Heterogeneous equilibrium | Chemical equilibrium | Chemistry | Khan Academy
TLDRThe video script discusses the concept of equilibrium constants in chemical reactions, particularly in heterogeneous equilibria involving different states of matter. It explains why the concentration of solids and solvents is not included in the equilibrium expression. The example of boron trifluoride reacting with water to form hydrofluoric acid and boric acid is used to illustrate this point, emphasizing that only the concentrations of the dissolved species are relevant. The script also introduces a second example involving water vapor, solid carbon, and the formation of hydrogen and carbon dioxide to further clarify the concept. The explanation is engaging and informative, aiming to solidify the viewer's understanding of equilibrium constants in chemistry.
Takeaways
- 🧪 The equilibrium constant expression for the reaction involving boron trifluoride, hydrofluoric acid, and boric acid is given by the concentration of HF to the third power times the concentration of H3BO3, divided by the concentration of BF3.
- 🌊 In the context of the reaction, the concentration of water (H2O) is considered to be in surplus and is not included in the equilibrium constant expression as it does not affect the rate of the reaction.
- 🔄 The equilibrium constant is a measure of the probability of the reaction happening or the likelihood of finding the reactants and products in the same place.
- 📈 When calculating equilibrium constants, the concentration of solids and solvents are typically ignored because they are either in excess or not participating in the reaction dynamics.
- 🌡️ Heterogeneous equilibria involve different states of matter, such as gaseous, liquid, and solid phases, and require special consideration when determining the equilibrium constant expressions.
- 🔄 For heterogeneous equilibria, the forward and reverse reaction rates are equal at equilibrium, allowing for the calculation of equilibrium constants based on the concentrations of the reacting species.
- 📝 The equilibrium constant expression for a reaction involving H2O in the gaseous state, solid carbon, hydrogen gas, and carbon dioxide gas does not include the solid carbon or the gaseous H2O, focusing only on the gaseous species.
- 🔄 The rate of the forward reaction is dependent on the concentration of the gaseous reactants, while the rate of the reverse reaction depends on the product of the concentrations of the gaseous products.
- 🎯 The equilibrium constant is a ratio of the rates of the forward and reverse reactions, providing a way to predict the position of equilibrium for a given reaction.
- 📚 Understanding the states of matter and their roles in reactions is crucial for correctly setting up and interpreting equilibrium constant expressions.
- 🚀 This knowledge will be further applied in future studies, such as exploring Le Chatelier's principle, which helps predict how changes in conditions will affect the position of equilibrium.
Q & A
What is the balanced chemical equation for the reaction between boron trifluoride and water?
-The balanced chemical equation is: BF3(g) + 3H2O(l) ⇌ 3HF(aq) + H3BO3(aq).
How is the equilibrium constant expression written for the reaction between BF3 and water?
-The equilibrium constant expression is K = [HF]^3 * [H3BO3] / [BF3].
Why is the concentration of water not included in the equilibrium constant expression for the reaction involving BF3 and water?
-The concentration of water is not included because water acts as the solvent and is present in excess, making its concentration effectively constant and not a limiting factor in the reaction.
What is the significance of the stoichiometric coefficients in the equilibrium constant expression?
-The stoichiometric coefficients indicate the molar ratios of the reactants and products at equilibrium. They are raised to the power in the expression to reflect the probability of the molecules being in close proximity to react.
How does the state of matter of reactants and products affect the equilibrium constant expression?
-The state of matter affects the expression in that solids and pure liquids are not included in the expression because their concentrations are not defined in the same way as solutions. Only the concentrations of dissolved species are considered.
What is a heterogeneous equilibrium?
-A heterogeneous equilibrium is a type of chemical equilibrium where the reactants and products are in different states of matter, such as gas and solid.
What are the key takeaways for calculating equilibrium constants in heterogeneous equilibria?
-In heterogeneous equilibria, one should ignore the presence of the solvent and solids in the equilibrium constant expression, focusing only on the species in solution.
What is the forward reaction rate dependent on for the reaction involving BF3 and water?
