanged, but the chemical reaction does not stop. That. – this is a dynamic (moving) state. For clarity, here is the following figure:

Let's say there is a certain reversible chemical reaction:

a A + b B = c C + d D

then, based on the law of mass action, we write down expressions for straightυ 1 and reverseυ 2 reactions:

v1 = k 1 ·[A] a ·[B] b

v2 = k 2 ·[C] c ·[D] d

Able chemical equilibrium, the rates of forward and reverse reactions are equal, i.e.:

k 1 ·[A] a ·[B] b = k 2 ·[C] c ·[D] d

we get

TO= k 1 / k 2 = [C] c [D] d ̸ [A] a [B] b

Where K =k 1 / k 2 – equilibrium constant.

For any reversible process, under given conditions k is a constant value. It does not depend on the concentrations of substances, because When the amount of one of the substances changes, the amounts of other components also change.

When the conditions of a chemical process change, the equilibrium may shift.

Factors influencing the shift in equilibrium:

changes in concentrations of reagents or products,
pressure change,
temperature change,
adding a catalyst to the reaction medium.

Le Chatelier's principle

All of the above factors influence the shift in chemical equilibrium, which obeys Le Chatelier's principle: If you change one of the conditions under which the system is in a state of equilibrium - concentration, pressure or temperature - then the equilibrium will shift in the direction of the reaction that counteracts this change. Those. equilibrium tends to shift in a direction leading to a decrease in the influence of the influence that led to a violation of the state of equilibrium.

So, let us consider separately the influence of each of their factors on the state of equilibrium.

Influence changes in concentrations of reactants or products let's show with an example Haber process:

N 2(g) + 3H 2(g) = 2NH 3(g)

If, for example, nitrogen is added to an equilibrium system consisting of N 2 (g), H 2 (g) and NH 3 (g), then the equilibrium should shift in a direction that would contribute to a decrease in the amount of hydrogen towards its original value, those. in the direction of the formation of additional ammonia (to the right). At the same time, the amount of hydrogen will decrease. When hydrogen is added to the system, the equilibrium will also shift towards the formation of a new amount of ammonia (to the right). Whereas the introduction of ammonia into the equilibrium system, according to Le Chatelier's principle , will cause a shift in equilibrium towards the process that is favorable for the formation of starting substances (to the left), i.e. The ammonia concentration should decrease through the decomposition of some of it into nitrogen and hydrogen.

A decrease in the concentration of one of the components will shift the equilibrium state of the system towards the formation of this component.

Influence pressure changes makes sense if gaseous components take part in the process under study and there is a change in the total number of molecules. If the total number of molecules in the system remains permanent, then the change in pressure does not affect on its balance, for example:

I 2(g) + H 2(g) = 2HI (g)

If the total pressure of an equilibrium system is increased by decreasing its volume, then the equilibrium will shift towards decreasing volume. Those. towards decreasing the number gas in system. In reaction:

N 2(g) + 3H 2(g) = 2NH 3(g)

from 4 gas molecules (1 N 2 (g) and 3 H 2 (g)) 2 gas molecules are formed (2 NH 3 (g)), i.e. the pressure in the system decreases. As a result, an increase in pressure will contribute to the formation of an additional amount of ammonia, i.e. the equilibrium will shift towards its formation (to the right).

If the temperature of the system is constant, then a change in the total pressure of the system will not lead to a change in the equilibrium constant TO.

Temperature change system affects not only the displacement of its equilibrium, but also the equilibrium constant TO. If additional heat is imparted to an equilibrium system at constant pressure, then the equilibrium will shift towards the absorption of heat. Consider:

N 2(g) + 3H 2(g) = 2NH 3(g) + 22 kcal

So, as you can see, the direct reaction proceeds with the release of heat, and the reverse reaction with absorption. As the temperature increases, the equilibrium of this reaction shifts towards the decomposition reaction of ammonia (to the left), because it appears and weakens the external influence - an increase in temperature. On the contrary, cooling leads to a shift in equilibrium in the direction of ammonia synthesis (to the right), because the reaction is exothermic and resists cooling.

Thus, an increase in temperature favors a shift chemical equilibrium towards the endothermic reaction, and the temperature drop towards the exothermic process . Equilibrium constants all exothermic processes decrease with increasing temperature, and endothermic processes increase.

