Question

Which of the following metals does not liberate hydrogen from acids? (a) \(\mathrm{Fe}\) (b) \(\mathrm{Cu}\) (c) \(\mathrm{Mg}\) (d) \(\mathrm{Zn}\)

Short answer

Copper (Cu) does not liberate hydrogen from acids.

Step-by-step solution

Understanding Metal Reactivity with Acids

For a metal to liberate hydrogen from an acid, it must be capable of undergoing a redox reaction with the acid. This property is determined by the metal's position in the electrochemical series, also known as the reactivity series.

Identifying if Metals can Liberate Hydrogen from Acids

Reactivity series order for the given options is generally: Mg > Zn > Fe > Cu. Metals like Magnesium (Mg), Zinc (Zn), and Iron (Fe) are above Hydrogen in the reactivity series and can displace Hydrogen from an acid. Copper (Cu), however, being below Hydrogen, cannot displace Hydrogen from an acid.

Determining the Metal That Does Not Liberate Hydrogen

Based on the reactivity series, Copper (Cu) is less reactive than Hydrogen and cannot liberate Hydrogen from acids, as opposed to Fe, Mg, and Zn which are more reactive and can liberate Hydrogen.

Key concepts

Electrochemical Series

When it comes to understanding how different metals react with acids, the electrochemical series is an important tool. It is essentially a ranking of elements according to their standard electrode potentials. Metals that are higher in the series, like Potassium and Sodium, are more reactive and have a greater tendency to lose electrons and form positive ions. Conversely, metals lower on the series, such as Gold and Platinum, are less reactive.

For students, it’s useful to comprehend that this series not only predicts whether a metal will react with acids to release hydrogen gas but also how vigorous that reaction will be. Magnesium, for example, reacts rapidly and exothermically with acids, which is consistent with its high position in the series. In your exercise, understanding this concept is crucial to determining that Copper (Cu) is the metal that does not liberate hydrogen when reacting with acids due to its lower position in the electrochemical series.

Redox Reactions

A redox reaction is a chemical process involving the transfer of electrons from one substance to another. It combines two separate processes – reduction, where an atom or ion gains electrons, and oxidation, where an atom or ion loses electrons.

In the context of metals reacting with acids, the metal atoms lose electrons (get oxidized) and the hydrogen ions in the acid gain electrons (get reduced), resulting in the liberation of hydrogen gas. The ability of a metal to oxidize and therefore reduce hydrogen ions to hydrogen gas is influenced by its position in the electrochemical series. For the metals listed in your exercise, Iron (Fe), Magnesium (Mg), and Zinc (Zn) can all oxidize readily, thereby serving as reducing agents that react with acids in redox reactions to produce hydrogen gas.

Hydrogen Displacement

Hydrogen displacement is a specific type of redox reaction where a metal displaces hydrogen from an acid. This process occurs based on the activity or reactivity of the metal involved. Typically, a more reactive metal can displace a less reactive metal from its compound; the same applies to hydrogen.

Metals above hydrogen in the electrochemical series, which includes most metals, can displace hydrogen from acids. This happens because these metals have a greater affinity for electrons compared to hydrogen, so when they react with acids (which contain hydrogen ions), the metal atoms can donate electrons to hydrogen ions, forming hydrogen gas and a salt. According to the exercise, only Copper (Cu) cannot displace hydrogen from acids as it is below hydrogen in the series. This ties back to the previous concepts, showing how the electrochemical series and redox reactions are crucial in understanding hydrogen displacement.