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Chemical equilibrium is a situation that occurs during a reversible chemical process in which there is no net change in the quantities of reactants and products. A reversible chemical reaction is one in which the products react to generate the original reactants as soon as they are produced. At equilibrium, the two opposing reactions occur at equal rates or velocities, and so no net change in the number of substances involved occurs. At this point, the reaction can be termed complete; that is, the maximal conversion of reactants to products has been achieved for any defined reaction state.
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Key Terms: Equilibrium, Dynamic and Static Equilibrium, Solid, Liquid, Gas
Also read: Relation Between Kp and Kc
Equilibrium involving Dissolution of Solid or Gases in Liquids
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Solids in liquids: The solubility of solute (solid) in liquids varies with temperature, liquid, and solid type, and, to a lesser degree, system pressure. When a solute is mixed with a solvent, solid particles dissolve in the solvent, raising the concentration of the solute in the solution. This is known as dissolution.
Some solid particles in the solution clash with other solid particles and precipitate out. This is referred to as crystallization. The dissolving process continues until the solution reaches a maximum concentration level, at which point the solute dissolution stops. A saturated solution is one in which no additional solute particles may dissolve at the same pressure and temperature.
Equilibrium involving Dissolution of Solid or Gases in Liquids
Equilibrium Involving Dissolution of Solids: When a solution reaches the saturated stage, a state of equilibrium is reached. The balance attained is between the crystallization and dissolution processes. The amount of solid particles entering the solution equals the number of solid particles separating at this point. This is the dynamic equilibrium state. If the temperature and pressure remain constant in a saturated solution, the concentration of solute particles in the solution will remain the same.
Sugar (solution) = Sugar (solid)
Rate of dissolution of sugar = Rate of crystallization of sugar
Solubility refers to the greatest number of solute particles that may dissolve in a solvent (solution) at a given temperature. The solubility of a solid (solute) in a liquid is determined by the following parameters:
- The nature of the solvent and solute
- Effect of temperature
Gases in Liquids
A homogeneous solution can be formed by dissolving gases into liquids. The solubility of a gas in a liquid (solvent) is determined by:
- Nature of the solute (gas).
- Nature of the solvents.
- The temperature of the solution.
- Pressure.
Example: The amount of gas fizzing out of a cold drink at room temperature is more than the amount of gas fizzing out of a refrigerated drink.
Gases such as oxygen, helium, hydrogen, nitrogen, and so on dissolve in smaller amounts, but ammonia and hydrogen chloride are very soluble. The solubility of gases in liquids is governed by the following factors:
Temperature effect: The solubility of gas reduces as temperature rises.
Pressure effect: The solubility of gas increases as pressure rises.
Gases in Liquids
Henry’s Law
Henry's Law describes the relationship between the amount of gas dissolved. The mass of a gas dissolved in a given amount of solvent at any temperature is proportional to the pressure of the gas above the solvent. According to this law, the solubility of gases reduces as temperature rises.
Henry’s Law
(Mass of the gas) (pressure of the gas)
m p
mm = kHp
Where kH is the proportionality constant, often known as Henry's law constant, which varies with temperature.
Characteristics of Equilibria Involving Physical Processes
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General characteristics of a physical process are as follows:
- Equilibrium is only attainable in a closed system at a certain temperature.
- Equilibrium is a dynamic state.
- The forward reaction rate equals the reverse reaction rate.
- All measurable attributes of the system stay constant while the system is in equilibrium.
- A physical process's equilibrium is defined by a constant value of one of its parameters, known as the equilibrium constant.
- This parameter's value predicts the extent of the process.
Also read: Buffer Solutions
Difference Between Homogeneous and Heterogeneous Equilibrium
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Differences between Homogeneous and Heterogeneous equilibrium are:
| Homogeneous Equilibrium | Heterogeneous Equilibrium |
|---|---|
| A homogeneous equilibrium exists when all of the reactants and products are present in the same phase. | A heterogeneous equilibrium is a system that has reactants and products in two or more phases. |
| The equilibrium constant for homogeneous equilibria includes all reactant and product concentrations | The equilibrium constant for heterogeneous equilibria must omit solid and pure liquid concentrations. |
| We must include the concentrations of all reactants and products when calculating the equilibrium constant for homogenous equilibria. | While estimating the equilibrium constant for heterogeneous equilibria, we must neglect the concentrations of solids and pure liquids and instead consider the concentrations of other reactants and products. |
| 2SO22(g) + O2(g) ⇔ 2SO3(g) is a homogeneous equilibrium | O2(g) + 2C(s) ⇔ 2CO(g) is a heterogeneous equilibrium. |
Things to Remember
- A heterogeneous solution is made up of different states of matter.
- A homogeneous solution is one made up of matter that is all in the same state.
- At equilibrium, the forward reaction rate equals the backward reaction rate.