-The forward reaction rate is dependent on the concentration of BF3 and the presence of water, but since water is the solvent and is in excess, its concentration is not specifically considered in the rate expression.
How does the concentration of water affect the probability of the forward reaction in the equilibrium involving BF3?
-Since water is the solvent and is everywhere, its high concentration does not limit the reaction. Therefore, the probability of finding water molecules is effectively 1, making its concentration irrelevant to the reaction rate.
What is an example of a heterogeneous equilibrium involving a solid and a gas?
-An example of a heterogeneous equilibrium involving a solid and a gas is the reaction: H2O(g) + C(s) ⇌ H2(g) + CO2(g).
How does the equilibrium constant expression differ for a heterogeneous equilibrium involving gases and solids?
-In such a case, the equilibrium constant expression includes only the concentrations of the gases, as the solid's presence is constant and does not affect the concentration-based equilibrium expression.
Outlines
🧪 Calculation of Equilibrium Constant for Aqueous Reaction
This paragraph discusses the process of determining the equilibrium constant for a reaction involving boron trifluoride in the gaseous state reacting with water to form hydrofluoric acid and boric acid in aqueous states. It explains why the concentration of water is not included in the equilibrium expression, as it is in surplus and its high concentration does not affect the reaction rate. The concept of probability in relation to the concentration of reactants is also explored, emphasizing that the forward reaction rate depends only on the concentration of boron trifluoride. The paragraph introduces the idea of heterogeneous equilibrium, where different states of matter are involved in the reaction.
🌡️ Heterogeneous Equilibrium with Solid and Gaseous States
The second paragraph delves into another example of heterogeneous equilibrium, this time with water in the gaseous state (steam) and carbon in the solid state. It explains how the forward and reverse reaction rates are dependent on the concentrations of the gaseous reactants and products. The paragraph clarifies that the solid state should not be included in the equilibrium expression since it is not dissolved and its presence is constant throughout the reaction. The explanation concludes with the rationale for omitting the solvent and solid state in equilibrium calculations, reinforcing the concept of equilibrium constants and setting the stage for future discussions on Le Chatelier's principle.
Mindmap
Keywords
💡Equilibrium Constant
💡Boron Trifluoride
💡Hydrofluoric Acid
💡Boric Acid
💡Molarity
💡Concentration
💡Heterogeneous Equilibrium
💡Probability
💡Le Chatelier's Principle
💡Stoichiometric Coefficients
💡Solvent
Highlights
The equilibrium constant expression for the reaction involving boron trifluoride, hydrofluoric acid, and boric acid is discussed.
In the equilibrium reaction, the concentration of the solvent (water in this case) is not included in the equilibrium constant expression.
The concentration of solid reactants or products is also not included in the equilibrium constant expression.
The equilibrium constant expression is written with the concentrations of the products raised to the power of their stoichiometric coefficients.
The probability of finding the molecules in the same place is related to their concentration, which is reflected in the equilibrium constant expression.
The forward reaction rate depends on the concentration of the reactants that are actually dissolved in the solution.
The backward reaction rate depends on the concentration of the products, which are in the gaseous state in this example.
At equilibrium, the forward and backward reaction rates are equal, which allows for the calculation of the equilibrium constant.
The equilibrium constant is a ratio of the forward to the reverse reaction rate constants.
In heterogeneous equilibria, the presence of different states of matter (solid, liquid, gas) is considered.
An example of a heterogeneous equilibrium is given involving gaseous water (steam), solid carbon, and the gaseous products hydrogen and carbon dioxide.
In the heterogeneous equilibrium example, the concentration of the gaseous reactants drives the forward reaction rate.
The equilibrium constant for the heterogeneous reaction is derived from the ratio of the concentrations of the gaseous reactants and products.
The concept of ignoring the solid state and solvent in equilibrium constant expressions is reinforced for heterogeneous equilibria.
These principles will be applied in future lessons when calculating equilibrium constants.
Le Chatelier's principle, which will be covered in the next video, is introduced as a related concept to equilibrium reactions.
Transcripts
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