Problem 39 - Chemistry and Chemical Reactivity

Question

Write an equation that describes the equilibrium that exists when nitric acid dissolves in water. Identify each of the four species in solution as either Bronsted acids or Bronsted bases. Does the equilibrium favor the products or the reactants?

Step-by-Step Solution

Answer

The equilibrium favors the products, with HNO₃ as a Brønsted acid and NO₃⁻ as a base.

Step 1: Write the Dissolution Reaction

When nitric acid (HNO₃) dissolves in water, it donates a proton (H⁺) to water, producing hydronium ions (H₃O⁺) and nitrate ions (NO₃⁻). The equilibrium equation for this reaction is: \[\text{HNO₃} + \text{H₂O} \rightleftharpoons \text{H₃O}^+ + \text{NO₃}^-\] This reaction shows nitric acid dissociating into hydronium ions and nitrate ions.

Step 2: Identify the Species as Brønsted Acids or Bases

In the equation, HNO₃ acts as a Brønsted acid because it donates a proton (H⁺) to water. Water ( H₂O) accepts the proton to become H₃O⁺ and acts as a Brønsted base. H₃O⁺, the hydronium ion, is the conjugate acid that forms, while NO₃⁻ is the conjugate base that results from the deprotonation of HNO₃.

Step 3: Determine the Equilibrium Direction

Nitric acid is a strong acid, meaning it ionizes completely in water. This implies that the equilibrium lies far to the right, favoring the formation of products (H₃O⁺ and NO₃⁻) over the reactants (HNO₃ and H₂O). Therefore, the equilibrium significantly favors the product side.

Key Concepts

Brønsted Acid

When discussing the concept of a Brønsted acid, we are focusing on a substance that donates a proton, which is essentially a hydrogen ion (H⁺), in a chemical reaction.
In the dissolution of nitric acid (HNO₃) in water, HNO₃ acts as the Brønsted acid, since it donates a proton to water.

• The formula for this reaction is: \[\text{HNO₃} + \text{H₂O} \rightleftharpoons \text{H₃O}^+ + \text{NO₃}^-\]
• After donating a proton, HNO₃ becomes NO₃⁻, the nitrate ion.
• This transformation demonstrates the principal behavior of a Brønsted acid—proton donation.

Understanding the role of HNO₃ as a Brønsted acid is crucial, as it sets up the basic framework for evaluating acid-base equilibriums.If you imagine acids as proton donors, it becomes easier to identify other acids by observing which substances donate hydrogen ions in reactions.

Brønsted Base

While the Brønsted acid in a reaction is the donor of H⁺ , a Brønsted base, in contrast, accepts a hydrogen ion. In the example of nitric acid dissolving in water, water ( H₂O ) functions as the Brønsted base.
This is because it accepts the hydrogen ion from HNO₃ and forms H₃O⁺ , the hydronium ion.

• During this process, H₂O acts as a proton acceptor.
• This proton acceptance transforms water into H₃O⁺ , linking the conversion to its role as a base.
• The ability of a substance to accept a proton defines it as a Brønsted base regardless of its original state.

Recognizing H₂O as a Brønsted base in this scenario not only helps classify the substances involved, but also clarifies the direction of proton movement in acid-base reactions. Having a clear understanding of Brønsted bases is especially important for solving equilibrium problems where both acids and bases must be identified to ascertain how reactions proceed.

Equilibrium Direction

In chemical reactions, equilibrium direction refers to the tendency of a reaction to favor either the reactants or products once the reaction has reached a stable state. This concept is vital in understanding how strong acids like nitric acid behave in solution.
Since HNO₃ is a strong acid, it dissociates almost completely in water.
This has a significant impact on the equilibrium of the reaction.

• In this scenario: \[\text{HNO₃} + \text{H₂O} \rightleftharpoons \text{H₃O}^+ + \text{NO₃}^-\]
• The equilibrium heavily favors the right side, meaning a majority of HNO₃ converts to H₃O⁺ and NO₃⁻.
• This is indicative of the strong acid’s propensity to release its protons almost completely.

Understanding the equilibrium direction helps predict the extent of acid ionization in solution and influences how we measure the strength of acids and equally, the weakness of associated bases in any given chemical environment.