Pure solids & Pure liquids can be ignored while writing expression for equilibrium constant Kc

Chemistry by Dr..Suresh Kumar

7 Dec 201802:59

Summary

TLDRThe script delves into the concept of equilibrium constants, particularly focusing on why the concentrations of pure solids and liquids are omitted in the expression for KC. It clarifies that molarity, which is the basis for KC, is calculated as moles per volume, and since the density of pure substances remains constant regardless of volume changes, their concentration is considered constant. This is why they are ignored in the equilibrium expression. The script uses examples, including ethyl acetate and acetic acid, to illustrate this principle, emphasizing the importance of understanding the behavior of gases in contrast to pure solids and liquids.

Takeaways

🧪 The equilibrium constant, KC, is used to express the concentrations of reactants and products at equilibrium in a chemical reaction.
📚 Solids and pure liquids are often omitted from the expression for KC because their concentration remains constant due to their intensive properties.
🔍 Molarity, or molar concentration, is defined as the number of moles of a substance divided by the volume of the solution.
📉 The concentration of pure solids and liquids does not change with volume because their density, an intensive property, remains the same regardless of the amount of substance.
🌡️ For gases, the concentration can vary with changes in volume and mass, which is why they are included in the expression for KC.
🧩 In the given example, copper oxide and copper nitrate are ignored in the KC expression because they are solids with constant molar concentration.
🌌 The concentration of gases like NO2 and O2 is considered in the KC expression because they can change with volume and mass.
🛑 The script emphasizes that the concentration of pure liquids, such as water in the example of ethyl acetate and acetic acid, is constant and thus not included in the KC expression.
🔄 The equilibrium constant is important for understanding the extent to which a reaction proceeds and the relative concentrations of products and reactants at equilibrium.
📈 The value of KC can be used to predict the direction a reaction will take and the position of equilibrium for a given set of conditions.
🌟 Understanding the principles behind the inclusion or exclusion of substances in the KC expression is crucial for correctly applying chemical equilibrium concepts.

Q & A

Why are solids and pure liquids typically not included in the expression for the equilibrium constant (KC)?
-Solids and pure liquids are not included in the KC expression because their concentration remains constant. This is due to their density being an intensive property, meaning it does not change with volume or mass changes, thus maintaining a constant molar concentration.
What is molar concentration or molarity?
-Molar concentration, also known as molarity, is the amount of substance (in moles) of a solute divided by the volume of the solution in liters. It is a measure of how much of a substance is present in a given volume of solution.
How does the density of a substance relate to its molar concentration?
-The density of a substance is an intensive property that does not change with the size of the sample. For pure solids and liquids, the density remains constant, which means that the mass per unit volume also remains constant, leading to a constant molar concentration.
Why is the concentration of gases included in the KC expression?
-The concentration of gases is included in the KC expression because it can vary with changes in volume and pressure. Gases do not have a constant density like solids and liquids, and their molar concentration can change with the conditions of the system.
What is the significance of the equilibrium constant (KC) in chemical reactions?
-The equilibrium constant (KC) is a measure of the extent to which a reaction proceeds at equilibrium. It provides information about the relative concentrations of reactants and products at equilibrium, indicating the direction in which the reaction favors.
Can you provide an example of a reaction where the concentration of pure liquids is ignored in the KC expression?
-An example given in the script is the reaction involving ethyl acetate, acetic acid, and ethanol. Water, which is a pure liquid in this context, is not included in the KC expression due to its constant concentration.
What is the difference between an intensive property and an extensive property?
-An intensive property is a characteristic of a substance that does not depend on the amount of material present, such as density or boiling point. An extensive property, on the other hand, depends on the amount of material, such as mass or volume.
Why is the concentration of copper oxide and copper nitrate ignored in the example given?
-Copper oxide and copper nitrate are solid substances, and their concentration is ignored in the KC expression because, as solids, they have a constant density and thus a constant molar concentration that does not affect the equilibrium position.
How does the equilibrium constant (KC) differ from the equilibrium constant in terms of pressure (KP)?
-The equilibrium constant (KC) is expressed in terms of molar concentrations of the reactants and products, while the equilibrium constant in terms of pressure (KP) is expressed in terms of partial pressures. KP is used for reactions involving gases.
What happens to the molar concentration of a substance when its volume changes?
-When the volume of a substance changes, its molar concentration also changes. For gases, an increase in volume leads to a decrease in molar concentration, and vice versa, because the number of moles per unit volume changes.
Can the equilibrium constant (KC) be used to predict the direction of a reaction?
-Yes, the value of KC can be used to predict the direction of a reaction. If KC is greater than 1, the reaction favors the formation of products. If KC is less than 1, the reaction favors the formation of reactants.

