Question

Which of the following metals will react with \(\mathrm{NaOH}\) and \(\mathrm{KOH}\) to liberate hydrogen gas? (a) \(\mathrm{Zn}, \mathrm{Al}, \mathrm{Fe}\) and \(\mathrm{Mg}\) (b) Al, Fe, \(\mathrm{Mg}\) and \(\mathrm{Sn}\) (c) \(\mathrm{Zn}, \mathrm{Sn}\) and \(\mathrm{Al}\) (d) \(\mathrm{Fe}, \mathrm{Mg}\) and \(\mathrm{Al}\)

Short answer

Metals that will react with NaOH and KOH to liberate hydrogen gas include Zn, Sn, and Al. Thus, the correct answer is option (c).

Step-by-step solution

Understanding the Reactivity with Alkali

Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are strong alkaline substances, commonly referred to as alkalis. Metals that react with NaOH and KOH to release hydrogen gas are those that are amphoteric or can behave as reducing agents. Amphoteric metals, like zinc and aluminum, can react with both acids and bases. Some of the less reactive metals such as iron (Fe) and tin (Sn) can also react with very strong bases under the right conditions to produce hydrogen gas.

Evaluating the Options

For each given group of metals, consider their chemical properties and whether they are known to react with NaOH and KOH to liberate hydrogen. (a) Zn, Al, Fe, and Mg: Zinc and aluminum are known to react with alkalis such as NaOH and KOH. Iron and magnesium can also react with them but usually under more specific conditions (e.g., high temperature) or with pure metals in powder form.(b) Al, Fe, Mg, and Sn: Aluminum and tin are amphoteric and hence will react with NaOH and KOH. For Fe and Mg, the similar considerations as in option (a) apply.(c) Zn, Sn, and Al: All of these metals are capable of reacting with alkalis, considering that tin and aluminum are amphoteric and zinc can act as a reducing agent.(d) Fe, Mg, and Al: While aluminum will definitely react with NaOH and KOH, Fe and Mg's reactivity is not as straightforward and depends on specific conditions.

Determining the Correct Answer

Based on the known chemical properties of the metals and the requirement that they should react with NaOH and KOH, we can rule out the options without zinc and aluminum, which are the most commonly known to react with these alkalis to liberate hydrogen. Therefore, the correct answer is the option that includes both zinc and aluminum and any other metal that can react under the right conditions. This is option (c): Zn, Sn, and Al.

Key concepts

Amphoteric Metals

When studying the reactivity of metals with alkalis, it's important to understand what amphoteric metals are. These metals have the unique ability to react with both acidic and basic substances. Two well-known examples of amphoteric metals are zinc (Zn) and aluminum (Al). In an educational context, visualizing the reactivity of amphoteric metals can be particularly helpful.

In their reactions with strong bases like sodium hydroxide (NaOH) and potassium hydroxide (KOH), amphoteric metals like Al and Zn can engage in a chemical process that results in the liberation of hydrogen gas. The reaction occurs because these metals can act as both Lewis acids and bases. They essentially 'donate' some of their electrons to the hydroxide ions in the alkalis, leading to the production of hydrogen.

• Zinc reacting with NaOH gives sodium zincate and hydrogen gas
• Aluminum reacting with NaOH forms sodium aluminate and hydrogen gas

Understanding this dual capability is crucial when predicting the reactivity patterns of metals with different substances.

Reducing Agents

Reducing agents play a pivotal role in chemical reactions, including those involving the release of hydrogen gas when metals react with alkalis. A reducing agent is a substance that donates electrons to another substance during a chemical reaction, thereby reducing the oxidation state of the other substance.

In the metal-alkali reaction scenario, the metal acts as the reducing agent. For instance, when zinc (Zn), which is a good reducing agent, reacts with sodium hydroxide (NaOH), it donates electrons to the hydroxide ions. This electron transfer is the force behind the reaction, ultimately leading to the formation of hydrogen gas and a salt.

• Zn, as a reducing agent, changes from an oxidation state of 0 to +2
• The hydroxide ion (OH-) is reduced to hydrogen gas (H2)

It's important to note that not all metals will act as reducing agents in these circumstances; this depends on their propensity to lose electrons and their position in the electrochemical series.

Hydrogen Gas Liberation

One of the key observations when metals react with alkalis is the liberation of hydrogen gas. This outcome is indicative of a chemical reaction taking place. Hydrogen gas is produced when certain metals react with strong bases like NaOH and KOH as part of a redox reaction. This is an important concept in both chemistry and practical applications such as hydrogen production.

The occurrence of hydrogen gas production during the reaction can be explained by the metal (acting as a reducing agent) transferring electrons to the hydroxide ions present in the alkali, which leads to the formation of water and the release of hydrogen gas. The ability to predict this reaction helps students understand not only the theoretical aspects of the chemical behavior of elements but also the practical implications in real-world applications such as clean energy.

• Zn + 2NaOH → Na2ZnO2 + H2↑
• 2Al + 2NaOH + 2H2O → 2NaAlO2 + 3H2↑