Problem 106 - Introductory Chemistry Atoms First

Question

You are trying to determine the identity of an unknown metal. You place a strip of it in a solution of \(\mathrm{Mg}^{2+}\), and no reaction occurs. You then place a strip of it in a solution of \(\mathrm{Zn}^{2+}\), and zinc metal plates out on the strip. Name one possibility for the identity of the unknown metal.

Step-by-Step Solution

Answer

One possibility for the identity of the unknown metal is aluminum (Al), as it is more reactive than zinc but less reactive than magnesium.

Step 1: Understand the reactivity series

The reactivity series is a list of metals arranged in order of their reactivity, from the most reactive to the least reactive. Reactivity here means how easily a metal loses electrons to form positive ions (cations). Metals with a higher reactivity will displace metals with a lower reactivity from their salts in solution.

Step 2: Evaluate the reaction with \(\mathrm{Mg}^{2+}\)

Since there is no reaction when the unknown metal is placed in a solution of \(\mathrm{Mg}^{2+}\), this means that the unknown metal is less reactive than magnesium. Otherwise, it would have displaced \(\mathrm{Mg}^{2+}\), causing a reaction.

Step 3: Evaluate the reaction with \(\mathrm{Zn}^{2+}\)

In the second reaction, the unknown metal is placed in a solution of \(\mathrm{Zn}^{2+}\), and zinc metal plates out on the strip. This means that the unknown metal is more reactive than zinc and displaces \(\mathrm{Zn}^{2+}\) in the solution.

Step 4: Identify a possible unknown metal using the reactivity series

Using the reactivity series, we can find a metal that is less reactive than magnesium but more reactive than zinc. One possibility for the unknown metal is aluminum (Al). Aluminum is more reactive than zinc but less reactive than magnesium, fitting the conditions given in the problem. Therefore, one possibility for the identity of the unknown metal is aluminum (Al).

Key Concepts

Displacement Reactions

Displacement reactions are fascinating chemical processes where a more reactive metal can push out a less reactive metal from a compound. This process happens because metals have different levels of reactivity. In a displacement reaction, the more reactive metal gives up electrons more easily, so it can effectively "take the place" of the less reactive metal in a solution. For instance, if you have a metal strip and dip it into a solution containing ions of another metal, the more reactive metal can cause the ions of the less reactive metal to deposit onto its surface. This is exactly what happened in the exercise when zinc metal plated out on the strip of the unknown metal. Key factors to consider in displacement reactions include:

• The identity of metals involved, as the reactivity level determines if a displacement will occur.
• The nature of the solution, which should contain ions of a less reactive metal for the displacement to take place.

Understanding displacement reactions is essential to identify unknown metals, through observing interactions and seeing which metals can replace others.

Metal Reactivity

Metal reactivity is defined by how easily a metal can lose electrons to form positive ions, known as cations. The most reactive metals are eager to give away their electrons and participate in chemical reactions, displacing less reactive metals from their compounds. The reactivity series lists metals from the most to least reactive. This ranking is crucial for predicting reactions. A highly active metal, such as potassium or magnesium, will react vigorously and is usually found higher up in the reactivity series. For example:

• Potassium is highly reactive and will quickly displace other metals lower in the series, like zinc or iron, from solutions.
• Zinc, being less reactive than magnesium, cannot displace magnesium from its ions.

Understanding the metal reactivity series helps determine what metals can replace others in chemical reactions and why certain reactions occur or don't occur when different metals are combined in solutions.

Cation Formation

Cation formation is a critical concept in understanding metal reactivity and displacement reactions. Metals tend to lose electrons during chemical reactions, forming positively charged ions called cations. When a metal loses electrons, it transforms into a cation, carrying a positive charge due to the imbalance between protons (positive) and electrons (negative). This process is fundamental in displacement reactions, where more reactive metals lose electrons and replace less reactive metals in solutions. Here's how it generally works:

• A metal in its elemental form contacts a solution with metal ions that are less reactive.
• The more reactive metal loses electrons to become cations, while the less reactive metal ions gain electrons and settle as a metal on the more reactive metal strip.

This exchange of electrons and formation of cations are what drive the displacement reactions, giving insight into which metals can or cannot be part of such chemical processes. Understanding these principles is key to identifying unknown metals based on their reactions with known solutions.