Main article: Le Chatelier-Brown principle

The position of chemical equilibrium depends on the following reaction parameters: temperature, pressure and concentration. The influence that these factors have on a chemical reaction is subject to a pattern that was expressed in general terms in 1885 by the French scientist Le Chatelier.

Factors influencing chemical equilibrium:

1) temperature

As the temperature increases, the chemical equilibrium shifts towards the endothermic (absorption) reaction, and when it decreases, towards the exothermic (release) reaction.

CaCO 3 =CaO+CO 2 -Q t →, t↓ ←

N 2 +3H 2 ↔2NH 3 +Q t ←, t↓ →

2) pressure

As pressure increases, the chemical equilibrium shifts towards a smaller volume of substances, and as pressure decreases towards a larger volume. This principle only applies to gases, i.e. If solids are involved in the reaction, they are not taken into account.

CaCO 3 =CaO+CO 2 P ←, P↓ →

1mol=1mol+1mol

3) concentration of starting substances and reaction products

With an increase in the concentration of one of the starting substances, the chemical equilibrium shifts towards the reaction products, and with an increase in the concentration of the reaction products, towards the starting substances.

S 2 +2O 2 =2SO 2 [S],[O] →, ←

Catalysts do not affect the shift of chemical equilibrium!

Basic quantitative characteristics of chemical equilibrium: chemical equilibrium constant, degree of conversion, degree of dissociation, equilibrium yield. Explain the meaning of these quantities using the example of specific chemical reactions.

In chemical thermodynamics, the law of mass action relates the equilibrium activities of the starting substances and reaction products, according to the relationship:

Activity of substances. Instead of activity, concentration (for a reaction in an ideal solution), partial pressures (a reaction in a mixture of ideal gases), fugacity (a reaction in a mixture of real gases) can be used;

Stoichiometric coefficient (negative for starting substances, positive for products);

Chemical equilibrium constant. The subscript "a" here means the use of the activity value in the formula.

The efficiency of a reaction is usually assessed by calculating the yield of the reaction product (section 5.11). At the same time, the efficiency of the reaction can also be assessed by determining what part of the most important (usually the most expensive) substance was converted into the target reaction product, for example, what part of SO 2 was converted into SO 3 during the production of sulfuric acid, that is, find degree of conversion original substance.

Let a brief diagram of the ongoing reaction

Then the degree of conversion of substance A into substance B (A) is determined by the following equation

Where n proreact (A) – the amount of substance of reagent A that reacted to form product B, and n initial (A) – initial amount of reagent A.

Naturally, the degree of transformation can be expressed not only in terms of the amount of a substance, but also in terms of any quantities proportional to it: the number of molecules (formula units), mass, volume.

If reagent A is taken in short supply and the loss of product B can be neglected, then the degree of conversion of reagent A is usually equal to the yield of product B

The exception is reactions in which the starting substance is obviously consumed to form several products. So, for example, in the reaction

Cl 2 + 2KOH = KCl + KClO + H 2 O

chlorine (reagent) is converted equally into potassium chloride and potassium hypochlorite. In this reaction, even with a 100% yield of KClO, the degree of conversion of chlorine into it is 50%.

The quantity you know - the degree of protolysis (section 12.4) - is a special case of the degree of conversion:

Within the framework of TED, similar quantities are called degree of dissociation acids or bases (also designated as the degree of protolysis). The degree of dissociation is related to the dissociation constant according to Ostwald's dilution law.

Within the framework of the same theory, the hydrolysis equilibrium is characterized by degree of hydrolysis (h), and the following expressions are used that relate it to the initial concentration of the substance ( With) and dissociation constants of weak acids (K HA) and weak bases formed during hydrolysis ( K MOH):

The first expression is valid for the hydrolysis of a salt of a weak acid, the second - salts of a weak base, and the third - salts of a weak acid and a weak base. All these expressions can only be used for dilute solutions with a degree of hydrolysis of no more than 0.05 (5%).

Typically, the equilibrium yield is determined by a known equilibrium constant, with which it is related in each specific case by a certain ratio.

