Question

Give a chemical explanation for each of the following: (a) When calcium metal is added to a sulfuric acid solution, hydrogen gas is generated. After a few minutes, the reaction slows down and eventually stops even though none of the reactants is used up. Explain. (b) In the activity series, aluminum is above hydrogen, yet the metal appears to be unreactive toward hydrochloric acid. Why? (Hint: Al forms an oxide, \(\mathrm{Al}_{2} \mathrm{O}_{3},\) on the surface.) (c) Sodium and potassium lie above copper in the activity series. Explain why \(\mathrm{Cu}^{2+}\) ions in a \(\mathrm{CuSO}_{4}\) solution are not converted to metallic copper upon the addition of these metals. (d) A metal M reacts slowly with steam. There is no visible change when it is placed in a pale green iron(II) sulfate solution. Where should we place \(\mathrm{M}\) in the activity series? (e) Before aluminum metal was obtained by electrolysis, it was produced by reducing its chloride \(\left(\mathrm{AlCl}_{3}\right)\) with an active metal. What metals would you use to produce aluminum in that way?

Short answer

Calcium reacts with sulfuric acid quickly but forms an insoluble coating; aluminum forms a protective oxide barrier; sodium and potassium react with water first; metal M is less reactive than iron; use sodium or calcium to reduce AlCl3.

Step-by-step solution

Reaction of Calcium with Sulfuric Acid

When calcium metal (Ca) is added to sulfuric acid (H₂SO₄), hydrogen gas (H₂) is initially produced by the reaction \( \text{Ca} + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2 \uparrow \). However, the reaction slows down over time because calcium sulfate (CaSO₄), an insoluble salt, forms a coating on the surface of the calcium metal. This coating acts as a barrier, preventing further contact between calcium and sulfuric acid, slowing down and eventually stopping the reaction.

Aluminum's Apparent Unreactivity with Hydrochloric Acid

Although aluminum (Al) is above hydrogen in the activity series and should react with acids like hydrochloric acid (HCl), its surface quickly oxidizes to form a protective aluminum oxide (Al₂O₃) layer. This oxide layer is very thin yet strong, effectively preventing further corrosion of the metal underneath by acting as a non-reactive barrier to the acid.

Inactivity of Sodium and Potassium with Copper(II) Ions

While sodium (Na) and potassium (K) are highly reactive and above copper (Cu) in the activity series, when added to copper sulfate (CuSO₄) solution, they do not reduce copper ions \(\text{Cu}^{2+}\) to metallic copper. This is because these metals react violently with water, forming hydroxides and hydrogen gas before they can interact with the copper ions effectively. The water reaction dominates in aqueous solutions.

Placement of Metal M in the Activity Series

The metal M reacts slowly with steam, suggesting it is above hydrogen but below the more reactive metals like sodium and potassium. Since there is no reaction with iron(II) sulfate (FeSO₄), it indicates that M is less reactive than iron. Therefore, M can be placed below iron and above hydrogen in the activity series.

Reducing Aluminum Chloride with Active Metals

Before electrolysis, aluminum was produced by reducing aluminum chloride (AlCl₃) with a more active metal. Metals such as sodium (Na) or calcium (Ca) can be used for this process since they are above aluminum in the activity series, meaning they can donate electrons to aluminum ions, reducing Al³⁺ to metallic aluminum.

Key concepts

Calcium Reaction with Sulfuric Acid

When calcium is introduced to sulfuric acid, an exciting chemical interaction occurs. The combination of calcium (\[ \text{Ca} \]) and sulfuric acid (\[ \text{H}_2\text{SO}_4 \]) results in the generation of hydrogen gas (\[ \text{H}_2 \]). The chemical reaction can be depicted as follows: \[ \text{Ca} + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2 \uparrow \] Initially, this reaction is quite vigorous with the visible release of bubbles. However, it slows and eventually halts. This decline in activity is due to the formation of calcium sulfate (\[ \text{CaSO}_4 \]), a compound that creates an insoluble layer on the calcium metal's surface.

• This layer acts as a barrier.
• It prevents further contact between the reactive components.
• The reaction stops once the barrier is complete.

This protective film is why the reaction doesn't consume all reactants, yet ceases after some time.

Aluminum Oxide Layer

Aluminum, although positioned above hydrogen in the activity series, seems surprisingly unreactive when it comes into contact with dilute hydrochloric acid (\[ \text{HCl} \]). The reason behind this anomaly lies in aluminum's ability to form a robust oxide layer. This oxide, aluminum oxide (\[ \text{Al}_2\text{O}_3 \]), covers the surface of the metal as soon as it is exposed to oxygen.

• The aluminum oxide layer is thin yet tough.
• It acts as a protective shield.
• This barrier prevents acid from corroding the aluminum beneath.

Thus, even though aluminum is theoretically reactive, its interaction with \[ \text{HCl} \] is minimal under normal conditions, due to this naturally occurring protective film.

Copper Sulfate Reaction

Copper has its own set of chemical mysteries. In a solution of copper sulfate (\[ \text{CuSO}_4 \]), ions of copper (\[ \text{Cu}^{2+} \]) are present. When more reactive metals, such as sodium or potassium, are added to this solution, one might expect them to reduce the copper ions to copper metal. However, sodium and potassium are so reactive with water that another process takes precedence. Upon contact with water, they form hydroxides and release hydrogen gas.

• The violent reaction with water dominates.
• This prevents interaction with copper ions.
• As a result, copper remains in ionic form.

The predominant activity of these metals with water overshadows any possibility of copper ion reduction in the solution.

Metal Reactivity with Water

A metal's position in the activity series significantly influences its interaction with water. When a metal, called M, reacts slowly with steam, it offers a clue about its chemical nature. This slow reaction implies that M is:

• More reactive than hydrogen, which doesn't react with steam.
• Less reactive than rapidly reacting metals like sodium or potassium.

Since M shows no visible change when placed in an iron(II) sulfate (\[ \text{FeSO}_4 \]) solution, it is less reactive than iron. Thus, M stands above hydrogen but below iron in the activity series, indicative of its moderate reactivity.

Reduction of Aluminum Chloride

Before the use of electrolysis, aluminum was obtained in a more traditional manner through chemical reduction. Aluminum chloride (\[ \text{AlCl}_3 \]) served as the reactant, but required a more reactive metal for its reduction into aluminum. Metals like sodium (\[ \text{Na} \]) or calcium (\[ \text{Ca} \]) were often used because they occupy a higher position than aluminum in the activity series. This higher placement allows them to donate electrons effectively, thus reducing aluminum ions (\[ \text{Al}^{3+} \]) into metallic aluminum.

• Active metals facilitate electron transfer.
• They reduce the aluminum ions in the process.
• This method preceded the modern electrolysis technique.

Through these chemical processes, pure aluminum was produced from its chloride form.