Problem 89 - Introductory Chemistry Atoms First

Question

When aqueous calcium hydroxide reacts with aqueous hydrochloric acid in a neutralization reaction to yield a neutral solution, the resulting solution is found to conduct electricity very well. (a) Explain why the neutral solution conducts electricity. (b) Write a complete ionic equation that goes along with your explanation in (a).

Step-by-Step Solution

Answer

(a) The neutral solution conducts electricity because it contains mobile ions from the dissolved salt. In an aqueous solution, salts dissociate into their constituent ions, which are free to move and carry an electric charge. (b) The complete ionic equation for the reaction between calcium hydroxide and hydrochloric acid is: \[ Ca^{2+}(aq) + 2OH^-(aq) + 2H^+(aq) + 2Cl^-(aq) \rightarrow Ca^{2+}(aq) + 2Cl^-(aq) + 2H_2O(l) \]

Step 1: Understanding neutralization reactions

In a neutralization reaction, an acid reacts with a base to produce a salt and water. In this case, aqueous calcium hydroxide (a base) reacts with aqueous hydrochloric acid (an acid) to form a neutral solution containing a salt. This process is a double displacement reaction, where the cation from the base and the anion from the acid will combine to form the salt.

Step 2: Explaining conductivity

The resulting neutral solution conducts electricity because it contains mobile ions. In an aqueous solution, salts dissociate into their constituent ions, which are free to move within the solution. These mobile ions can carry an electric charge, allowing the solution to conduct electricity.

Step 3: Writing the balanced molecular equation

First, let's write the balanced molecular equation for the reaction between calcium hydroxide and hydrochloric acid. The reaction can be written as: \[ Ca(OH)_2(aq) + 2HCl(aq) \rightarrow CaCl_2(aq) + 2H_2O(l) \]

Step 4: Writing the complete ionic equation

Next, write the complete ionic equation, which shows all of the ions involved in the reaction individually. Calcium hydroxide and hydrochloric acid dissociate into their ions in solution: \[ Ca^{2+}(aq) + 2OH^-(aq) + 2H^+(aq) + 2Cl^-(aq) \rightarrow Ca^{2+}(aq) + 2Cl^-(aq) + 2H_2O(l) \]

Step 5: Identifying spectator ions

Spectator ions are ions that are the same on both sides of the equation and do not take part in the reaction. In this case, the spectator ion is the calcium ion: \[ Ca^{2+}(aq) \]

Step 6: Writing the net ionic equation

Finally, to complete the explanation for conducting electricity, we can remove the spectator ions from the complete ionic equation to form the net ionic equation: \[ 2OH^-(aq) + 2H^+(aq) \rightarrow 2H_2O(l) \] This net ionic equation shows that hydroxide and hydrogen ions combine to form water during the neutralization reaction. The remaining ions, \( Ca^{2+}(aq) \) and \( 2Cl^-(aq) \), are present in the solution, allowing it to conduct electricity.

Key Concepts

Electrical Conductivity in Solutions

Understanding the electrical conductivity in solutions is pivotal when studying neutralization reactions like the one between calcium hydroxide and hydrochloric acid. The key lies in the generation of mobile ions within the aqueous solution. Ion mobility occurs when a substance, typical of salts - like calcium chloride in the discussed reaction, dissociates into ions that are positively or negatively charged.

These ions are crucial because they can transport the electric charge, essentially serving as carriers for electricity through the water, thus conducting electricity. For students wanting to grasp this concept, think of ions as little boats and the solution as a river. If the river has boats (ions), it's easy for people (electric charge) to travel across it. Without boats, the river doesn't help much, and likewise, a solution with no ions lacks electrical conductivity. This is why the neutral solution of calcium chloride in water is able to conduct electricity so well; it's full of these charge-carrying 'boats'.

Complete Ionic Equations

A complete ionic equation provides a more detailed perspective of what's actually happening in the solution during a chemical reaction, such as a neutralization process. This expanded view allows us to see all the players involved, particularly the ions that are actually participating in the reaction.

Let's take the equation from our exercise, which includes calcium hydroxide and hydrochloric acid. The complete ionic equation breaks this down into individual ions before and after the reaction takes place. For visual learners, imagine a dance floor where dancers (ions) are initially with their original partners (compounds). As the music (reaction) starts, they switch partners to form new pairs, which are the products. Writing this down as we did with calcium hydroxide and hydrochloric acid gives us a vivid snapshot of the chemical dance that's unfolding. It's this degree of detail in the complete ionic equation that reveals the presence of spectator ions, those dancers that never change partners, helping us move toward understanding the reaction in its entirety.

Net Ionic Equations

Reflecting the essence of a chemical reaction, net ionic equations cut through the complexity to highlight the fundamental change. In the context of our neutralization example, it distills the complete ionic equation by removing spectator ions, akin to focusing a camera solely on the key action in a bustling scene.

From the complete ionic equation, we eliminate ions that have not undergone any change - the spectators. These ions are important for the reaction's environment but are not 'acting' in our chemical 'play'. By focusing on the hydroxide and hydrogen ions combining to form water, the net ionic equation succinctly conveys the main event of neutralization. This simplicity aids in conceptual understanding, emphasizing not only the outcome but also clarifying the underlying processes that allow solutions, such as those formed from neutralization reactions, to conduct electricity.