The yield of the product can be changed by shifting the equilibrium of the reaction in reversible processes, under the influence of factors such as temperature, pressure, concentration.

In accordance with Le Chatelier's principle, the equilibrium degree of conversion increases with increasing pressure during simple reactions, and in other cases the volume of the reaction mixture does not change and the yield of the product does not depend on pressure.

The effect of temperature on the equilibrium yield, as well as on the equilibrium constant, is determined by the sign of the thermal effect of the reaction.

For a more complete assessment of reversible processes, the so-called yield from the theoretical (yield from the equilibrium) is used, equal to the ratio of the actually obtained product to the amount that would be obtained in a state of equilibrium.

THERMAL DISSOCIATION chemical

a reaction of reversible decomposition of a substance caused by an increase in temperature.

With Etc., several (2H2H+ OCaO + CO) or one simpler substance are formed from one substance

Equilibrium etc. is established according to the law of mass action. It

can be characterized either by an equilibrium constant or by the degree of dissociation

(the ratio of the number of decayed molecules to the total number of molecules). IN

In most cases, etc. is accompanied by the absorption of heat (increase

enthalpy

DN>0); therefore, in accordance with Le Chatelier-Brown principle

heating enhances it, the degree of displacement etc. with temperature is determined

absolute value of DN. The pressure interferes with etc., the more strongly, the greater

change (increase) in the number of moles (Di) of gaseous substances

the degree of dissociation does not depend on pressure. If solids are not

form solid solutions and are not in a highly dispersed state,

then the pressure etc. is uniquely determined by the temperature. To implement T.

d. solids (oxides, crystalline hydrates, etc.)

It is important to know

temperature at which the dissociation pressure becomes equal to the external one (in particular,

atmospheric) pressure. Since the gas released can overcome

ambient pressure, then upon reaching this temperature the decomposition process

immediately intensifies.

Dependence of the degree of dissociation on temperature: the degree of dissociation increases with increasing temperature (increasing temperature leads to an increase in the kinetic energy of dissolved particles, which promotes the disintegration of molecules into ions)

The degree of conversion of starting substances and the equilibrium yield of the product. Methods for their calculation at a given temperature. What data is needed for this? Give a scheme for calculating any of these quantitative characteristics of chemical equilibrium using an arbitrary example.

The degree of conversion is the amount of reacted reagent divided by its original amount. For the simplest reaction, where is the concentration at the inlet to the reactor or at the beginning of the periodic process, is the concentration at the outlet of the reactor or the current moment of the periodic process. For a voluntary response, for example, , in accordance with the definition, the calculation formula is the same: . If there are several reagents in a reaction, then the degree of conversion can be calculated for each of them, for example, for the reaction The dependence of the degree of conversion on the reaction time is determined by the change in the concentration of the reagent over time. At the initial moment of time, when nothing has transformed, the degree of transformation is zero. Then, as the reagent is converted, the degree of conversion increases. For an irreversible reaction, when nothing prevents the reagent from being completely consumed, its value tends (Fig. 1) to unity (100%). Fig. 1 The greater the rate of reagent consumption, determined by the value of the rate constant, the faster the degree of conversion increases, as shown in the figure. If the reaction is reversible, then as the reaction tends to equilibrium, the degree of conversion tends to an equilibrium value, the value of which depends on the ratio of the rate constants of the forward and reverse reactions (on the equilibrium constant) (Fig. 2). Fig. 2 Yield of the target product Yield of the product is the amount of the target product actually obtained, divided by the amount of this product that would have been obtained if all the reagent had passed into this product (to the maximum possible amount of the resulting product). Or (through the reagent): the amount of the reagent actually converted into the target product, divided by the initial amount of the reagent. For the simplest reaction, the yield is , and keeping in mind that for this reaction, , i.e. For the simplest reaction, the yield and the degree of conversion are the same value. If the transformation takes place with a change in the amount of substances, for example, then, in accordance with the definition, the stoichiometric coefficient must be included in the calculated expression. In accordance with the first definition, the imaginary amount of product obtained from the entire initial amount of the reagent will be for this reaction two times less than the initial amount of the reagent, i.e. , and the calculation formula. In accordance with the second definition, the amount of the reagent actually transferred into the target product will be twice as large as this product was formed, i.e. , then the calculation formula is . Naturally, both expressions are the same. For a more complex reaction, the calculation formulas are written in exactly the same way in accordance with the definition, but in this case the yield is no longer equal to the degree of conversion. For example, for the reaction, . If there are several reagents in a reaction, the yield can be calculated for each of them; if there are also several target products, then the yield can be calculated for any target product for any reagent. As can be seen from the structure of the calculation formula (the denominator contains a constant value), the dependence of the yield on the reaction time is determined by the time dependence of the concentration of the target product. So, for example, for the reaction this dependence looks like in Fig. 3. Fig.3

