Carbonic acid – The kinetics and thermodynamics: Why can’t carbonic acid replace sulfuric acid in the cooling tower?

Carbonic acid is just one example of many that cannot be produced at STP due to thermodynamic constraints. This only demonstrates how effective the second law of thermodynamics is.

I recently wrote two posts about alkalinity and its role in calcium and silica precipitation with reference to a cooling tower. Alkalinity is produced by carbonic acid. This post examines the thermodynamics and kinetics of carbonic acid and attempts to explain why is carbonic acid a nuisance.

Why is carbonic acid problematic and hard to tackle? Why is it that once carbonic acid enters your system it does not leave the system easily?  Why while we can successfully handle highly corrosive sulfuric acid and hydrochloric acid but do not know how to stop a weak carbonic it from entering our water? These are some of the most frequently asked questions. We will answer all these questions.

Carbonic acid is all around us. It is present and spreading everywhere. It is spreading through the air.  It has spread so far that no single technology can stop it. While it does some good things, such as providing alkalinity to water and buffering pH swings, it is best known for its dreaded corrosion reactions with metals. It slowly poisons most metals.

 "Why not use CO2 as an acid replacement in cooling towers?" is a frequently asked and debated question. "What is the reason that carbonic acid is not a stable acid?"

This post will address these concerns by delving into the fundamentals of carbonic acid [1] thermodynamics and [2] kinetics.

We will investigate its thermodynamics and kinetics to determine what causes the CO2 to carbonic acid conversion to be less than 1%. We will explain why CO2 has a self-sustaining reaction in water that produces unstable carbonic acid, which decomposes to CO2 and makes room for further CO2 dissolution. This is a never-ending process.

The post has two parts

[1] Chemistry, thermodynamics, and kinetics of carbonic acid

[2] Why can’t carbonic acid replace sulfuric acid in the cooling tower?

Part 1

The chemistry of carbonic acid

Carbonic acid is an inorganic compound with the chemical formula H2CO3 in chemistry. It is pervasive as a dilute solution in water and can be found everywhere. Carbonic acid is a weak acid that's formed from the reaction of carbon dioxide dissolved in water. By definition, a weak acid is only partially ionized in a solution. In other words, weak acids don't completely dissociate, or break apart, into ions in a solution. Like all weak acids, carbonic acid stays in a state of equilibrium between dissociation and recombination. Only at temperatures around - 80 degc can pure carbonic acid be obtained. Its pH is 4.18. The molecule quickly degrades into water and carbon dioxide. It's a very unstable acid.

The problem lies with its thermodynamics and kinetics

Thermodynamics

The reaction, CO2 (G) + 1 H2O (ℓ) → H2CO3 (AQ), is an endothermic reaction with enthalpy +2.2 KJ.

The reaction violates the second law of thermodynamics. The gas transforming to liquid generates a negative entropy, - 340.49 J/k

As a result, the Gibbs free energy of the reaction is at 298.15k is +103.67 kJ [nonspontaneous at 298.15k].

dG = dH – T ds = 2.2 - 298x [- 0.3405] = 2.2 + 101.47 = + 103.67 KJ

Explanation

This shows that this reaction even at 25 degc is not a favorable spontaneous reaction. Positive Gibbs free energy further says it is a reversible reaction. It further says it favors moving in the reverse direction than moving forward.

Kinetics

Equilibrium is established between the dissolved CO2 and H2CO3, carbonic acid. CO2 (l) + H2O (l) < = > H2CO3 (l)

This reaction is kinetically slow. It is a reaction self-sustaining reaction between CO2 and water. As the unstable carbonic acid decomposes the equilibrium is maintained by dissolving more CO2. The process goes on.

In the absence of a catalyst, the equilibrium is reached quite slowly. At equilibrium, only a small fraction (ca. 0.2 - 1%) of the dissolved CO2 is actually converted to H2CO3. Most of the CO2 remains as solvated molecular CO2. As equation:

k = [H2CO3] / [CO2] = 1.7 X 10^-3

Explanation

The equilibrium constant of the reaction at 298.15 k at 1.70×10^−3 is an indication that the majority of the carbon dioxide is not converted into carbonic acid and stays as CO2 molecules.

The rate constants are 0.039 s−1 for the forward reaction (CO2 + H2O → H2CO3) and 23 s−1 for the reverse reaction (H2CO3 → CO2 + H2O). This again shows it is a non-spontaneous reaction at 298.15 k. Its reverse reaction is quicker than forward reaction.

Part2

Why can’t carbonic acid replace sulfuric acid in a cooling tower?

Many plants have attempted to substitute CO2 for H2SO4. That can work, but it's important to understand CO2's limitations when using it to adjust pH.

Explanation

Injecting CO2 into cooling water temporarily shifts carbonate alkalinity to bicarbonate as carbonic acid formation lowers pH. This works with lower pH circulating water, but the liberated CO2 is stripped in the cooling tower. Consequently, cooling water pH returns to equilibrium as the water drops through the cooling tower fill and the CO 2 is released. Stripping the CO2 from the cooling water limits its usefulness for pH control. Tower M-alkalinity stays the same.

 Credit: Google