Question

When aluminium is added to potassium hydroxide solution (1) No reaction takes place. (2) Oxygen is evolved. (3) Water is produced. (4) Hydrogen is evolved.

Short answer

4. Hydrogen is evolved.

Step-by-step solution

Analyze the Given Chemicals

Identify the reactants: Aluminium (Al) and Potassium Hydroxide (KOH). Aluminium is a metal, and potassium hydroxide is a strong base.

Understand the Reaction Context

Metallic aluminium can react with strong bases like KOH to form aluminate ions and release hydrogen gas. The general reaction is: \[2Al + 2KOH + 6H_2O \rightarrow 2K[Al(OH)_4] + 3H_2\]

Determine Products and Gases Released

From the balanced chemical equation, observe that the reaction produces potassium aluminate (K[Al(OH)_4]) and releases hydrogen gas. No oxygen or water is produced as primary products of this reaction.

Evaluate Each Option

Evaluate the given options based on the products of the reaction: 1. No reaction takes place (Incorrect, reaction occurs).2. Oxygen is evolved (Incorrect, hydrogen is evolved, not oxygen).3. Water is produced (Incorrect, water is a reactant, not a product).4. Hydrogen is evolved (Correct, based on the chemical equation).

Conclusion

The correct answer is that hydrogen is evolved when aluminium reacts with potassium hydroxide.

Key concepts

Reactivity of metals

The reactivity of a metal determines how well it can react with other substances. Aluminium might seem unreactive because it forms a protective oxide layer when exposed to air. However, under certain conditions, like in the presence of a strong base, it can react quite vigorously. Metals like aluminium are more reactive with bases rather than acids. This is why, when aluminium is added to potassium hydroxide (KOH), an active reaction occurs. The aluminium displaces hydrogen from water molecules present in the base solution, highlighting its reactive nature. Understanding the reactivity series helps predict such reactions: metals above hydrogen tend to react with acids and bases to liberate hydrogen gas.

Strong bases

A base is a substance that can accept hydrogen ions (protons) or more generally, donate electron pairs. Strong bases, like potassium hydroxide (KOH), dissociate completely in water, producing a high concentration of hydroxide ions (OH⁻). When these hydroxide ions come into contact with aluminium, they disrupt the oxide layer on the aluminium, allowing the metal to react with water in the solution. This produces aluminate ions. The complete dissociation of KOH impels the reaction forward because there are plenty of OH⁻ ions to interact with the aluminium. Strong bases are typically very reactive and are capable of breaking down materials that seem stable otherwise.

Balanced chemical equations

A balanced chemical equation ensures that the number of atoms of each element is the same on both sides of the equation. This balances the law of conservation of mass. By writing the balanced equation: \[2Al + 2KOH + 6H_2O \rightarrow 2K[Al(OH)_4] + 3H_2\], we see that two atoms of aluminium (Al) react with two molecules of potassium hydroxide (KOH) and six molecules of water (H₂O). The reaction produces two molecules of potassium aluminate (K[Al(OH)_4]) and three molecules of hydrogen gas (H₂). Balancing equations is essential because it reflects the actual stoichiometry of the reactants and products. Unbalanced equations can lead to incorrect predictions about the amounts and types of substances involved in chemical reactions.

Hydrogen evolution

In the reaction between aluminium and potassium hydroxide, hydrogen gas is evolved. This is because the aluminium reacts with the hydroxide ions from the potassium hydroxide solution, which results in the displacement of hydrogen from water (H₂O) molecules. The equation \[2Al + 2KOH + 6H_2O \rightarrow 2K[Al(OH)_4] + 3H_2\] shows that three molecules of hydrogen gas are produced for every two atoms of aluminium and two molecules of potassium hydroxide used. This gas evolution is an indicator of a chemical reaction taking place. The hydrogen evolution can be detected by the appearance of bubbles in the solution, and it’s also why hydrogen is checked as the correct answer in the given options.