Problem 105 - Introductory Chemistry Atoms First

Question

The oxide ion, \(\mathrm{O}^{2-}\), present in sodium oxide \(\left(\mathrm{Na}_{2} \mathrm{O}\right)\) reacts violently with water to produce a highly basic solution. The hydride ion, \(\mathrm{H}^{-}\), in sodium hydride (NaH) does the same. (a) Write a balanced total ionic equation for the reaction of sodium oxide with water. (b) In terms of the Bronsted-Lowry definition, how are oxide and hydride similar? (c) What is it about the hydride and oxide ions that allow them to do what they do in water?

Step-by-Step Solution

Answer

(a) The balanced total ionic equation for the reaction of sodium oxide with water is: \(O^{2-} + 2H^+ \rightarrow 2OH^-\) (b) Oxide and hydride ions are similar because they both act as Bronsted-Lowry bases, as they can accept protons (H+ ions). (c) Hydride and oxide ions are highly reactive with water due to their high electron density and the presence of lone pairs of electrons, which enable them to easily act as Lewis and Bronsted-Lowry bases, accepting H+ ions from water molecules and producing highly basic solutions.

Step 1: (a) Write a balanced total ionic equation for the reaction of sodium oxide with water.

First, we need to write the chemical reaction between sodium oxide (Na2O) and water (H2O): Na2O + H2O -> 2NaOH Now let's break this equation down into its ionic components: 2Na+ + O2- + 2H+ + 2OH- -> 2Na+ + 2OH- Finally, we can cancel out the spectator ions (2Na+) to get the balanced total ionic equation for the reaction of sodium oxide with water: O2- + 2H+ -> 2OH-

Step 2: (b) In terms of the Bronsted-Lowry definition, how are oxide and hydride similar?

According to the Bronsted-Lowry definition, an acid is a substance that can donate a proton (H+), and a base is a substance that can accept a proton. Oxide ions (O2-) can accept two protons, as seen in the reaction of sodium oxide with water, where it accepts 2 H+ ions to form 2OH- ions. Similarly, hydride ions (H-) can accept a proton and form hydrogen gas, as seen in the reaction between sodium hydride and water: NaH + H2O -> Na+ + H2 + OH- In this reaction, the hydride ion (H-) accepts a proton (H+) from water: H- + H2O -> H2 + OH- So, oxide and hydride ions are similar in that they both act as Bronsted-Lowry bases because they can accept protons (H+ ions).

Step 3: (c) What is it about the hydride and oxide ions that allow them to do what they do in water?

Hydride (H-) and oxide (O2-) ions are highly reactive with water because of their high electron density and the presence of lone pairs of electrons on their respective atoms. These lone pairs make it easy for them to act as Lewis bases, which in turn makes them good Bronsted-Lowry bases as they can easily accept H+ ions from the water molecules. Their high electron density also stabilizes the OH- ions that are produced in the reaction, making the solutions highly basic.

Key Concepts

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons between reactants. In these reactions, one species loses electrons (gets oxidized) while another gains electrons (gets reduced). The key concept of redox reactions is the change in oxidation states of the molecules involved, which reflects the electron transfer process. Knowing this is essential because it allows us to predict how different substances will interact chemically.

• Oxidation involves the loss of electrons.
• Reduction involves the gain of electrons.
• In any redox reaction, the substance that donates electrons is called the reducing agent, and the substance that accepts electrons is the oxidizing agent.

While the exercise provided is not directly about redox reactions, the foundational understanding of electrons' role in chemical reactions is crucial. For a complex ion or molecule, identifying oxidation numbers for each element helps keep track of electron transfers during chemical exchanges. This skill can assist in identifying the role of substances like oxide and hydride in different chemical contexts.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+ ions) between substances. The Bronsted-Lowry theory states that:

• An acid is a proton donor.
• A base is a proton acceptor.

In the context of the given exercise, the oxide ( O^{2-} ) and hydride ( H^{-} ) ions act as Bronsted-Lowry bases. They are capable of accepting protons. When sodium oxide reacts with water, the oxide ion accepts protons to form hydroxide ions ( OH^{-} ), creating a basic solution. Similarly, the hydride ion in sodium hydride reacts with water by accepting a proton to produce hydrogen gas and hydroxide ions.
This understanding is pivotal for recognizing how different ions behave as acids or bases based on their ability to accept or donate protons. Such reactions are important in many fields, including biochemistry and environmental science, as they underlie the behavior of solutions in different pH environments.

Ionic Equations

Ionic equations help break down a chemical reaction into its component ions, showing the true reactants and products in an aqueous solution. This approach often reveals the core changes taking place in a chemical reaction, especially in polar solvents like water.

• A complete ionic equation lists all ions present in a reaction.
• A net ionic equation shows only the ions that undergo a chemical change.
• Spectator ions are those that do not participate directly in the reaction and are cancelled out in the net ionic equation.

In the sodium oxide and water reaction, the complete ionic form includes sodium ions ( Na^+ ), oxide ions ( O^{2-} ), and hydroxide ions ( OH^{-} ). However, the net ionic equation only includes active participants, such as the oxide ion accepting protons to form hydroxide ions. This simplification helps focus on the transformation of reactants to products, enhancing our understanding of the chemical process. Writing ionic equations strengthens the understanding of how ions in a solution rearrange during chemical reactions.