Question

Phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) is an important industrial chemical used in fertilizers, detergents, and the food industry. It is produced by two different methods. In the electric furnace method elemental phosphorus \(\left(\mathrm{P}_{4}\right)\) is burned in air to form \(\mathrm{P}_{4} \mathrm{O}_{10}\), which is then combined with water to give \(\mathrm{H}_{3} \mathrm{PO}_{4}\). In the wet process the mineral phosphate rock \(\left[\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{~F}\right]\) is combined with sulfuric acid to give \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (and \(\mathrm{HF}\) and \(\mathrm{CaSO}_{4}\) ). Write equations for these processes, and classify each step as precipitation, acid-base, or redox reaction.

Short answer

Electric furnace: Redox and hydration. Wet process: Precipitation.

Step-by-step solution

Electric Furnace Method - Burning Phosphorus

In the electric furnace method, we start with the burning of elemental phosphorus in air to form phosphorus pentoxide. The chemical equation for this reaction is: \[ \text{P}_4 + 5 \text{O}_2 \rightarrow \text{P}_4\text{O}_{10} \]This reaction involves a change in the oxidation state of phosphorus and oxygen, and therefore it is classified as a redox reaction.

Electric Furnace Method - Formation of Phosphoric Acid

Next, phosphorus pentoxide (\(\text{P}_4\text{O}_{10}\)) is reacted with water to form phosphoric acid. The chemical equation for this reaction is: \[ \text{P}_4\text{O}_{10} + 6 \text{H}_2\text{O} \rightarrow 4 \text{H}_3\text{PO}_4 \]There is no change in oxidation states in this step, so it is not a redox reaction. However, it can be considered a hydration reaction.

Wet Process - Reaction with Sulfuric Acid

In the wet process, phosphate rock combines with sulfuric acid to produce phosphoric acid, hydrogen fluoride, and calcium sulfate. The chemical equation for this reaction is: \[ \text{Ca}_5(\text{PO}_4)_3\text{F} + 5 \text{H}_2\text{SO}_4 \rightarrow 3 \text{H}_3\text{PO}_4 + 5 \text{CaSO}_4 + \text{HF} \]This reaction is complex, involving multiple steps where the products separate out. The reaction primarily involves exchange of ions. Calcium sulfate (\(\text{CaSO}_4\)) precipitates, thus the step can be classified as a precipitation reaction.

Final Summary: Classify the Reactions

- Burning phosphorus to \(\text{P}_4\text{O}_{10}\): Redox reaction- Formation of \(\text{H}_3\text{PO}_4\) from \(\text{P}_4\text{O}_{10}\) and water: Hydration reaction- Reaction of phosphate rock with sulfuric acid: Precipitation reaction

Key concepts

Electric Furnace Method

The electric furnace method is one way to produce phosphoric acid, which is crucial for various industrial applications. In this method, elemental phosphorus \(\text{P}_4 \) is burned in air to create phosphorus pentoxide \(\text{P}_4\text{O}_{10} \). This process involves a reaction between phosphorus and oxygen. Afterwards, phosphorus pentoxide reacts with water, forming phosphoric acid \(\text{H}_3\text{PO}_4 \). This process is used because it provides a high purity product. Although more expensive than other methods, it is favored in certain applications that require very pure phosphoric acid.
- Step 1: \[ \text{P}_4 + 5 \text{O}_2 \rightarrow \text{P}_4\text{O}_{10} \] - Step 2: \[ \text{P}_4\text{O}_{10} + 6 \text{H}_2\text{O} \rightarrow 4 \text{H}_3\text{PO}_4 \]
Each step involves different types of reactions, which are key to understanding the chemistry behind this method.

Wet Process

The wet process is another prevalent method used for producing phosphoric acid. It begins with mineral phosphate rock, such as \(\text{Ca}_5\left(\text{PO}_4\right)_3\text{F}\), reacting with sulfuric acid. This forms phosphoric acid, calcium sulfate, and hydrogen fluoride. Some advantages of the wet process include lower production costs and the ability to utilize raw materials rich in calcium phosphate.
- Reaction: \[ \text{Ca}_5(\text{PO}_4)_3\text{F} + 5 \text{H}_2\text{SO}_4 \rightarrow 3 \text{H}_3\text{PO}_4 + 5 \text{CaSO}_4 + \text{HF} \]
This method focuses on breaking down the mineral structure and facilitates the separation of different by-products from the main product, phosphoric acid.
The industrial application of the wet process lies mostly in fertilizers, where large quantities of phosphoric acid are needed without the stringent purity demanded in other sectors.

Redox Reaction

Redox reactions, short for reduction-oxidation reactions, are processes where one substance is oxidized, and another is reduced. In the electric furnace method, an important redox reaction occurs when phosphorus burns in oxygen.
- \[ \text{P}_4 + 5 \text{O}_2 \rightarrow \text{P}_4\text{O}_{10} \]
In this reaction:

• Phosphorus is oxidized as its oxidation state increases from 0 in \(\text{P}_4\) to +5 in \(\text{P}_4\text{O}_{10}\).
• Oxygen is reduced as its oxidation state decreases, facilitating the bonding in \(\text{P}_4\text{O}_{10}\).

Understanding redox reactions is essential because they highlight how electrons are transferred during the reaction. They also help clarify the energy changes involved in chemical processes.

Acid-Base Reaction

Though not highlighted prominently in the initial exercise, understanding acid-base reactions is fundamental in the formation of phosphoric acid, especially in the wet process. When sulfuric acid reacts with phosphate rock, a series of reactions occur that can involve acidic and basic interactions.
- Acid: Sulfuric acid (\(\text{H}_2\text{SO}_4\))- Base: The phosphate compounds within the rock
These interactions typically result in the decomposition of base structures, forming new acids like phosphoric acid while also producing by-products.
Acid-base reactions play a crucial role in modifying the pH of substances during such chemical transformations, enabling mineral dissolution and creating favorable conditions for the desired reactions.

Precipitation Reaction

Precipitation reactions occur when soluble ions in separate solutions combine to form an insoluble compound, or precipitate. In the wet process of producing phosphoric acid, calcium sulfate is formed as a primary precipitate.
- Reaction: \[ \text{Ca}_5(\text{PO}_4)_3\text{F} + 5 \text{H}_2\text{SO}_4 \rightarrow 3 \text{H}_3\text{PO}_4 + 5 \text{CaSO}_4 + \text{HF} \]
In this context:

• The calcium ions released from the reaction with phosphate rock bind with sulfate ions from sulfuric acid.
• Calcium sulfate precipitates out of the solution, facilitating separation from the liquid phosphoric acid.

Precipitation reactions are integral to chemical manufacturing as they allow separation of desired products from mixtures and solutions.