Problem 48 - Chemistry: Principles and Reactions

Question

It is desired to produce tin from its ore, cassiterite, \(\mathrm{SnO}_{2}\), at as low a temperature as possible. The ore could be (a) decomposed by heating, producing tin and oxygen. (b) heated with hydrogen gas, producing tin and water vapor. (c) heated with carbon, producing tin and carbon dioxide. Solely on the basis of thermodynamic principles, which method would you recommend? Show calculations.

Step-by-Step Solution

Answer

Answer: To determine the most suitable method, compare the Gibbs free energy changes, ΔG, for each reaction. The method with the most negative ΔG is the thermodynamically preferred method, and hence, the most suitable for producing tin from cassiterite at the lowest temperature.

Step 1: Determine the balanced equations for each method

For each of the three reactions: decomposition, heating with hydrogen, and heating with carbon, let's write the balanced chemical equations: (a) \(\mathrm{SnO}_2(s) \rightarrow \mathrm{Sn}(s) + \mathrm{O}_2(g)\) (b) \(\mathrm{SnO}_2(s) + 2\mathrm{H}_2(g) \rightarrow \mathrm{Sn}(s) + 2\mathrm{H}_2\mathrm{O}(g)\) (c) \(\mathrm{SnO}_2(s) + 2\mathrm{C}(s) \rightarrow \mathrm{Sn}(s) + 2\mathrm{CO}(g)\)

Step 2: Determine \(\Delta G\) for each reaction

In order to determine how favorable each reaction is based on thermodynamics principles, we need to calculate the Gibbs free energy change \(\Delta G\) for each reaction. We can do this by using the following equation: \(\Delta G = \Delta H - T\Delta S\) Where: - \(\Delta G\) is the Gibbs free energy change - \(\Delta H\) is the enthalpy change - \(T\) is the temperature in Kelvin - \(\Delta S\) is the entropy change We also assume that the standard G values of each of the species in the reaction, \(\Delta G^\circ\), are known and have been given to us. For each reaction, we will calculate the \(\Delta G\) using the equation above: (a) \(\Delta G_a = \Delta G^\circ_{SnO_2} - [\Delta G^\circ_{Sn} + \Delta G^\circ_{O_2}]\) (b) \(\Delta G_b = [\Delta G^\circ_{SnO_2} + 2\Delta G^\circ_{H_2}] - [\Delta G^\circ_{Sn} + 2\Delta G^\circ_{H_2O}]\) (c) \(\Delta G_c = [\Delta G^\circ_{SnO_2} + 2\Delta G^\circ_{C}] - [\Delta G^\circ_{Sn} + 2\Delta G^\circ_{CO}]\)

Step 3: Compare the Gibbs free energy changes

Now that we have the expressions for the Gibbs free energy changes, \(\Delta G\), for each reaction, we can compare these values to determine the thermodynamically favored method. (a) If \(\Delta G_a < 0\), then the decomposition of cassiterite into tin and oxygen is thermodynamically favored. (b) If \(\Delta G_b < 0\), then reacting cassiterite with hydrogen gas to produce tin and water vapor is thermodynamically favored. (c) If \(\Delta G_c < 0\), then reacting cassiterite with carbon to produce tin and carbon dioxide is thermodynamically favored. The method with the most negative Gibbs free energy change is the thermodynamically preferred method. So, in this case, we would choose the method with the lowest (most negative) value for \(\Delta G\).

Key Concepts

Gibbs Free Energy

Gibbs free energy, symbolized as \(\Delta G\), plays a crucial role in predicting the spontaneity of chemical reactions in thermodynamics. It is a thermodynamic potential that helps to determine the maximum amount of work that a thermodynamic system can perform at constant temperature and pressure. When we talk about spontaneous reactions, these are processes that can occur without the input of additional energy.

For a chemical reaction to be spontaneous, \(\Delta G\) needs to be negative. This concept directly applies to metallurgical processes such as the extraction of metals from their ores. In the context of the exercise regarding the production of tin from cassiterite, we calculate \(\Delta G\) for each proposed method to find which one is more thermodynamically favorable — the one yielding the most negative value of \(\Delta G\). We use the following equation to calculate \(\Delta G\):
\[\Delta G = \Delta H - T\Delta S\]
where \(\Delta H\) is the change in enthalpy, \(T\) is the temperature in Kelvin, and \(\Delta S\) is the change in entropy. By determining the \(\Delta G\) for the three reactions involving cassiterite, we can decide the best method for producing tin based on thermodynamic principles.

Chemical Reactions

Chemical reactions are processes where reactants transform into products through the breaking and forming of chemical bonds. Each reaction is characterized by a unique conversion pathway and exhibits changes in energy and matter properties. Thermodynamics looks into these energy changes, focusing particularly on heat and work exchange during chemical reactions.

Considering metallurgical processes, like in our exercise, knowledge of thermodynamics clarifies which reactions are feasible and which are not. In evaluating the reactions for producing tin from cassiterite, understanding the energetic favorability through Gibbs free energy of each proposed reaction—decomposition, heating with hydrogen, and heating with carbon—is vital. This dictates not just the possibility of a reaction occurring but also its practicality from an energy consumption standpoint.

Enthalpy

Enthalpy, symbolized as \(\Delta H\), refers to the total heat content of a system and represents the energy involved in bond breaking and making—although we typically measure only the change in enthalpy. In an exothermic reaction, energy is released into the surroundings, resulting in a negative \(\Delta H\). Conversely, in an endothermic reaction, energy is absorbed from the surroundings, causing a positive \(\Delta H\).

When assessing the thermodynamics of producing tin from cassiterite, we consider \(\Delta H\) of the reactions, as it is a crucial component of the Gibbs free energy equation. It provides insights into the heat exchange associated with the transformation of cassiterite into tin and the byproducts, oxygen, water vapor, or carbon dioxide, depending on the method used.

Entropy

Entropy, represented by \(\Delta S\), measures the degree of disorder or randomness in a system. The second law of thermodynamics posits that for any spontaneous process, the entropy of the universe will increase. During a chemical reaction, entropy can either increase or decrease; however, the overall trend when taking the universe into account must be towards greater disorder.

In our metallurgical context, determining the entropy change involved in converting cassiterite to tin is essential. This helps predict the direction a process will naturally take. When a reaction causes the entropy of the system to rise, there is a larger possibility of it happening spontaneously. Calculating the entropy change is a part of understanding the suitability of each method for producing tin, involving the heating of cassiterite alone, with hydrogen, or with carbon.