Question

The following table shows the enthalpies and Gibbs energies of formation of three metal oxides at \(25^{\circ} \mathrm{C}\). (a) Which of these oxides can be most readily decomposed to the free metal and \(\mathrm{O}_{2}(\mathrm{g}) ?\) (b) For the oxide that is most easily decomposed, to what temperature must it be heated to produce \(\mathrm{O}_{2}(\mathrm{g})\) at 1.00 atm pressure? $$\begin{array}{lll} \hline & \Delta \mathrm{H}_{7}, \mathrm{kJ} \mathrm{mol}^{-1} & \Delta \mathrm{G}_{7}, \mathrm{kJ} \mathrm{mol}^{-1} \\ \hline \mathrm{PbO}(\mathrm{red}) & -219.0 & -188.9 \\ \mathrm{Ag}_{2} \mathrm{O} & -31.05 & -11.20 \\ \mathrm{ZnO} & -348.3 & -318.3 \\ \hline \end{array}$$

Short answer

The oxide that can be most readily decomposed into the metal and \( \mathrm{O}_2 \) gas is \( \mathrm{Ag}_2 \mathrm{O} \). However, without the entropy change (\( \Delta S \)), it is not possible to determine the temperature needed to decompose \( \mathrm{Ag}_2 \mathrm{O} \) into its components.

Step-by-step solution

Identify the most easily decomposable oxide

The most easily decomposable oxide will be the one with the highest (or least negative) Gibbs free energy (\( \Delta G \)) of formation. Searching the table for the highest \( \Delta G \) value, we find it is for \( \mathrm{Ag}_2 \mathrm{O} \) with \( \Delta G = -11.20 \mathrm{kJ/mol} \). Thus, \( \mathrm{Ag}_2 \mathrm{O} \) is the oxide which can most readily be decomposed to the metal and \( \mathrm{O}_2 \) gas.

Calculate the required temperature for decomposition

We use the equation for Gibbs free energy: \( \Delta G = \Delta H - T\Delta S \) to calculate the required temperature. In this scenario, we aim to have \( \Delta G = 0 \) since the reaction could occur spontaneously at this state. We., therefore, have equation \( 0 = \Delta H - T\Delta S \). The equation can be rewritten as \( T = \frac{\Delta H}{\Delta S} \). Thus, the temperature required is the change in enthalpy (\( \Delta H \)) divided by the change in entropy (\( \Delta S \)) for the formation of \( \mathrm{Ag}_2 \mathrm{O} \). However, we do not have the value for \( \Delta S \) and can not move further with this part of the question.

Key concepts

Enthalpy of Formation

Enthalpy of formation, denoted as \( \Delta H_f \) or simply \( \Delta H \) when the context is clear, is a crucial concept in chemical thermodynamics. It refers to the change in enthalpy when one mole of a substance is formed from its elements under standard conditions of temperature and pressure (usually \( 25^\circ \mathrm{C} \) and 1 atm). For a chemical reaction, it reflects the total heat content that is absorbed or released. Generally, if the value of \( \Delta H \) is negative, it implies that the reaction is exothermic and heat is released during the formation of a compound from its elements.

In our exercise, we compare the \( \Delta H \) values of different metal oxides to infer about their stability and how much energy is required to form them from their base elements. The less negative the enthalpy of formation, the more likely a compound can decompose back into its constituent elements, since it indicates a lower energy stability.

Metal Oxide Decomposition

Metal oxide decomposition refers to the process by which a metal oxide is broken down into its elements: the metal and oxygen. The ease with which a metal oxide decomposes is linked to its thermodynamic properties, specifically the Gibbs free energy of formation. The Gibbs free energy, \( \Delta G \), indicates the spontaneity of a reaction at constant temperature and pressure.

In our case, the objective is to determine which metal oxide can most readily be reduced to its elemental form. When comparing metal oxides, such as \( \mathrm{PbO} \) (red), \( \mathrm{Ag}_2\mathrm{O} \) and \( \mathrm{ZnO} \) in the given table, \( \mathrm{Ag}_2\mathrm{O} \) has the least negative \( \Delta G \) value. This tells us that among the given oxides, \( \mathrm{Ag}_2\mathrm{O} \) is the most prone to decompose into silver metal and oxygen gas under the right conditions.

Chemical Thermodynamics

Chemical thermodynamics involves the study of energy changes associated with chemical reactions, specifically relating to heat (enthalpy) and work. It incorporates the laws of thermodynamics to predict the feasibility of reactions and equilibrium states. A fundamental concept within this field is the interplay between enthalpy (\( \Delta H \)), entropy (\( \Delta S \)), and temperature (\( T \)), which together determine the Gibbs free energy (\( \Delta G \)) of a system.

The value of \( \Delta G \) helps chemists understand whether a process will occur spontaneously under certain conditions. If \( \Delta G \) is negative, the process is spontaneous and can occur without the input of added energy. For a reaction to be spontaneous at any temperature, not just at standard conditions, we must consider changes in \( \Delta S \) and \( T \) as well, and how they affect \( \Delta G \).

Gibbs Free Energy Equation

The Gibbs free energy equation is pivotal in predicting the spontaneity of a process. It is expressed as \( \Delta G = \Delta H - T\Delta S \). This equation connects the concepts of enthalpy (\( \Delta H \)), temperature (\( T \)), and entropy (\( \Delta S \)), offering a quantifiable means of determining the free energy change during a reaction. A negative \( \Delta G \) value suggests that a process or reaction can occur spontaneously.

In our table's context, to find the temperature at which the oxide will decompose, we set \( \Delta G = 0 \), indicating a state of equilibrium where the reaction can spontaneously proceed. Unfortunately, without additional data on the entropy change (\( \Delta S \)) for the formation of \( \mathrm{Ag}_2\mathrm{O} \), we cannot resolve the equation to find the exact temperature required for its decomposition. This demonstrates how each variable in the Gibbs free energy equation plays a critical role in understanding chemical reactivity and stability.