Question

Determine whether or not each metal dissolves in 1 \(\mathrm{M}\) HCl. For those metals that dissolve, write a balanced redox reaction showing what happens when the metal dissolves. a. Al \(\quad\) b. Ag \(\quad\) c. Pb

Short answer

Al dissolves in 1 M HCl with reaction: 2Al(s) + 6H^{+}(aq) -> 2Al^{3+}(aq) + 3H_{2}(g). Ag does not dissolve. Pb dissolves with reaction: Pb(s) + 2H^{+}(aq) -> Pb^{2+}(aq) + H_{2}(g).

Step-by-step solution

Check the Reactivity Series

Determine if Al, Ag, and Pb are above hydrogen in the reactivity series. Metals above hydrogen can displace it from acids like HCl.

Write the Reactions for Reactive Metals

For metals more reactive than hydrogen, write the half-reaction for the metal dissolving and the half-reaction for hydrogen ions being reduced to hydrogen gas, then combine them.

Determine the Reaction for Aluminum (Al)

Aluminum (Al) is more reactive than hydrogen. The half-reaction is: Al(s) -> Al^{3+}(aq) + 3e^{-}. The hydrogen half-reaction is: 2H^{+}(aq) + 2e^{-} -> H_{2}(g). Combine them in a way that cancels out electrons to get the balanced redox reaction.

Determine the Reaction for Silver (Ag)

Silver (Ag) is less reactive than hydrogen. It will not react with 1M HCl.

Determine the Reaction for Lead (Pb)

Lead (Pb) is more reactive than hydrogen. The half-reaction for lead is: Pb(s) -> Pb^{2+}(aq) + 2e^{-}. The balanced redox reaction when combined with the hydrogen reduction reaction is given by: Pb(s) + 2H^{+}(aq) -> Pb^{2+}(aq) + H_{2}(g).

Key concepts

Reactivity Series

Understanding the reactivity series is crucial when predicting the outcomes of reactions involving metals and acids like HCl. The reactivity series is a list of metals arranged in order of decreasing reactivity. Metals at the top, such as potassium and lithium, are highly reactive with acids and water, while those at the bottom like gold and platinum are much less so.

In the context of dissolving metals in hydrochloric acid (HCl), the position of a metal in the reactivity series predicts its ability to displace hydrogen from the acid. Metals that rank above hydrogen in this series can easily donate electrons to hydrogen ions, causing the hydrogen to be released as gas and the metal to form its corresponding ion in solution. For example, aluminum (Al) is above hydrogen, indicating that it can dissolve in HCl by displacing hydrogen gas, while silver (Ag) is below hydrogen, showing it will not react with HCl.

Redox Reaction

A redox reaction, short for reduction-oxidation reaction, involves the transfer of electrons between two species. These reactions are fundamental to many processes, including the dissolving of metals in HCl. In a redox reaction, one substance gets oxidized by losing electrons, while another gets reduced by gaining those electrons.

Looking at the reactions provided in the exercise for aluminum and lead dissolving in HCl, we see a clear example of a redox reaction. The metal atoms lose electrons and become ions, this is oxidation. Concurrently, hydrogen ions gain electrons to form hydrogen gas, this is reduction. For instance, when aluminum reacts, it loses three electrons: \[\text{Al}(s) \rightarrow \text{Al}^{3+}(aq) + 3e^{-}\] . These electrons are then gained by hydrogen ions from the HCl: \[2\text{H}^{+}(aq) + 2e^{-} \rightarrow \text{H}_{2}(g)\].Together, these half-reactions combine to form the complete redox reaction, balancing out the movement of electrons and maintaining charge equilibrium.

Chemical Displacement

Chemical displacement, or a single replacement reaction, occurs when an element replaces another in a compound, often driven by the differing reactivity of the elements involved. To predict whether a displacement reaction will occur, the reactivity series can again be employed.

When a metal that is more reactive than hydrogen is placed in an acidic solution, it can displace hydrogen from the solution as gas, forming the metal's positively charged ion. This type of reaction is observed in the dissolving of metals like aluminum and lead in HCl. In the exercise, we see that lead undergoes a displacement reaction, represented by the half-reaction: \[\text{Pb}(s) \rightarrow \text{Pb}^{2+}(aq) + 2e^{-}\], and combines with the reduction of hydrogen ions as follows: \[\text{Pb}(s) + 2\text{H}^{+}(aq) \rightarrow \text{Pb}^{2+}(aq) + \text{H}_{2}(g)\]. This equation demonstrates lead's ability to displace hydrogen from HCl due to its higher position in the reactivity series. However, it's important to balance the reaction stoichiometry, ensuring the number of atoms and charges are equal on both sides of the equation.