Engineer’s choice for a chemical process

The two chemical processes that are under focus today [1] Carbon dioxide capture and [2] Hydrolysis of water have fundamental issues

Carbon dioxide capture

Thermodynamic Limitations of Negative Entropy

CO2 + H2O ? HCO3- + H+

The conversion of CO2 to bicarbonate ions decreases the degree of randomness (entropy) in the system, it is accompanied by an increase in negative entropy.

Hydrolysis of Bicarbonate to revert to CO2

HCO3- + H2O ? [CO3]2- + H3O+

[CO3]2- + H3O+ ? HCO3- + H2O

Hydrolysis of water: Thermodynamic limitation dG>0

2H2O = 2H2 + O2

This is an endothermic reaction.

Energy requirement

Internal energy and work

?H = ?U + P?V

?H = + 285.83 KJ

TdS = 48.7 KJ

dG = dH - T?S = 285.83 – 48.7 = + 237 KJ

The reaction is not thermodynamically compliant, dG>0

Details

An engineer's top two preferences for a process are generally two things. Entropy, which measures how far a process is from equilibrium, and its equilibrium constant K, which is related to the process' activation energy.

To have a reaction that is both kinetically and thermodynamically compliant, an engineer manipulates the situation. The second law of thermodynamics and Le?Chatelier's principles. Le?Chatelier’s principles are frequently regarded as useful instruments for dynamic equilibrium manipulation techniques

Explanation

Entropy: Away from equilibrium

?"Away from equilibrium" refers to a negative free energy change (ΔG < 0). If a chemical process has a negative free energy change, it indicates that the reaction will proceed spontaneously in the forward direction and tends to reach equilibrium, where the free energy is minimized.

Equilibrium constant

A low activation energy is a desirable characteristic in chemical processes as it implies a faster reaction rate and the ability to achieve the desired product more efficiently.

The equilibrium constant, K, is defined as the ratio of the rate constants for the forward and reverse reactions at equilibrium. A higher value of K signifies a greater concentration of products at equilibrium, indicating a faster forward reaction compared to the reverse reaction. Therefore, if the goal is to have a chemical process that proceeds toward completion with a high concentration of products, engineers would typically prefer a reaction with a high equilibrium constant (K) value. This signifies that the forward reaction is favored, and the system is closer to equilibrium with a higher concentration of products. Regarding a low activation energy, it is generally desirable for a chemical process as it allows the reaction to occur more easily and at a faster rate. A low activation energy enables a larger number of reactant molecules to overcome the energy barrier and convert into products. This ultimately leads to a more efficient and faster reaction.

The relationship between Gibbs free energy (ΔG) and the equilibrium constant (K) is given by the equation ΔG = ΔG° + RT lnK, where ΔG° is the standard Gibbs free energy change, R is the gas constant, and T is the temperature.

Do not mix up Free energy with Activation energy

free energy (G) and activation energy (Ea) are different concepts but they are connected and relevant in thermodynamics and kinetics. Free energy (G) is a thermodynamic property that determines the spontaneity of a reaction or process. It takes into account both the enthalpy (H) and entropy (S) changes in a system: G = H - TS, where T is the temperature. The change in free energy (ΔG) between reactants and products determines whether a reaction is spontaneous (ΔG < 0) or non-spontaneous (ΔG > 0). The standard free energy change (ΔG°) is the value calculated under standard conditions. The activation energy (Ea) is a kinetic concept that represents the energy barrier that must be overcome for a reaction to occur. It reflects the energy required for reactant molecules to reach an intermediate, unstable state called the transition state before forming products. The activation energy determines the rate of a reaction - the higher the activation energy, the slower the reaction rate. Although free energy and activation energy are conceptually different, they are connected through the equilibrium constant (K). The equilibrium constant (K) is a measure of the extent to which a reaction has reached equilibrium, and it is related to the free energy change (ΔG) by the equation: ΔG = ΔG° + RT ln K. Here, R is the gas constant and T is the temperature.

[The para below is very important]

This equation allows us to relate the thermodynamic property of free energy (ΔG) to the equilibrium constant (K), which in turn provides information about the rate of a reaction. It indicates that while thermodynamics provides information about the spontaneous direction of a reaction, kinetics (represented by Ea) determines the speed at which the reaction occurs.

What does an engineer look for in a chemical process?

An engineer looks for a self-driven non-equilibrium process with dG < 0, in addition to low activation energy, they are searching for a reaction or system that not only proceeds spontaneously but also has a favorable change in free energy. Here are some reasons why engineers prioritize such processes:

Increased efficiency: A self-driven non-equilibrium process with dG < 0 implies that energy is being released or produced during the reaction. This can be harnessed to enhance the overall efficiency of the system. For example, in exothermic reactions, the released heat can be used to generate electricity or provide heating.

Sustainability: By focusing on self-driven non-equilibrium processes with dG < 0, engineers can design systems that utilize renewable energy sources and minimize the consumption of scarce resources. This aligns with sustainable development goals and reduces environmental impact. ?

Cost-effectiveness: Utilizing self-driven non-equilibrium processes with dG < 0 often leads to cost savings. These processes can either reduce the expenditure on external energy sources or enable the recovery of valuable resources that would otherwise go to waste.

Non-equilibrium systems can exhibit unique properties: Non-equilibrium systems can have emerging properties that are not present in equilibrium systems. Engineers may be interested in exploring and exploiting these properties for various applications, such as materials science, drug delivery systems, or nanotechnologies. Overall, engineers who focus on self-driven non-equilibrium processes with dG < 0 and low activation energy aim to maximize efficiency, promote sustainability, reduce costs, and harness the unique properties of non-equilibrium systems.