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The article provides readers with the most detailed general information about chemical equilibrium and chemical equilibrium shifts as well as how to apply Le Satellie's principle to judge equilibrium shifts. degree in chemistry. Find out with timhieulichsuquancaugiay.edu.vn now!

What is chemical equilibrium? Unidirectional and reversible reactions

To clearly understand what chemical equilibrium is, we will learn about one-way reactions and reversible reactions.

What is a one-way reaction?

Clarifying the concept of a one-way reaction, we will consider the following reaction:

2KClO3 -> 2KCl + 3O2 (Condition: MnO2, temperature)

When heating Potassium Chlorate (KClO3) crystals in the presence of a Manganese dioxide (MnO2) catalyst, KClO3 will decompose into potassium chloride (KCl) and oxygen (O2). Also under those conditions, Potassium Chloride (KCl) and oxygen (O2) cannot react with each other to form KClO3. Thus, the reaction only occurs in one direction from left to right, such a reaction is called a one-way reaction.

In the chemical equation of a one-way reaction, an arrow is used to indicate the direction of the reaction.

Reversible reaction

To clarify what is the concept of reversible reaction? We will consider the following reaction:

Cl2 + H2O ⇌ HCl + HClO

Under normal conditions, Cl2 will react with water (H2O) to form HCl and HClO, and the produced HCl and HClO can also react with each other to form Cl2 and H2O. Thus, under the same conditions, the reaction occurs in two opposite directions. Such a reaction is called a reversible reaction.

Reversible reaction. (Photo: Internet Collection)

In the chemical equation of a reversible reaction, people use arrows in opposite directions instead of arrows for a one-way reaction.

What is chemical equilibrium?

Chemistry textbook 10 (Vietnam Education Publishing House) has a detailed definition of chemical equilibrium as follows: “Chemical equilibrium is the state of a reversible reaction when the rate of the forward reaction is equal to the rate of the reverse reaction.”.

Experiment to identify reaction equilibrium shifts. (Photo: Screenshot of Chemistry Textbook Grade 10)

To better understand this concept, we will consider the following reversible reaction:

H2 (k) + I2 (k) ⇌ 2HI (k)

Place H2 and I2 in a sealed vessel at high and constant temperature. Initially, the forward reaction rate (Vt) is large because the H2 and I2 concentrations are large, while the reverse reaction rate (Vn) is zero, because the HI concentration is zero. During the reaction, the concentrations of H2 and I2 gradually decrease, so Vt gradually decreases and Vn will gradually increase due to the increasing HI concentration. At some point, Vt becomes equal to Vn, then the concentration of substances in the above reversible reaction remains the same, if the temperature does not change. This state of a reversible reaction is called chemical equilibrium.

In the equilibrium state, the reaction does not stop, but the forward and reverse reactions still occur but at equal rates (Vt = Vn). This means that in a single time, the concentration of reactants is reduced by the forward reaction but is produced by the reverse reaction. Therefore, chemical equilibrium is a dynamic equilibrium.

The characteristic of a reversible reaction is that the reactants are not completely converted into products, so in the equilibrium system there are always reactants and products present.

For example, put 0.5 mol/l H2 and 0.5 mol/l I2 in a closed vessel at a temperature of 430 degrees Celsius. If the reaction is one-way, then H2 and I2 will react completely to form 1 mol/l HI. However, this is a reversible reaction so only 0.786 mol/l HI is obtained and the remaining 0.107 mol/l H2, 0.107 mol/l I2.

Shift in chemical balance

Perform the experiment by assembling a kit consisting of two test tubes with branches (a) and (b), connected together by a soft plastic tube, with an open K lock. Continue filling both tubes (a) and (b) with NO2 gas at normal temperature. Plug both tubes, which have the following balance:

2NO2 (k – reddish brown) ⇌ N2O4 (gas – colorless)

The color of the gas mixture in equilibrium in both tubes (a) and (b) is the same. Close lock K to prevent the gas in the two pipes from diffusing into each other. Soak tube (a) in ice water for a while, then take it out and compare the color of tube (a) with tube (b). Notice that tube (a) has a lighter color.

When tube (a) was cooled, the NO2 molecules in the tube reacted further to produce N2O4, causing the NO2 concentration to decrease and the N2O4 concentration to increase. That phenomenon is called a chemical equilibrium shift.

Conclude: Chemical equilibrium shift is the movement from one equilibrium state to another due to the impact of external factors on the balance. (Chemistry Textbook 10, page 158, Vietnam Education Publishing House).

Factors affecting chemical balance

The factors that shift the equilibrium are concentration, pressure and temperature. These are called factors that affect chemical equilibrium. Specifically as follows:

Effect of concentration

Consider the following equilibrium system in a closed vessel at high and constant temperature:

C (r) + CO2 (k) ⇌ 2CO (k)

When the reaction system is in equilibrium, meaning Vt = Vn, the concentration of substances in the reaction does not change anymore. If we add an amount of CO2 gas to the system, the concentration of CO2 in the system will increase, causing Vt to equal Vn, then equilibrium is established. In the new equilibrium, the concentrations of substances will be different from the old equilibrium.

So when adding CO2 to the equilibrium system, the equilibrium will shift from left to right (in the forward direction), the direction that reduces the concentration of added CO2. The process of shifting the equilibrium occurs similarly when we remove CO gas from the equilibrium, because then Vn

Conclusion: When increasing or decreasing the concentration of a substance in equilibrium, the equilibrium always shifts in the direction that reduces the impact of increasing or decreasing the concentration of that substance. In case the equilibrium system contains solids in pure form, adding or removing the amount of solids does not affect the equilibrium, meaning the equilibrium does not shift.