The degree of conversion as a quantitative characteristic of chemical equilibrium. How will an increase in total pressure and temperature affect the degree of conversion of the reagent ... in a gas-phase reaction: ( the equation is given)? Provide a rationale for your answer and appropriate mathematical expressions.

The state in which the rates of forward and reverse reactions are equal is called chemical equilibrium. The equation for a reversible reaction in general form:

Forward reaction rate v 1 =k 1 [A] m [B] n, reverse reaction speed v 2 =k 2 [C] p [D] q, where in square brackets are equilibrium concentrations. By definition, at chemical equilibrium v 1 =v 2, where from

K c =k 1 /k 2 = [C] p [D] q / [A] m [B] n,

where Kc is the chemical equilibrium constant, expressed in terms of molar concentrations. The given mathematical expression is often called the law of mass action for a reversible chemical reaction: the ratio of the product of the equilibrium concentrations of the reaction products to the product of the equilibrium concentrations of the starting substances.

If an external influence is exerted on a system in a state of equilibrium, the system will move to another state in such a way as to reduce the effect of the external influence.

Factors influencing chemical equilibrium.

1. Effect of temperature. In each reversible reaction, one of the directions corresponds to an exothermic process, and the other to an endothermic process.

As the temperature increases, the chemical equilibrium shifts in the direction of the endothermic reaction, and as the temperature decreases, in the direction of the exothermic reaction.

2. Effect of pressure. In all reactions involving gaseous substances, accompanied by a change in volume due to a change in the amount of substance during the transition from starting substances to products, the equilibrium position is influenced by the pressure in the system.
The influence of pressure on the equilibrium position obeys the following rules:

As pressure increases, the equilibrium shifts towards the formation of substances (initial or products) with a smaller volume.

3. Effect of concentration. The influence of concentration on the state of equilibrium is subject to the following rules:

When the concentration of one of the starting substances increases, the equilibrium shifts towards the formation of reaction products;
When the concentration of one of the reaction products increases, the equilibrium shifts towards the formation of the starting substances.

Questions for self-control:

1. What is the rate of a chemical reaction and what factors does it depend on? What factors does the rate constant depend on?

2. Create an equation for the reaction rate of the formation of water from hydrogen and oxygen and show how the rate changes if the concentration of hydrogen is increased threefold.

3. How does the reaction rate change over time? What reactions are called reversible? What characterizes the state of chemical equilibrium? What is called the equilibrium constant, on what factors does it depend?

4. What external influences can disrupt the chemical balance? In which direction will the equilibrium mix when the temperature changes? Pressure?

5. How can a reversible reaction be shifted in a certain direction and completed?

Lecture No. 12 (problematic)

Solutions

Target: Give qualitative conclusions about the solubility of substances and a quantitative assessment of solubility.

Keywords: Solutions – homogeneous and heterogeneous; true and colloidal; solubility of substances; concentration of solutions; solutions of non-electroyls; Raoult's and van't Hoff's laws.

Plan.

1. Classification of solutions.

2. Concentration of solutions.

3. Solutions of non-electrolytes. Raoult's laws.

Classification of solutions

Solutions are homogeneous (single-phase) systems of variable composition, consisting of two or more substances (components).

According to the nature of their state of aggregation, solutions can be gaseous, liquid and solid. Typically, a component that, under given conditions, is in the same state of aggregation as the resulting solution is considered a solvent, while the remaining components of the solution are considered solutes. In the case of the same state of aggregation of the components, the solvent is considered to be the component that predominates in the solution.

