Question

Chemical tests of four metals \(\mathrm{A}, \mathrm{B}, \mathrm{C},\) and \(\mathrm{D}\) show the following results: (a) Only \(\mathrm{B}\) and \(\mathrm{C}\) react with \(0.5 \mathrm{M}\) \(\mathrm{HCl}\) to give \(\mathrm{H}_{2}\) gas. (b) When \(\mathrm{B}\) is added to a solution containing the ions of the other metals, metallic \(\mathrm{A}, \mathrm{C},\) and \(\mathrm{D}\) are formed. (c) A reacts with \(6 \mathrm{M} \mathrm{HNO}_{3}\), but \(\mathrm{D}\) does not. Arrange the metals in the increasing order as reducing agents. Suggest four metals that fit these descriptions.

Short answer

Order: D, A, C, B. Metals: Hg, Cu, Fe, Zn.

Step-by-step solution

Analyze reactivity with HCl

Metals B and C react with 0.5 M HCl to form H2 gas, indicating they are more reactive than A and D, which do not liberate hydrogen.

Determine reactivity based on replacement reactions

Metal B can displace ions of A, C, and D from their solutions, suggesting that B is more reactive than A, C, and D.

Analyze reactivity with concentrated nitric acid

Metal A reacts with 6 M HNO3, while D does not react, implying A is more reactive than D.

Arrange metals by decreasing reactivity

Combining all facts, B is the most reactive since it reacts with HCl and displaces others, C reacts with HCl, A reacts with HNO3, and D is the least reactive. Therefore, the order from most to least reactive as reducing agents is B, C, A, D.

Suggest appropriate metals

Metals fitting the description are: - B could be Zinc (Zn), as it reacts with HCl and displaces others in solutions. - C could be Iron (Fe), as it reacts with HCl. - A could be Copper (Cu), which reacts with HNO3. - D could be Mercury (Hg), which does not react with HCl or HNO3.

Key concepts

Metal Reactivity

Metal reactivity refers to the tendency of a metal to undergo chemical reactions. Some metals are more reactive, easily partaking in reactions, while others are less inclined to react. The reactivity series, or activity series, is a list of metals arranged from most reactive to least reactive. Understanding this helps in predicting how metals will behave in different chemical reactions.
Factors influencing metal reactivity include:

• The ease with which a metal atom can lose electrons to form positive ions.
• The metal’s position on the reactivity series; metals at the top are more reactive than those at the bottom.

In the original exercise, Metals B and C are more reactive because they can react with 0.5 M HCl, which showcases their ability to displace hydrogen from acids. This behavior indicates a higher position in the reactivity series.

Chemical Reactions

Chemical reactions involve the transformation of one or more substances into new products. These processes can be as simple as combining molecules or as complex as restructuring entire compounds. Reactivity plays a crucial role in chemical reactions, as it dictates how different substances interact with one another.
Chemical reactions include several types, such as:

• Combination reactions, where two reactants form a single product.
• Decomposition reactions, where one reactant breaks down into multiple products.
• Displacement reactions, where an element in a compound is replaced by another element.

For instance, when Metal B reacts with HCl, a chemical reaction occurs where hydrogen gas (\(\mathrm{H}_{2}\)) is released, demonstrating that B plays an active role in the reaction by replacing hydrogen from the acid.

Displacement Reactions

Displacement reactions occur when a more reactive metal displaces a less reactive metal from its compound. This is a type of redox reaction where one element is oxidized and another is reduced.
In the displacement reactions:

• A more reactive metal loses electrons (oxidation) faster than a less reactive one.
• The less reactive metal typically gains electrons (reduction) and is displaced.

In the exercise, Metal B can displace Metals A, C, and D from their solutions, which shows B's higher reactivity. These reactions highlight the importance of understanding the reactivity series to predict which metals can displace others from a compound.

Reducing Agents

A reducing agent is a substance that donates electrons to another substance during a chemical reaction, resulting in the reduction of the other component. The more an element is ready to lose electrons, the stronger a reducing agent it becomes.
In the context of the given problem, metals act as reducing agents. The reactivity order helps determine which metal is the best reducing agent:

• Metal B, being the most reactive, is the strongest reducing agent as it most readily donates electrons.
• Metal D, being the least reactive, is the weakest reducing agent.

Recognizing reducing agents helps in predicting reaction outcomes, as the sequence from strongest to weakest reducing agent aligns with their reactivity: B > C > A > D.