Effect of pressure

Consider the following equilibrium system in a closed cylinder with a piston at normal and constant temperature:

N2O4 (k) ⇌ 2NO2 (k)

Experiment on the effect of pressure on balance. (Photo: Screenshot of Chemistry Textbook Grade 10)

Experiments show that when the system is in equilibrium, if you increase the overall pressure of the system by pushing the piston in to reduce the overall volume of the system, the number of moles of NO2 gas will decrease, and at the same time The number of moles of N2O4 gas will increase, meaning the equilibrium shifts in the opposite direction.

Conclude: When increasing or decreasing the overall pressure of the equilibrium system, the balance always shifts in the direction that reduces the impact of increasing or decreasing that pressure.

Thus, if the reaction has an equal number of moles of gas on both sides of the chemical equation or the reaction has no gas, pressure does not affect the equilibrium. For example, the pressure reaches the following equilibria:

H2 (k) + I2 (k) ⇌ 2HI (k)

Fe2O3 (r) + 3CO (k) ⇌ 2Fe (r) + 3CO2 (k)

Effect of temperature

To study the effect of temperature on chemical equilibrium, we need to know the concept of exothermic and endothermic reactions.

Learn about the exothermic reaction from quicklime. (Photo: Internet Collection)

  • Exothermic reaction: A chemical reaction that usually involves the release or absorption of energy in the form of heat. For example: CaO quicklime reacts with H2O water, the resulting mixture boils.

  • Endothermic reaction: It is a chemical reaction where heat must be continuously supplied to the reaction. For example: Calculate limestone (CaCO3) to produce quicklime.

To indicate the amount of heat accompanying chemical reactions, people use the quantity heat of reaction, denoted ΔH0 (positive value).

To study the effect of temperature on chemical equilibrium, consider the equilibrium in a closed vessel:

N2O4 (gas – colorless) ⇌ 2NO2 (gas – reddish brown); ΔH = 58 kJ

Endothermic reaction ΔH = 58 kJ >0; The reverse reaction is exothermic ΔH = -58 kJ

Conclude:

  • When the temperature increases, the equilibrium shifts in the direction of the endothermic reaction, that is, the direction that reduces the impact of increasing temperature, and when the temperature decreases, the equilibrium shifts in the direction of the exothermic reaction, that is, the direction that reduces the impact of the temperature reduction.

  • The three factors concentration, pressure and temperature that affect chemical equilibrium were summarized by French chemist H. Le Chatelier into the principle of equilibrium shifting by Le Satlieer: “A reversible reaction is in a state of equilibrium when subjected to an external influence such as a change in concentration, pressure, or temperature, the equilibrium will shift in the direction that reduces that external impact.” (According to Chemistry Textbook 10, page 161).

Involvement of catalysts

A catalyst increases the rate of the forward reaction and the rate of the reverse reaction by an equal amount. Therefore, the catalyst does not affect the chemical equilibrium. When the reversible reaction is not yet in equilibrium, the catalyst helps to establish the equilibrium more quickly.

See more:

Significance of reaction rate and chemical equilibrium in chemical production

To see the meaning of reaction rate and chemical equilibrium in chemical production, let's take the following example:

For example: During the production of sulfuric acid, the following reaction must be performed:

2SO2 (k) + O2 (k) ⇌ 2SO3 (k)

In this reaction, atmospheric oxygen is used.

At normal temperature, the reaction occurs very slowly. Therefore, to increase the reaction rate, you must use a catalyst and carry out the reaction at quite high temperatures. Because this is an exothermic reaction, when the temperature increases, the equilibrium shifts in the opposite direction, reducing the efficiency of the reaction. To limit this effect, people used an excess amount of air, which means increasing the oxygen concentration, causing the balance to shift in the positive direction.

Exercises on chemical equilibria in Chemistry Textbook Grade 10 with detailed explanations

Based on the theoretical knowledge timhieulichsuquancaugiay.edu.vn has compiled from the Grade 10 Chemistry Textbook above, please continue to practice doing the exercises and compare with the suggested detailed answers!

Exercises on chemical balance. (Photo: Shutterstock.com)

Lesson 1 (Chemistry textbook grade 10, page 162)

Which of the following is correct:

A. Any reaction must reach chemical equilibrium.

B. When the reversible reaction is at equilibrium, the reaction stops.

C. Only reversible reactions have chemical equilibrium.

D. In equilibrium, the masses of substances on both sides of the reaction equation must be equal.

Suggested answer:

The correct answer is C.

Lesson 2 (Chemistry Textbook, page 162)

The following equilibrium system is carried out in a closed vessel:

2SO2(k) + O2 (k) ⇄ 2SO3 (k) ΔH

Which of the following factors does not change the concentration of substances in an equilibrium system?

A. Temperature change.

B. Pressure change.

C. Presence of catalyst.

D. Change the capacity of the reaction vessel.

Suggested answer:

The answer is C. The presence of a catalyst. Catalysts increase the rate of the forward reaction and the rate of the reverse reaction by an equal amount. Therefore, the catalyst does not shift the chemical equilibrium or change the concentration of substances in the equilibrium

Lesson 3 (Chemistry Textbook Grade 10, page 163)

What is chemical equilibrium? Why is chemical equilibrium said to be dynamic equilibrium?

Suggested answer:

Chemical equilibrium is the state of a reversible reaction when the rate of the forward reaction is equal to the rate of the reverse reaction.

Chemical equilibrium is dynamic equilibrium because: In an equilibrium state, the reaction does not stop, but the forward and reverse reactions still occur, but at equal rates (V forward = V reverse). This means that in one unit of time, the number of moles of reactant reduced by the forward reaction is produced by the reverse reaction. Therefore chemical equilibrium is dynamic equilibrium.

Above is a detailed summary of theory and some exercises on chemical balance. Wishing you effective revision and high results in your studies!

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