Depending on the particle size, solutions are divided into true and colloidal. In true solutions (often called simply solutions), the solute is dispersed to the atomic or molecular level, the particles of the solute are not visible either visually or under a microscope, and move freely in the solvent environment. True solutions are thermodynamically stable systems that are indefinitely stable in time.

The driving forces for the formation of solutions are entropy and enthalpy factors. When gases are dissolved in a liquid, entropy always decreases ΔS< 0, а при растворении кристаллов возрастает (ΔS >0). The stronger the interaction between the solute and the solvent, the greater the role of the enthalpy factor in the formation of solutions. The sign of the change in the enthalpy of dissolution is determined by the sign of the sum of all thermal effects of the processes accompanying dissolution, of which the main contribution is made by the destruction of the crystal lattice into free ions (ΔH > 0) and the interaction of the resulting ions with solvent molecules (soltivation, ΔH< 0). При этом независимо от знака энтальпии при растворении (абсолютно нерастворимых веществ нет) всегда ΔG = ΔH – T·ΔS < 0, т. к. переход вещества в раствор сопровождается значительным возрастанием энтропии вследствие стремления системы к разупорядочиванию. Для жидких растворов (расплавов) процесс растворения идет самопроизвольно (ΔG < 0) до установления динамического равновесия между раствором и твердой фазой.

The concentration of a saturated solution is determined by the solubility of the substance at a given temperature. Solutions with lower concentrations are called unsaturated.

Solubility for various substances varies widely and depends on their nature, the interaction of solute particles with each other and with solvent molecules, as well as on external conditions (pressure, temperature, etc.)

In chemical practice, the most important solutions are those prepared on the basis of a liquid solvent. Liquid mixtures in chemistry are simply called solutions. The most widely used inorganic solvent is water. Solutions with other solvents are called non-aqueous.

Solutions are of extremely great practical importance; many chemical reactions take place in them, including those underlying metabolism in living organisms.

Concentration of solutions

An important characteristic of solutions is their concentration, which expresses the relative amount of components in the solution. There are mass and volume concentrations, dimensional and dimensionless.

TO dimensionless concentrations (shares) include the following concentrations:

Mass fraction of solute W(B) expressed as a fraction of a unit or as a percentage:

where m(B) and m(A) are the mass of solute B and the mass of solvent A.

The volume fraction of the solute σ(B) is expressed in fractions of a unit or volume percent:

where Vi is the volume of the solution component, V(B) is the volume of the dissolved substance B. Volume percentages are called degrees *).

*) Sometimes the volume concentration is expressed in parts per thousand (ppm, ‰) or in parts per million (ppm), ppm.

The mole fraction of the dissolved substance χ(B) is expressed by the relation

The sum of the mole fractions of the k components of the solution χ i is equal to unity

TO dimensional concentrations include the following concentrations:

The molality of the solute C m (B) is determined by the amount of substance n(B) in 1 kg (1000 g) of solvent, the dimension is mol/kg.

Molar concentration of substance B in solution C(B) – content of the amount of dissolved substance B per unit volume of solution, mol/m3, or more often mol/liter:

where μ(B) is the molar mass of B, V is the volume of the solution.

Molar concentration of equivalents of substance B C E (B) (normality - outdated) is determined by the number of equivalents of a dissolved substance per unit volume of solution, mol/liter:

where n E (B) is the amount of substance equivalents, μ E is the molar mass of the equivalent.

Titer of solution of substance B( T B) is determined by the mass of the solute in g contained in 1 ml of solution:

G/ml or g/ml.

Mass concentrations (mass fraction, percentage, molal) do not depend on temperature; volumetric concentrations refer to a specific temperature.

All substances are capable of dissolving to one degree or another and are characterized by solubility. Some substances are unlimitedly soluble in each other (water-acetone, benzene-toluene, liquid sodium-potassium). Most compounds are sparingly soluble (water-benzene, water-butyl alcohol, water-table salt), and many are slightly soluble or practically insoluble (water-BaSO 4, water-gasoline).

The solubility of a substance under given conditions is its concentration in a saturated solution. In such a solution, equilibrium is achieved between the solute and the solution. In the absence of equilibrium, a solution remains stable if the concentration of the solute is less than its solubility (unsaturated solution), or unstable if the solution contains a solute more than its solubility (supersaturated solution).