Outlines

00:00

🧪 Chemistry of Equilibrium Constants

This paragraph discusses the concept of equilibrium constants in chemistry, particularly focusing on why the concentration of solids and pure liquids can be ignored when writing the expression for the equilibrium constant (KC). It explains that molarity, which is the basis for the equilibrium constant expression, is calculated as the number of moles divided by volume. For pure solids and liquids, the density remains constant because both mass and volume change proportionally, thus maintaining a constant molar concentration. The paragraph uses the example of the reaction involving copper oxide and copper nitrate to illustrate this point, emphasizing that the concentration of these solids is not included in the KC expression.

Mindmap

Keywords

💡Equilibrium Constant (KC)

The equilibrium constant, denoted as KC, is a measure of the extent of a chemical reaction at equilibrium. It is defined as the ratio of the concentrations of products to the concentrations of reactants, each raised to the power of their stoichiometric coefficients. In the video, the concept of KC is central to explaining why the concentrations of solids and pure liquids are not included in the expression for KC, as their concentrations remain constant.

💡Concentration

Concentration in chemistry refers to the amount of a particular substance in a given volume. It is typically expressed in molarity, which is the number of moles of solute per liter of solution. The video script discusses how the concentration of pure solids and liquids is constant because their density, an intensive property, does not change with volume or mass.

💡Molarity

Molarity is the concentration of a substance in a solution, measured as the number of moles of solute per liter of solution. It is used in the script to explain why the concentration of pure solids and liquids is not included in the KC expression, as their molar concentration remains constant regardless of the volume.

💡Density

Density is an intensive property of matter that describes the mass per unit volume. In the context of the video, it is mentioned to illustrate why the concentration of pure solids and liquids does not change with volume, as their density remains constant.

💡Intensive Property

An intensive property is a characteristic of a substance that does not depend on the amount of material present. In the video, density is given as an example of an intensive property, which is why the concentration of pure solids and liquids remains constant regardless of the sample size.

💡Solids

Solids are one of the three main states of matter, characterized by a fixed shape and volume. The script explains that in the context of writing the expression for KC, the concentration of solids is not included because their density and thus their concentration remains constant.

💡Pure Liquids

Pure liquids are liquids that are not mixed with any other substances. The video script mentions that the concentration of pure liquids is not included in the KC expression because their density, and thus their concentration, remains constant regardless of the volume.

💡Copper Oxide

Copper oxide is a chemical compound with the formula CuO or Cu2O, depending on the oxidation state of copper. In the script, copper oxide is given as an example of a solid that is not included in the KC expression due to its constant concentration.

💡Copper Nitrate

Copper nitrate is a chemical compound that can be used in various chemical reactions. The script mentions copper nitrate as another example of a compound where the concentration of its solid form is not considered in the KC expression.

💡Ethyl Acetate

Ethyl acetate is an ester that is used as a solvent and in the production of certain types of adhesives. In the video, it is mentioned as part of a reaction where water, a pure liquid, is involved, and its concentration is not included in the KC expression.

💡Acetic Acid

Acetic acid is the main component of vinegar and is also used in various industrial processes. The script uses acetic acid as an example of a reactant in a reaction where the concentration of water, a pure liquid, is not considered in the KC expression.

Highlights

Solids and pure liquids can be ignored in the equilibrium constant expression because their concentration remains constant due to their density being an intensive property.

The equilibrium constant KC does not account for the concentration of solids and pure liquids as their molar concentration is constant.

Molarity, which is the number of moles per unit volume, is used in the expression for KC.

For pure solids and liquids, the number of moles is proportional to mass per unit volume, which is density.

Density is an intensive property, meaning it remains the same regardless of the size of the sample.

The concentration of pure solids and liquids is assumed to be constant because their density does not change with volume.

In the context of chemical equilibrium, the concentration of gases is variable and must be included in the KC expression.

The transcript explains why copper oxide and copper nitrate are not included in the KC expression due to their solid state.

The example of ethyl acetate, acetic acid, and water is used to illustrate the concept of ignoring pure liquids in KC calculations.

Water, being a pure liquid, is not included in the KC expression for the reaction involving ethyl acetate and acetic acid.

The concentration of gases must be included in the KC expression as it can vary with changes in mass and volume.

The explanation emphasizes the importance of understanding the physical state of reactants and products in equilibrium calculations.

The concept of molar concentration is fundamental to writing the expression for the equilibrium constant KC.

The transcript provides a clear distinction between the treatment of solids/liquids and gases in equilibrium constant expressions.

Understanding the properties of intensive and extensive variables is crucial for correctly writing KC expressions.

The transcript clarifies why the concentration of pure substances in their natural state does not affect the equilibrium constant.

The explanation of why pure liquids are ignored in KC expressions helps in simplifying the calculation of chemical equilibria.

The transcript offers practical examples to aid in the understanding of equilibrium constant expressions in chemistry.