Question

Based on the following information arrange four metals \(A, B, C\) and \(D\) in order of decreasing ability to act as reducing agents: (I) Only \(A, B\) and \(C\) react with \(1 M \mathrm{HCl}\) to give \(\mathrm{H}_{2}(g)\) (II) When \(C\) is added to solutions of the other metal ions, metallic \(B\) and \(D\) are formed (III) Metal \(C\) does not reduce \(A^{n+}\). (a) \(C>A>B>D\) (b) \(C>A>D>B\) (c) \(A>C>D>B\) (d) \(A>C>B>D\)

Short answer

A>C>B>D

Step-by-step solution

Understanding Reducing Agent Ability

To be a good reducing agent, a metal must easily lose electrons to reduce other substances. The reactivity of metals with HCl to produce hydrogen gas indicates reducing ability. Metals reacting directly with HCl and releasing hydrogen gas are stronger reducing agents than those that do not.

Analyzing the Reactions with HCl

Since A, B, and C react with HCl to release hydrogen gas, but D does not, D is the weakest reducing agent among the four. Therefore, D will be placed last in the order.

Considering the Reaction of Metal C with Metal Ions

Metal C causes the reduction of metal ions B and D to their corresponding metallic forms, but does not reduce A^n+. This means C can reduce B and D but not A, placing C above B and D, but below A in terms of reducing power.

Ordering Based on Reducing Agent Ability

Based on the analysis, metal A does not get reduced by C but B and D do, meaning A is the strongest reducing agent. Since C reduces B and D, C is the second strongest. Because B is reduced by C, it is stronger than D but weaker than A and C. Therefore, the correct decreasing order of their ability to act as reducing agents is A > C > B > D.

Key concepts

Chemical Reactivity

Chemical reactivity is a crucial concept in chemistry that defines how easily a chemical species can participate in a reaction to form new substances. This is especially important when considering reduction-oxidation (redox) processes, where the transfer of electrons is at the core of the reaction.

Elements and compounds show varying levels of reactivity based on their tendencies to either gain or lose electrons. For metals, their ability to serve as reducing agents is a direct measure of their chemical reactivity. A good reducing agent is one that readily loses electrons, thereby reducing another reactant, and in doing so, becomes oxidized itself.

Factors that influence chemical reactivity include the element's position in the periodic table, the nature of its electron configurations, and the potential energy changes involved in a chemical reaction. For metals, their activity in reactions with acids, such as hydrochloric acid (HCl), also provides valuable insight into their reactivity. For instance, metals reacting with HCl to displace hydrogen indicate a higher level of reactivity compared to those that do not easily engage in such reactions.

Metal Reduction Reactions

Metal reduction reactions involve the process of gaining electrons, typically from a reducing agent. In such reactions, metal ions are transformed into a pure metallic state, a process that is fundamental to many areas of chemistry and industrial applications.

Understanding the ability of metals to act as reducing agents can help comprehend their behavior in reduction reactions. A metal that is a stronger reducing agent will have a greater tendency to lose electrons when presented with an opportunity such as in contact with metal ions in a solution.

Thus, in the context of our problem, when metal C is added to solutions with ions of metals A, B, and D, the observed reactions help determine the reducing strength of metal C. It can reduce ions B and D to the metal, thus making it a better reducing agent than both. However, since it cannot reduce A^n+, A is indicated to have a stronger reducing power than C.

Standard Reduction Potential

The standard reduction potential is a quantitative measure of the tendency of a chemical species to acquire electrons and thereby be reduced. This value is determined under standard conditions, which are 25 degrees Celsius, 1 atm pressure, and 1 M concentration for all reactants and products involved.

Standard reduction potentials are tabulated values that can predict the spontaneity of redox reactions. A higher reduction potential means a species is more likely to gain electrons and be reduced, making it a poorer reducing agent. Conversely, a species with a lower or more negative reduction potential is more likely to lose electrons and act as a reducing agent.

In textbook exercises like the one we're discussing, we often use the concept of standard reduction potential indirectly. By observing reactions between metals and metal ions, we infer the relative tendencies of metals to become oxidized. We deduce which metals are better reducing agents through their chemical behavior and then can associate these behaviors with their respective standard reduction potentials without necessarily referring to the tabulated values.