Problem 68 - Chemistry The Molecular Nature of Matter

Question

The following reactions occur spontaneously. $$ \begin{aligned} 2 \mathrm{Y}+3 \mathrm{Ni}^{2+} & \longrightarrow 2 \mathrm{Y}^{3+}+3 \mathrm{Ni} \\ 2 \mathrm{Mo}+3 \mathrm{Ni}^{2+} & \longrightarrow 2 \mathrm{Mo}^{3+}+3 \mathrm{Ni} \\ \mathrm{Y}^{3+}+\mathrm{Mo} \longrightarrow & \mathrm{Y}+\mathrm{Mo}^{3+} \end{aligned} $$ List the metals Y, \(\mathrm{Ni}\), and Mo in order of increasing ease of oxidation.

Step-by-Step Solution

Answer

The metals in order of increasing ease of oxidation are \text{Ni}, \text{Y}, \text{Mo}.

Step 1: Identify the Oxidation and Reduction Reactions

First, we identify the oxidation and reduction in the given reactions. Oxidation is the loss of electrons (increase in oxidation state), and reduction is the gain of electrons (decrease in oxidation state). In the first reaction, Y is oxidized as it goes from Y to Y^{3+}, while Ni^{2+} is reduced to Ni. In the second reaction, Mo is oxidized as it goes from Mo to Mo^{3+}, while Ni^{2+} is again reduced to Ni. In the third reaction, Y^{3+} is reduced to Y, while Mo is oxidized to Mo^{3+}.

Step 2: Analyze the Spontaneity of Reactions

Since all reactions are given to be spontaneous, we know that the species that go from a neutral state to a charged state (oxidation) do so spontaneously, indicating they are easier to oxidize than the species being reduced in that reaction.

Step 3: Order the Metals by Ease of Oxidation

From the first equation, Y is easier to oxidize than Ni^{2+}. From the second equation, Mo is easier to oxidize than Ni^{2+}. From the third equation, since Y^{3+} is being reduced and Mo is being oxidized, Mo is harder to oxidize than Y. Therefore, the ease of oxidation is Ni < Y < Mo.

Key Concepts

Oxidation States

Understanding oxidation states is crucial in identifying which atoms are participating in redox reactions and determining if the reaction can occur spontaneously. An oxidation state, often known as oxidation number, is a measure of the degree of oxidation of an atom in a chemical compound. It is an indicator of the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic.

An atom's oxidation state can change during a reaction as electrons are transferred between atoms. For instance, when an atom loses an electron during a reaction, its oxidation state increases, indicating oxidation. Conversely, when an atom gains an electron, its oxidation state decreases, which is a sign of reduction. In the given exercise, you can see this evidenced in the reactions where Y, Mo, and Ni change their oxidation states as they undergo oxidation or reduction.

Identifying Oxidation States

First, look at the reactants and products to determine the initial and final oxidation states. For the metals in the exercise (Y, Ni, and Mo), the oxidation states change as follows: Y goes from 0 to +3, Ni goes from +2 to 0, and Mo goes from 0 to +3. Recognizing these changes allows us to follow electron transfer in the reactions and provides a foundation for predicting the reactivity of these metals.

Chemical Spontaneity

Chemical spontaneity is a concept denoting whether a reaction can proceed without needing to be driven by external energy. Spontaneous reactions occur due to the system's natural tendency to achieve a state of lower energy and increased disorder or entropy. In the context of redox reactions, spontaneous processes are often characterized by an exchange of electrons that naturally progresses towards equilibrium.

A reaction's spontaneity is influenced by the Gibb's free energy change, \( \Delta G \) - a thermodynamic quantity. If \( \Delta G \) is negative, the process is spontaneous, and if \( \Delta G \) is positive, the reaction is non-spontaneous and requires outside energy to proceed. For the reactions presented, it is indicated they occur spontaneously, implying that without intervention, the atoms of Y, Ni, and Mo will naturally undergo oxidation and reduction as described.

Factors Affecting Spontaneity

Several factors can affect the spontaneous nature of a redox reaction, including the relative ease of oxidation or reduction of the participating elements, their concentrations, and the environmental conditions such as temperature and pressure. By evaluating these factors, one can predict the likelihood of a reaction, which is especially helpful in applications like battery technology and corrosion prevention.

Oxidation and Reduction

Oxidation and reduction are two halves of a whole in redox (reduction-oxidation) chemistry. These processes involve the transfer of electrons between chemical species, altering their oxidation states and driving the overall reaction.

Oxidation refers to the loss of electrons by a molecule, atom, or ion, resulting in an increase in its oxidation state. It's easy to remember with the phrase 'LEO' from 'Loss of Electrons is Oxidation'. Conversely, reduction refers to the gain of electrons, which you can recall with 'GER' from 'Gain of Electrons is Reduction'. The two processes always occur together because when one species loses electrons, another must necessarily gain them.

Tracking Electron Movement

In the exercise, we observe that Y and Mo are oxidized as they lose electrons and their oxidation states increase, while Ni is reduced as it gains electrons and its oxidation state decreases. Redox reactions play a significant role in many biological processes, industrial applications, and everyday chemical occurrences (like rusting). By studying instances of oxidation and reduction in various reactions, we gain a deeper understanding of the chemical changes taking place and can utilize this knowledge in technologies like batteries, where control of redox reactions is essential for storing and releasing energy.