Question

Predict which of the following metals reacts with hydrochloric acid. (a) \(\mathrm{Hg}\) (b) \(\mathbf{P b}\) (c) Ni (d) \(\mathrm{Cr}\)

Short answer

Chromium (Cr) reacts with hydrochloric acid.

Step-by-step solution

Understand the Problem

We need to predict which of the given metals: Mercury (Hg), Lead (Pb), Nickel (Ni), and Chromium (Cr), will react with hydrochloric acid (HCl). This relies on understanding the activity series of metals, which is a list of metals arranged in order of decreasing reactivity.

Recall the Activity Series

The activity series helps us determine if a metal will react with an acid. Metals higher in the series will displace hydrogen from acids, while those lower will not. Metals like Potassium, Sodium, and Calcium are highly reactive, while metals like Gold and Platinum are less reactive.

Identify Metal Placement in the Activity Series

In the activity series, Chromium (Cr) and Nickel (Ni) are above hydrogen, Mercury (Hg) is below hydrogen, and Lead (Pb) is slightly above. Lead's reaction can be slow or inhibited due to passivation.

Apply the Activity Series

A metal will react with hydrochloric acid if it is above hydrogen in the activity series. Since Chromium ( Cr) and Nickel ( Ni) are above hydrogen, they will react with hydrochloric acid. Although Lead ( Pb) is sometimes just above hydrogen, it might only react weakly or not at all due to the formation of a protective oxide or chloride layer.

Conclude Based on Activity

Only Chromium ( Cr) will reliably react with hydrochloric acid. Nickel ( Ni) may also react but not as vigorously, and Lead ( Pb) might not react effectively due to protective oxide layers.

Key concepts

Metal Reactivity

The concept of metal reactivity is crucial for understanding how different metals interact with various substances, including acids. The activity series of metals is a list that ranks metals from the most reactive to the least reactive. This order is determined by how easily metals can lose electrons to form positive ions. For example, metals like potassium and sodium are at the top of this series because they easily give up their electrons and react vigorously. In contrast, metals such as gold and platinum are at the bottom because they don't lose electrons easily, making them less reactive.

This hierarchy helps predict reactions; metals more reactive than hydrogen can usually displace hydrogen in reactions. Those that are less reactive can't displace hydrogen, making them inert in such scenarios. Understanding this reactivity sequence enables us to anticipate how metals will behave in chemical processes, particularly with acids.

Hydrochloric Acid Reactions

Hydrochloric acid, often represented as HCl, is a strong acid commonly used in laboratories and industry. When metals react with hydrochloric acid, the primary reaction is the metal displacing hydrogen from the acid, releasing hydrogen gas and forming a metal chloride. This is a key characteristic of metals in the activity series above hydrogen.

For instance, when a reactive metal such as zinc is introduced to hydrochloric acid, the reaction will produce zinc chloride and hydrogen gas. However, an unreactive metal won't engage in this reaction and the acid will remain unchanged. The outcome depends on the metal's position in the activity series. Chromium, being above hydrogen, reacts with hydrochloric acid, causing hydrogen to evolve, while mercury, below hydrogen in the series, does not react under normal conditions.

Displacement Reactions

Displacement reactions are a fascinating type of chemical reaction where an element reacts with a compound, displacing another element from it. In the context of metal and acid reactions, a displacement reaction occurs when a metal atom replaces the hydrogen ion in the acid, releasing hydrogen gas and forming a new compound, often a metal salt.

These reactions are reliant upon the activity series of metals, as only metals higher than hydrogen are capable of displacing it from acids. For example, chromium and nickel, being higher than hydrogen in the activity series, can engage in displacement reactions with hydrochloric acid. The metal atoms donate electrons to the hydrogen ions, reducing them to hydrogen gas and forming their respective metal chlorides. This sequence is a practical illustration of the reactivity hierarchy and is a foundational principle in chemistry for predicting the outcomes of reactions involving metals and acids.