Problem 90 - Chemistry

Question

A piece of magnesium ribbon and a copper wire are partially immersed in a \(0.1 M \mathrm{HCl}\) solution in a beaker. The metals are joined externally by another piece of metal wire. Bubbles are seen to evolve at both the \(\mathrm{Mg}\) and Cu surfaces. (a) Write equations representing the reactions occurring at the metals. (b) What visual evidence would you seek to show that Cu is not oxidized to \(\mathrm{Cu}^{2+} ?(\mathrm{c})\) At some stage, \(\mathrm{NaOH}\) solution is added to the beaker to neutralize the HCl acid. Upon further addition of \(\mathrm{NaOH},\) a white precipitate forms. What is it?

Step-by-Step Solution

Answer

a) Mg oxidizes and H+ reduces. b) No blue solution for Cu2+ shows Cu isn't oxidized. c) White precipitate is Mg(OH)2.

Step 1: Understanding the System

We have a piece of magnesium and a piece of copper immersed in hydrochloric acid, with an external connection. Magnesium is more reactive than copper, so it is expected to release electrons and undergo oxidation. Copper typically doesn't participate actively in this setup as its reduction potential is higher than that of hydrogen gas liberation from acid by magnesium.

Step 2: Writing the Redox Reactions

At the magnesium electrode, magnesium is oxidized. The reaction is: \[ \mathrm{Mg} \rightarrow \mathrm{Mg}^{2+} + 2e^{-} \]At the copper electrode, hydrogen ions in the solution gain electrons released from the oxidation of magnesium and form hydrogen gas. The reaction is: \[ 2\mathrm{H}^{+} + 2e^{-} \rightarrow \mathrm{H}_2(g) \]

Step 3: Visual Evidence at Copper Surface

Since copper is not oxidized to \(\mathrm{Cu}^{2+}\), there will be no blue coloration in the solution (which would indicate the presence of \(\mathrm{Cu}^{2+}\) ions). The presence of bubbles alone at the copper surface only suggests hydrogen gas evolution, confirming that copper is acting simply as a surface for hydrogen reduction rather than being oxidized.

Step 4: Effect of Neutralization with NaOH

When \(\mathrm{NaOH}\) is added to neutralize the \(\mathrm{HCl}\), it interacts with \(\mathrm{Mg}^{2+}\) ions in solution. The additional \(\mathrm{NaOH}\) reacts to form magnesium hydroxide when the solution becomes slightly basic: \[ \mathrm{Mg}^{2+} + 2\mathrm{OH}^{-} \rightarrow \mathrm{Mg(OH)}_2 \]Magnesium hydroxide is insoluble in water and will precipitate as a white solid.

Key Concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between chemical species. In the given exercise, magnesium acts as the reducing agent. It loses electrons, a process referred to as oxidation. The equation describing this process is: \[ \mathrm{Mg} \rightarrow \mathrm{Mg}^{2+} + 2e^{-} \].
On the other side, the hydrogen ions (\(\mathrm{H}^+\)) from hydrochloric acid gain these electrons, which is called reduction. This process forms hydrogen gas, shown in the reaction: \[ 2\mathrm{H}^{+} + 2e^{-} \rightarrow \mathrm{H}_2(g) \].
Thus, these two reactions form the basis of a redox process, where magnesium undergoes oxidation and hydrogen ions undergo reduction.

Reactivity Series

The reactivity series is an important concept to understand which metals can displace other elements from compounds. It's a list of metals arranged in order of decreasing reactivity. Magnesium is more reactive than copper and hydrogen, making it able to displace hydrogen ions in the acid and release electrons.

• Because magnesium is higher in the reactivity series than hydrogen, it readily reacts with hydrochloric acid.
• Copper, being lower in the series than hydrogen, does not react with the acid, confirming why copper does not oxidize in this setup.

Understanding the reactivity series allows us to predict that in this experiment, magnesium will undergo oxidation while copper remains unchanged.

Precipitation Reaction

Precipitation reactions occur when soluble ions in different solutions react to form an insoluble solid, called a precipitate. When \(\mathrm{NaOH}\) is added to the solution in this exercise, it reacts with dissolved \(\mathrm{Mg}^{2+}\) ions. This reaction forms magnesium hydroxide as a white precipitate: \[ \mathrm{Mg}^{2+} + 2\mathrm{OH}^{-} \rightarrow \mathrm{Mg(OH)}_2 \].
Magnesium hydroxide is insoluble in water, and its formation signifies that the solution has become slightly basic, as \(\mathrm{NaOH}\) neutralizes the \(\mathrm{HCl}\) acid. This visual cue of a precipitate helps confirm reactions at a basic pH level.

Hydrogen Evolution

Hydrogen evolution is the process where hydrogen gas is produced during a chemical reaction. In the context of this experiment, this occurs at the copper surface, where hydrogen ions from \(\mathrm{HCl}\) are reduced: \[ 2\mathrm{H}^{+} + 2e^{-} \rightarrow \mathrm{H}_2(g) \].
The presence of bubbles on the copper wire indicates hydrogen gas is forming and not a sign of copper oxidizing to \(\mathrm{Cu}^{2+}\).

• This process confirms the role of copper as a site for electron transfer and hydrogen gas formation.
• Observing bubbles on copper shows hydrogen is being reduced, maintaining copper's unreacted state.

The experiment visually demonstrates this fundamental electrochemical process.

Neutralization Reaction

Neutralization reactions involve an acid and a base reacting to form water and a salt. In this experiment, adding \(\mathrm{NaOH}\) to the beaker neutralizes the \(\mathrm{HCl}\) present.

• The neutralization is crucial in changing the \(\text{pH}\) of the solution from acidic to neutral, further affecting the solubility dynamics.
• As the \(\text{pH}\) shifts, it sets the stage for the precipitation reaction to occur.

Though the immediate result of the neutralization isn't visually striking, the subsequent formation of the white precipitate, magnesium hydroxide, is a clear indicator of this chemical balancing act.