Problem 43
Question
Hydrofluoric acid is a weak acid. Write the mass action expression for its acid ionization reaction.
Step-by-Step Solution
Verified Answer
The mass action expression for the acid ionization reaction of hydrofluoric acid is: ([H+][F-]) / [HF].
1Step 1: Write the chemical reaction for hydrofluoric acid ionization
Hydrofluoric acid (HF) is a weak acid, which means that it does not completely ionize in water. When added to water, it will partially ionize, forming hydrogen ions (H+) and fluoride ions (F-). The chemical reaction can be written as:
HF (aq) <-> H+ (aq) + F- (aq)
2Step 2: Write the equilibrium expression for the reaction
The equilibrium expression is the mathematical representation of the reaction. It is written using the equilibrium concentrations of the reaction species and is expressed as the product of the concentration of products divided by the concentration of reactants. In this case, the equilibrium constant, denoted by 'K_a', is given by:
K_a = ([H+][F-]) / [HF]
3Step 3: Write the mass action expression for the acid ionization reaction
The mass action expression is similar to the equilibrium expression and is derived from it. In the mass action expression, we replace the equilibrium concentrations of the species with their generic concentrations. This gives us the mass action expression for the ionization of hydrofluoric acid:
Mass action expression = ([H+][F-]) / [HF]
And that is our final answer.
Key Concepts
Hydrofluoric AcidWeak AcidEquilibrium ExpressionMass Action Expression
Hydrofluoric Acid
Hydrofluoric acid (HF) is notable for its unique properties among the other hydrohalic acids. Unlike its counterparts, which are strong acids like hydrochloric acid (HCl), hydrofluoric acid is classified as a weak acid. This distinction is essential in understanding its behavior when dissolved in water.
HV is used in various applications, from glass etching to chemical synthesis. Its weak acidic nature does not mean it is less hazardous; in fact, HF poses significant risks due to its ability to penetrate skin and cause deep tissue damage.
When HF is diluted with water, it exists in a state of dynamic equilibrium, where the amount of undissociated HF and the ions H+ and F- it forms are constant over time, but the actual particles are in continuous exchange. This is crucial when approaching problems that require understanding how HF behaves in solution, particularly its ionization process.
HV is used in various applications, from glass etching to chemical synthesis. Its weak acidic nature does not mean it is less hazardous; in fact, HF poses significant risks due to its ability to penetrate skin and cause deep tissue damage.
When HF is diluted with water, it exists in a state of dynamic equilibrium, where the amount of undissociated HF and the ions H+ and F- it forms are constant over time, but the actual particles are in continuous exchange. This is crucial when approaching problems that require understanding how HF behaves in solution, particularly its ionization process.
Weak Acid
A weak acid is an acid that does not completely dissociate into its ions in a solution. In contrast to strong acids, which nearly fully ionize, weak acids only partially give off hydrogen ions (H+) in water. The strength of an acid is quantified by its dissociation constant (Ka), which is a reflection of its tendency to donate H+. The weaker the acid, the lower the Ka value.
Understanding the concept of acidity strength is foundational in topics ranging from titration to buffer solutions. A weak acid like hydrofluoric acid has a Ka value that reflects its limited ionization in solution. The degree of ionization directly affects the pH of the solution, which is a measure of its acidity.
Understanding the concept of acidity strength is foundational in topics ranging from titration to buffer solutions. A weak acid like hydrofluoric acid has a Ka value that reflects its limited ionization in solution. The degree of ionization directly affects the pH of the solution, which is a measure of its acidity.
Equilibrium Expression
Equilibrium in chemistry refers to the state where the forward and reverse reactions occur at the same rate, leading to constant concentrations of reactants and products over time. The equilibrium expression is a way to express the balance between these reactants and products for a reversible chemical reaction.
For the ionization of weak acids, the equilibrium expression is formulated as the ratio of the product of the concentrations of the products to the concentration of the reactant, and is signified by the equilibrium constant, Ka, for acid dissociation reactions. For hydrofluoric acid, this can be written as:
\[ K_a = \frac{{[H+][F-]}}{{[HF]}} \]
The brackets indicate the concentration of each species in moles per liter (M). In the classroom or laboratory, this expression helps predict the extent of the ionization process and the final concentrations of ions at equilibrium.
For the ionization of weak acids, the equilibrium expression is formulated as the ratio of the product of the concentrations of the products to the concentration of the reactant, and is signified by the equilibrium constant, Ka, for acid dissociation reactions. For hydrofluoric acid, this can be written as:
\[ K_a = \frac{{[H+][F-]}}{{[HF]}} \]
The brackets indicate the concentration of each species in moles per liter (M). In the classroom or laboratory, this expression helps predict the extent of the ionization process and the final concentrations of ions at equilibrium.
Mass Action Expression
Rooted in the Law of Mass Action, mass action expression explains how the rate of a chemical reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its stoichiometric coefficient.
In the context of acid ionization, such as that of hydrofluoric acid, the mass action expression looks very similar to the equilibrium expression but represents the concentrations at any given point, not necessarily at equilibrium. Therefore, the generic mass action expression for the ionization of HF can be showcased as:
\[ \text{Mass action expression} = \frac{{[H+][F-]}}{{[HF]}} \]
This expression is used to calculate the changes in concentration of reactants and products over time until equilibrium is achieved. It's an essential tool for chemists to predict the direction of the reaction and to calculate equilibrium concentrations when the initial conditions are known.
In the context of acid ionization, such as that of hydrofluoric acid, the mass action expression looks very similar to the equilibrium expression but represents the concentrations at any given point, not necessarily at equilibrium. Therefore, the generic mass action expression for the ionization of HF can be showcased as:
\[ \text{Mass action expression} = \frac{{[H+][F-]}}{{[HF]}} \]
This expression is used to calculate the changes in concentration of reactants and products over time until equilibrium is achieved. It's an essential tool for chemists to predict the direction of the reaction and to calculate equilibrium concentrations when the initial conditions are known.
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