Question

Various thermochemical cycles are being explored as possible sources of \(\mathrm{H}_{2}(\mathrm{g}) .\) The object is to find a series of reactions that can be conducted at moderate temperatures (about \(500^{\circ} \mathrm{C}\) ) and that results in the decomposition of water into \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2} .\) Show that the following series of reactions meets these requirements. $$\begin{aligned}\mathrm{FeCl}_{2}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{Fe}_{3} \mathrm{O}_{4}+\mathrm{HCl}+\mathrm{H}_{2} \\\\\mathrm{Fe}_{3} \mathrm{O}_{4}+\mathrm{HCl}+\mathrm{Cl}_{2} & \longrightarrow \mathrm{FeCl}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2} \\\\\mathrm{FeCl}_{3} \longrightarrow & \mathrm{FeCl}_{2}+\mathrm{Cl}_{2}\end{aligned}$$

Short answer

The series of thermochemical reactions indeed meets the requirements for the decomposition of water to hydrogen and oxygen gases, thanks to the cycling of substances within the reactions. The overall reaction is 2H2O -> 2H2(g) + O2(g).

Step-by-step solution

Understanding the First Reaction

In the first reaction, FeCl2 reacts with water to form Fe3O4, HCl, and hydrogen gas. This means water is broken down, producing hydrogen gas which is one of the desired outcomes. However, it also produces Fe3O4 and HCl, which are needed for the following reactions.

Understanding the Second Reaction

In the second step, the product Fe3O4 reacts with HCl and Cl2 to form FeCl3, water and oxygen gas. This reaction produces the other desired product, oxygen gas. It also reconstitutes water, which however isn't a problem because net water consumption occurs in the first step. This step also produces FeCl3, which is used in the next reaction.

Understanding the Third Reaction

The third reaction decomposes FeCl3 to form FeCl2 and Cl2. This allows the reaction cycle to continue, because FeCl2 can now repeat the first step again and so on.

Combining the Reactions

When combining these reactions, the intermediate formations (e.g., Fe3O4, HCl, FeCl3), cancel out because they are used in the following reaction. What remains is the net equation: 2H2O -> 2H2(g) + O2(g). This means that two molecules of water are decomposed into two molecules of hydrogen gas and one molecule of oxygen gas.

Key concepts

Thermochemical Water Splitting

Thermochemical water splitting is a method employed to generate hydrogen and oxygen gases from water using chemical reactions driven by heat. Unlike electrolysis, which uses electricity to break water molecules into hydrogen and oxygen, thermochemical cycles utilize heat (in this case around (500^{{circ}} C)) to power a series of chemical reactions that ultimately decompose water (H_2O).

This process is being studied extensively because of the potential to harness energy from renewable sources such as solar and geothermal heat. For the water splitting to be feasible, it must occur at moderate temperatures and produce the desired gases without requiring excessive energy input. Thermochemical cycles are designed to facilitate this while continuously recycling reactants, thereby making it an energy-efficient method for sustainable hydrogen production.

In the described series of reactions, a reaction chain is crafted to both maximize the production of hydrogen (H_2) and oxygen (O_2) and to ensure that reactants can be regenerated. This sustainable loop makes the thermochemical approach particularly promising for large-scale hydrogen production.

Hydrogen and Oxygen Production

The ultimate aim of thermochemical water splitting is the efficient and sustainable production of hydrogen and oxygen. Hydrogen, in particular, is of interest as a clean fuel that, when combusted, yields water, making it an attractive energy carrier for a low-carbon economy.

For each cycle of the given reactions, hydrogen is produced directly in the first step while oxygen is produced in the second step. The synthesis of hydrogen occurs alongside the formation of Fe_3O_4 and HCl, and oxygen is produced with the reconstitution of water and FeCl_3. Importantly, each of these products plays a pivotal role in sustaining the reaction cycle and hence does not represent waste material.

To assess the efficiency and effectiveness of the cycle, these gas products are the focus, and the reactions are crafted in a way to maximize their output. The cyclical nature of this process allows for continuous production without the need for additional water aside from that which is depleted by conversion into hydrogen and oxygen gases.

Chemical Reaction Steps

Understanding chemical reaction steps involved in thermochemical cycles is key to grasping how water is chemically decomposed into hydrogen and oxygen. Each step in the process serves a specific purpose and plays a strategic role in the efficient production of gases and the sustainability of the cycle.

In the exercise provided, the first step involved FeCl_2 reacting with water to yield H_2, a desired product, along with Fe_3O_4 and HCl which are used in subsequent steps. The second step cleverly utilizes Fe_3O_4 along with HCl and Cl_2 to generate O_2 and regenerate water and FeCl_3. Finally, the third step closes the cycle by breaking down FeCl_3 back into FeCl_2 and Cl_2, making the materials ready to initiate the cycle again.

This chain of reactions is carefully designed so that when combined, intermediate substances like Fe_3O_4, HCl, and FeCl_3 are neither lost nor accumulated but instead cycled through. This optimizes the use of materials and ensures the only net products are hydrogen and oxygen gases, aligning perfectly with the goals of a carbon-neutral energy cycle.