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Chapter 18: Problem 123

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 \(\mathrm{Cu}\) surfaces. (a) Write equations representing the reactions occurring at the metals. (b) What visual evidence would you seek to show that \(\mathrm{Cu}\) is not oxidized to \(\mathrm{Cu}^{2+} ?\) (c) At some stage, 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?

Short Answer

Expert verified
The reactions at the metal surfaces are \( \mathrm{Mg} (s) + 2 \mathrm{HCl} (aq) \rightarrow \mathrm{MgCl}_2 (aq) + \mathrm{H}_2 (g) \) and \( \mathrm{Cu} (s) + 2 \mathrm{HCl} (aq) \rightarrow \mathrm{CuCl}_2 (aq) + \mathrm{H}_2 (g)\), though the latter does not actually occur. The absence of a blue-green color in the solution would serve as visual evidence that the copper has not been oxidized to \( \mathrm{Cu}^{2+} \). The white precipitate that forms upon the addition of NaOH is magnesium hydroxide, \( Mg(OH)_2 \).

Step by step solution

01

Construct Chemical Reaction Equations

The reactions at the two metals, magnesium and copper, can be represented as follows: At the magnesium surface: \( \mathrm{Mg} (s) + 2 \mathrm{HCl} (aq) \rightarrow \mathrm{MgCl}_2 (aq) + \mathrm{H}_2 (g) \) . This equation represents the reaction of magnesium metal with hydrochloric acid to form magnesium chloride and hydrogen gas. At the copper surface: \( \mathrm{Cu} (s) + 2 \mathrm{HCl} (aq) \rightarrow \mathrm{CuCl}_2 (aq) + \mathrm{H}_2 (g) \) . This represents the reaction of copper metal with hydrochloric acid to form copper (II) chloride and hydrogen gas. However, in this scenario, this reaction does not actually happen, as copper doesn't normally react with hydrochloric acid to form Cu2+ ions.
02

Visual Evidence for Absence of Copper Oxidation

A visual evidence would be the absence of a blue-green color in the solution. If copper is oxidized to \( \mathrm{Cu}^{2+} \), it would react with chloride ions available in the solution to form \( \mathrm{CuCl}_2 \), which gives a blue-green color. If the solution remains colorless, it indicates that the copper has not been oxidized to \( \mathrm{Cu}^{2+} \).
03

Identify the White Precipitate

Upon further addition of NaOH, a white precipitate forms which is magnesium hydroxide, \( Mg(OH)_2 \). Remember, the NaOH neutralizes the HCl acid first, and after that it reacts with the Mg2+ ions which are present in the solution (from the MgCl2 which was the product of the initial reaction between magnesium and HCl). This reaction can be formulated as follows: Mg2+ (aq) + 2OH- (aq) -> \(Mg(OH)_2 (s)\)

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Magnesium and Hydrochloric Acid Reaction
When magnesium comes in contact with hydrochloric acid, a chemical reaction occurs. The magnesium metal reacts with hydrochloric acid producing magnesium chloride and hydrogen gas. The balanced chemical equation for this reaction is:
  • Magnesium: \( \mathrm{Mg} (s) + 2 \mathrm{HCl} (aq) \rightarrow \mathrm{MgCl}_2 (aq) + \mathrm{H}_2 (g) \)
This reaction is visible as bubbles form on the magnesium, indicating hydrogen gas is being released.
The effervescence is a key sign that a chemical reaction is taking place, showcasing the active nature of magnesium in acidic solutions.
Copper and Hydrochloric Acid Reaction
Copper is known for its resistance to reacting with hydrochloric acid. Practically, copper does not dissolve in HCl to form \( \mathrm{Cu}^{2+} \) ions. Therefore, the theoretical reaction:
  • Copper: \( \mathrm{Cu} (s) + 2 \mathrm{HCl} (aq) \rightarrow \mathrm{CuCl}_2 (aq) + \mathrm{H}_2 (g) \)
...does not occur under normal conditions. This non-volatility of copper in hydrochloric acid helps us conclude why the experiment remains circular without cupric ions. Despite writing the equation, the absence of the reaction supports copper鈥檚 stability in acid.
Visual Evidence of Copper Oxidation
Understanding copper's potential oxidation requires visual observation of the solution's color. If copper indeed oxidized to \( \mathrm{Cu}^{2+} \), a blue-green tint would appear in the solution, resulting from the formation of copper (II) chloride(\( \mathrm{CuCl}_2 \)).
The solution's continuing transparency signifies that oxidation did not occur; copper remains unreacted.
Thus, a lack of blue-green coloration assures the absence of copper oxidation within the solution.
Magnesium Hydroxide Precipitation
When sodium hydroxide is added continuously to the reaction mix, it begins to neutralize any leftover hydrochloric acid.Once neutralization occurs, any extra hydroxide ions in the solution will start interacting with magnesium ions present from the initial magnesium reaction with HCl.At this point, the reaction for magnesium hydroxide formation can be written as:
  • \( \mathrm{Mg}^{2+} (aq) + 2\mathrm{OH}^{-} (aq) \rightarrow \mathrm{Mg(OH)}_2 (s) \)
This forms a white precipitate, reaching at the bottom, and serves as an indication that the solution contains free magnesium ions.
Hence, the formation of this white solid represents an essential step in recognizing what occurs after neutralizing acid with NaOH.

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Most popular questions from this chapter

Industrially, copper is purified by electrolysis. The impure copper acts as the anode, and the cathode is made of pure copper. The electrodes are immersed in a CuSO \(_{4}\) solution. During electrolysis, copper at the anode enters the solution as \(\mathrm{Cu}^{2+}\) while \(\mathrm{Cu}^{2+}\) ions are reduced at the cathode. (a) Write halfcell reactions and the overall reaction for the electrolytic process. (b) Suppose the anode was contaminated with \(\mathrm{Zn}\) and \(\mathrm{Ag} .\) Explain what happens to these impurities during electrolysis. (c) How many hours will it take to obtain \(1.00 \mathrm{~kg}\) of \(\mathrm{Cu}\) at a current of \(18.9 \mathrm{~A} ?\)

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Write the Nernst equation and explain all the terms.

What is the potential of a cell made up of \(\mathrm{Zn} / \mathrm{Zn}^{2+}\) and \(\mathrm{Cu} / \mathrm{Cu}^{2+}\) half-cells at \(25^{\circ} \mathrm{C}\) if \(\left[\mathrm{Zn}^{2+}\right]=0.25 \mathrm{M}\) and \(\left[\mathrm{Cu}^{2+}\right]=0.15 \mathrm{M} ?\)
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