Question

What reaction can occur, if any, when the following experiments are carried out under standard-state conditions? (a) A strip of zinc is dipped into an aqueous solution of \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) (b) An acidic solution of \(\mathrm{FeSO}_{4}\) is exposed to oxygen. (c) A silver wire is immersed in an aqueous solution of \(\mathrm{NiCl}_{2}\). (d) Hydrogen gas is bubbled through aqueous \(\mathrm{Cd}\left(\mathrm{NO}_{3}\right)_{2}\).

Short answer

(a) Zn reacts with Pb²⁺; (b) Fe²⁺ is oxidized by O₂; (c) no reaction; (d) no reaction.

Step-by-step solution

Determine Standard Reduction Potentials

Look up standard reduction potentials for the half-reactions involving the elements and compounds in each scenario. Gather the following standard reduction potentials (in volts, V): - Zn²⁺ + 2e⁻ → Zn: -0.76 V - Pb²⁺ + 2e⁻ → Pb: -0.13 V - Fe²⁺ + 2e⁻ → Fe: -0.44 V - O₂ + 4H⁺ + 4e⁻ → 2H₂O: 1.23 V - Ni²⁺ + 2e⁻ → Ni: -0.25 V - Ag⁺ + e⁻ → Ag: +0.80 V - Cd²⁺ + 2e⁻ → Cd: -0.40 V - 2H⁺ + 2e⁻ → H₂: 0.00 V.

Compare Standard Reduction Potentials for Each Reaction

For each reaction, compare the standard reduction potentials. A spontaneous redox reaction is possible if the reduction potential of the ion in solution is greater than the reduction potential of the solid metal. Conversely, if the reduction potential of the solid metal is higher, no reaction will occur.

Analyze Scenario (a)

Zinc metal is in contact with \(\text{Pb(NO}_3)_2\), which contains Pb²⁺ ions. The reduction potential for Pb²⁺ (−0.13 V) is greater than that for Zn²⁺ (−0.76 V), so Pb²⁺ can be reduced by zinc. Reaction: Zn(s) + Pb²⁺(aq) → Zn²⁺(aq) + Pb(s).

Analyze Scenario (b)

In acidic solution, Fe²⁺ can react with O₂. The reduction potential for O₂ (1.23 V) is much higher than that for Fe²⁺ (-0.44 V), allowing O₂ to oxidize Fe²⁺. Reaction: 2Fe²⁺(aq) + O₂(g) + 4H⁺(aq) → 2Fe³⁺(aq) + 2H₂O(l).

Analyze Scenario (c)

Silver metal is immersed in \(\text{NiCl}_2\), which contains Ni²⁺ ions. The reduction potential for Ag⁺ (+0.80 V) is higher than for Ni²⁺ (-0.25 V), so no reaction occurs as nickel cannot oxidize silver.

Analyze Scenario (d)

Hydrogen gas is bubbled through \(\text{Cd(NO}_3)_2\), which contains Cd²⁺ ions. The reduction potential for H₂ (0.00 V) is higher than that for Cd²⁺ (-0.40 V), making Cd unable to oxidize H₂. Hence, no reaction occurs.

Key concepts

Standard Reduction Potentials

Standard reduction potentials are vital in understanding how redox reactions work. Redox reactions involve electron transfer between two substances. The standard reduction potential is a measure of the tendency of a chemical species to gain electrons and be reduced. It is measured in volts (V) under standard conditions (25°C, 1 M concentration, 1 atm pressure).

To find out if a spontaneous redox reaction can occur, we look at the standard reduction potentials for each half-reaction. This is typically given for ions in aqueous solution and metals in solid form.

• The higher the reduction potential, the greater the substance’s ability to gain electrons and be reduced.
• If a specific ion has a higher reduction potential than another potential half-reaction (often a metal or a different ion), it means that it's easier for that ion to "steal" electrons and get reduced.

By comparing these values, students can predict the direction in which electrons will flow in a redox reaction, helping to determine if the reaction will be spontaneous.

Redox Reactions

Redox reactions, also known as oxidation-reduction reactions, involve the transfer of electrons between substances. In a redox reaction, one species gets oxidized (loses electrons) and another gets reduced (gains electrons).

Understanding redox reactions helps in identifying which substances are acting as electron donors and which are electron acceptors.

• Oxidation can be remembered with the mnemonic OIL: Oxidation is Loss of electrons.
• Reduction can be remembered with RIG: Reduction is Gain of electrons.

In these types of reactions, the substance that gets reduced often has a higher reduction potential. Knowing which substance has a higher reduction potential helps predict which side of the chemical equation the products will form.

In electrochemistry, redox reactions are vital as they lay the foundation for understanding batteries and corrosion, among other processes. By analyzing the changes in oxidation states of elements in a given reaction, one can decide the role and the flow of electrons.

Spontaneous Reactions

A reaction is defined as spontaneous when it occurs naturally under specific conditions without needing an external source of energy. This concept is crucial when studying redox reactions and electrochemistry.

To determine if a redox reaction is spontaneous, students compare the standard reduction potentials of the involved species:

• If the reduction potential of the ion in solution is greater than that of the metal or another substance, the reaction is spontaneous. Electrons will naturally flow to the species with the higher reduction potential.
• If the reverse is true, and the solid's reduction potential is higher, the reaction won't proceed on its own.

Spontaneity is often associated with a negative change in free energy (ΔG), meaning energy is released during the reaction. This released energy can be harnessed in electrical circuits, illustrating the practical importance of spontaneous reactions in generating electric power.