Problem 57 - Chemistry The Central Science In Si Units

Question

The metal cadmium tends to form Cd \(^{2+}\) ions. The following observations are made: (i) When a strip of zinc metal is placed in \(\mathrm{CdCl}_{2}(a q),\) cadmium metal is deposited on the strip. (ii) When a strip of cadmium metal is placed in \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}(a q),\) nickel metal is deposited on the strip. (a) Write net ionic equations to explain each of the preceding observations. (b) Which elements more closely define the position of cadmium in the activity series? (c) What experiments would you need to perform to locate more precisely the position of cadmium in the activity series?

Step-by-Step Solution

Answer

(a) \( \mathrm{Zn}(s) + \mathrm{Cd}^{2+}(aq) \rightarrow \mathrm{Zn}^{2+}(aq) + \mathrm{Cd}(s) \); \( \mathrm{Cd}(s) + \mathrm{Ni}^{2+}(aq) \rightarrow \mathrm{Cd}^{2+}(aq) + \mathrm{Ni}(s) \). (b) Cadmium is between zinc and nickel. (c) Test cadmium against copper and iron solutions.

Step 1: Understand the Problem

We need to write net ionic equations for the reactions described in observations (i) and (ii), determine which elements are near cadmium's position in the activity series, and propose experiments for more precise placement.

Step 2: Write the Net Ionic Equation for Zinc and Cadmium Reaction

Observation (i) states that when zinc metal is placed in CdCl₂ solution, cadmium is deposited. This suggests a redox reaction where zinc is oxidized and cadmium is reduced. The net ionic equation is: \( \mathrm{Zn}(s) + \mathrm{Cd}^{2+}(aq) \rightarrow \mathrm{Zn}^{2+}(aq) + \mathrm{Cd}(s) \).

Step 3: Write the Net Ionic Equation for Cadmium and Nickel Reaction

For observation (ii), cadmium reacts with Ni(NO₃)₂ solution to deposit nickel. In this redox reaction, cadmium is oxidized and nickel is reduced. The net ionic equation is: \( \mathrm{Cd}(s) + \mathrm{Ni}^{2+}(aq) \rightarrow \mathrm{Cd}^{2+}(aq) + \mathrm{Ni}(s) \).

Step 4: Determine Cadmium's Position in the Activity Series

Based on the reactions, cadmium displaces nickel from its solution, indicating that cadmium is more reactive than nickel. However, zinc displaces cadmium, showing that zinc is more reactive than cadmium. Therefore, cadmium is positioned between zinc and nickel in the activity series.

Step 5: Propose Experiments to Precisely Locate Cadmium

To more precisely determine cadmium's position, compare it with other metals such as copper and iron. Conduct experiments by placing cadmium in solutions of these metals' ions to see if cadmium can displace them or vice versa, providing further information on its relative reactivity.

Key Concepts

Redox Reactions

Redox reactions are chemical processes where two species undergo simultaneous reduction and oxidation. These reactions involve the transfer of electrons between substances. In a redox reaction, one element or compound gets oxidized, meaning it loses electrons, while another species is reduced, meaning it gains electrons.

• Oxidation: This involves loss of electrons. The substance that loses electrons is said to be oxidized.
• Reduction: This refers to the gain of electrons. The substance that gains electrons undergoes reduction.

For example, in the reaction between zinc and cadmium ions, zinc acts as the reducing agent, losing electrons and forming Zn²⁺ ions. Concurrently, the cadmium ions gain these electrons to form metallic cadmium. Redox reactions are fundamental in understanding how different substances interact through electron exchange, often revealed through changes in oxidation states and material deposition.

Net Ionic Equations

Net ionic equations are simplified chemical equations that focus only on the species that participate in a reaction. These equations exclude spectator ions—ions that do not partake in the actual chemical transformation.
To write a net ionic equation, follow these steps:

• Write the balanced molecular equation for the reaction.
• Split the soluble ionic compounds into individual ions, known as a complete ionic equation.
• Identify and eliminate spectator ions, leaving only the species that undergo change.

In the reactions discussed:- The zinc and cadmium reaction simplifies to: \( \mathrm{Zn}(s) + \mathrm{Cd}^{2+}(aq) \rightarrow \mathrm{Zn}^{2+}(aq) + \mathrm{Cd}(s) \). - For cadmium and nickel, the equation is: \( \mathrm{Cd}(s) + \mathrm{Ni}^{2+}(aq) \rightarrow \mathrm{Cd}^{2+}(aq) + \mathrm{Ni}(s) \).These equations highlight the substances undergoing electron transfer, providing a clearer understanding of the redox process at play.

Metal Reactivity

Metal reactivity refers to a metal's tendency to undergo chemical reactions, often by displacing another ion from solution in redox reactions. Metals higher on the reactivity series are more likely to donate electrons and participate in oxidation processes.
Key aspects of metal reactivity include:

• Reactivity Series: A list of metals ranked by their reactivity. More reactive metals can displace less reactive metals from their compounds.
• Displacement Reactions: A type of redox reaction where a more reactive metal displaces a less reactive metal from a solution.

Cadmium sits in a middle position in the activity series, capable of displacing nickel (less reactive) but being displaced by zinc (more reactive). Understanding metal reactivity helps predict reaction outcomes and guide experiments, crucial in fields like metallurgy and chemistry.

Displacement Reactions

Displacement reactions are a subtype of redox reactions characterized by one metal replacing another in a compound. They are vital in determining the relative reactivity of metals and help in constructing the activity series.
The principles of displacement reactions are:

• A more reactive metal can displace a less reactive one from its compound in solution.
• The process results in a solid metal forming when ions in solution become reduced.

In the example of zinc and cadmium, zinc displaces cadmium from its chloride solution, indicating higher reactivity. When cadmium is placed with nickel ions, cadmium displaces nickel, showing its position between zinc and nickel in reactivity. These interactions confirm and refine knowledge about metal positions within the activity series, demonstrated through experimental observations and empirical data.