- Henry’s Law states that at any temperature, the mass of a gas that dissolves in a given mass of a solvent is proportional to the pressure of the gas above the surface of the solvent.
- Three types of reactions may be distinguished based on the extent to which they move toward equilibrium:
- Reactions that are almost complete and the concentrations of the reactants remaining are insignificant.
- Reactions in which the majority of the reactants remain unaltered, resulting in only little quantities of products being generated.
- Reactions in which the concentrations of reactants and products are comparable when the system is in equilibrium.
Also read: Le Chatelier’s Principle
Sample Questions
Ques: Write down the difference between a homogeneous mixture and a heterogeneous mixture. [2 marks]
Ans: Homogeneous mixtures are usually assumed to be indistinguishable from the pure material when seen at a macroscopic level. The reaction between the solutes is a component of a single homogenous equilibrium. Homogeneous mixtures include sugar, salt, water, color, air, and blood.
A heterogeneous mixture has a particular distinguishing property that allows the individual components of the mixture to be seen. It is a reactive system with two or more phases of reactants and products. Pizza, cookies, and rocks are all examples of heterogeneous mixes.
Ques: What is a solubility product? [2 marks]
Ans: The solubility product of salt is defined as the product of the concentration of its ions raised to the power equal to the number of ions generated by dissociation of one mole of the material at any given temperature.
Some key parameters influencing the solubility product constant are
- The impact of common ions (the presence of a common ion lowers the value of Ksp).
- The ion-diversity effect (if the ions of the solutes are uncommon, the value of Ksp will be high).
- Ion-pairs are present.
Ques: What is Dynamic Equilibrium? [2 marks]
Ans: Dynamic equilibrium is described as the condition of a given system in which the reversible reaction occurring in it stops altering the ratio of reactants and products, but there is substantial movement between the reactants and the products. This movement happens at the same rate, and there is no net change in the reactant and product ratio.
Example: The concentration of carbon dioxide in the liquid phase of a fresh bottle of aerated drink has a specified value. When the bottle is opened and half of the drink is spilled out, the liquid carbon dioxide slowly converts into gaseous carbon dioxide until a new point of equilibrium is achieved, and the rate of CO2 conversion from gas to liquid equals the rate of CO2 conversion from liquid to gaseous phase.
Ques: Why does boric acid act as Lewis acid? [2 marks]
Ans: Boric acid acts as a Lewis acid because, in boric acid, boron does not have its octet complete. It accepts OH- from water in aqueous solution B(OH)3 + H2O → [B(OH)4]- + H+.
Ques: Give one example of everyday life in which there is gas ? solution equilibrium. [2 marks]
Ans: Soda-water bottle.
The pressure of carbon dioxide gas in a sealed soda bottle will be high. The amount of CO2 gas dissolved in water increases, and carbon dioxide is in equilibrium with the water. When the pressure is reduced, the equilibrium shifts in the direction where the number of gaseous molecules is greater.
Ques: Illustrate the qualitative information that can be acquired from the magnitude of the equilibrium constant? [2 marks]
Ans:
- Large values of the equilibrium constant (>103) indicate that the forward direction is preferred, i.e. the concentration of products is substantially greater than the concentration of equilibrium reactants.
- Intermediate K values (10-3 to 103) indicate that the reactant and product concentrations are comparable.
- The low value of K (10-3) indicates that the backward reaction is preferred, implying that the concentration of reactants is substantially greater than that of products.
Ques: Why is common salt used to precipitate soap from a solution during the manufacturing process? [2 marks]
Ans: Soap is sodium salt of higher fatty acids [RCOONa]. On adding common salt, Na+
ion concentration increases. Hence the equilibrium RCOONa(s) ⇔ RCOO- + Na+ shifts in in the backward direction i.e., soap precipitates out.
To remove all of the soap from the aqueous solution, common salt is added. The addition of common salt to a soap solution reduces the solubility of the soap, causing all of the soap to precipitate out of the solution in solid form. This is known as salting out of soap.
Ques: Which of the following will function as an acid in the reaction between BF3 and C2H5OC2H5? Justify your answer. [2 marks]
Ans: The reaction between BF3 and C2H5OC2H5 is
BF3 is the Lewis acid because it is electron-deficient and receives a pair of electrons from C2H5OC2H5.
Ques: An aqueous solution of 0.02 g of NaOH in 50 mL has been prepared. Determine its pH and pOH. [2 marks]
Ans: 50 mL of NaOH contains 0.02 g of it
1000 mL of it contains = 0.02/50× 1000 = 0.4 g
Strength of NaOH = 0.4 gL-1
Ques: Prove that the pressure necessary to obtain 50% dissociation of PCl5 at 500 K is numerically equal to the three times the value of the equilibrium constant, Kp. [2 marks]
Ans: Total no. of moles = 1.5
If P is the total required pressure, then
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