In Section 11.5, we defined the vapor pressure of a liquid in terms of an equilibrium. (a) Write the equation representing the equilibrium between liquid water and water vapor and the corresponding expression for Kp.

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Identify the equilibrium process: The equilibrium between liquid water and water vapor can be represented by the phase change process where liquid water (H2O(l)) is in equilibrium with its vapor (H2O(g)).

Write the chemical equation for the equilibrium: H2O(l) \rightleftharpoons H2O(g).

Understand the concept of equilibrium constant (Kp): Kp is the equilibrium constant for reactions involving gases and is expressed in terms of partial pressures.

Write the expression for Kp: For the equilibrium H2O(l) \rightleftharpoons H2O(g), the expression for Kp is Kp = P_{H2O(g)}, where P_{H2O(g)} is the partial pressure of water vapor.

Note that the concentration of the liquid phase is not included in the Kp expression because it is a pure liquid and its activity is considered to be 1.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of particles to escape from the liquid phase into the vapor phase. The higher the vapor pressure, the more volatile the substance, indicating a greater number of molecules transitioning into the vapor state.

Equilibrium Constant (Kp)

The equilibrium constant, Kp, is a numerical value that expresses the ratio of the partial pressures of the products to the reactants at equilibrium for a given reaction. For the equilibrium between liquid water and water vapor, Kp can be expressed as the ratio of the vapor pressure of water to the pressure of the liquid phase, reflecting the balance between the two states.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time. In the context of vapor pressure, this means that the rate at which water molecules evaporate into vapor equals the rate at which vapor molecules condense back into liquid, establishing a stable vapor pressure.

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In Section 11.5, we defined the vapor pressure of a liquid in terms of an equilibrium. (b) By using data in Appendix B, give the value of Kp for this reaction at 30 C.

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In Section 11.5, we defined the vapor pressure of a liquid in terms of an equilibrium. (c) What is the value of Kp for any liquid in equilibrium with its vapor at the normal boiling point of the liquid?