Problem 23 - Chemistry in 30 days

Question

Which acid on heating produces phosphine? (a) phosphoric acid (b) phosphorous acid (c) peroxymonophosphorie acit' (d) metaphosphoric acid

Step-by-Step Solution

Answer

Phosphorous acid (b) produces phosphine on heating.

Step 1: Recall the Characteristics

Phosphine (PH₃) is a colorless, flammable, and toxic gas. It can be produced by heating certain phosphorus-containing acids. It is important to recall which specific phosphorus acid decomposes to release phosphine when heated.

Step 2: Analyze the Acid Options

There are several acids containing phosphorus: phosphoric acid (H₃PO₄), phosphorous acid (H₃PO₃), peroxymonophosphoric acid (H₅P₃O₸), and metaphosphoric acid (HPO₃). Of these, phosphorous acid is known for its ability to decompose and release phosphine gas when heated.

Step 3: Chemical Reaction Awareness

Understand the chemical reaction involved: Heating phosphorous acid leads to the decomposition into phosphine and phosphoric acid as follows:\[ 4 ext{H}_3 ext{PO}_3 ightarrow 3 ext{H}_3 ext{PO}_4 + ext{PH}_3 \]This reaction shows that phosphorous acid is the correct precursor for phosphine preparation through heating.

Step 4: Confirm the Answer

Based on the information gathered and the reaction equation, the acid that produces phosphine upon heating is phosphorous acid, which is option (b).

Key Concepts

Phosphorus-Containing Acids

Phosphorus-containing acids are an important class of chemicals in chemistry. They are derived from phosphorus and vary based on the oxidation state of phosphorus and the number of oxygen atoms bonded to it.
Here are the most common phosphorus-containing acids you might come across:

• Phosphoric Acid (H₃PO₄): This is the most common phosphorus acid, used widely in the industry and known for its role in fertilizers.
• Phosphorous Acid (H₃PO₃): Known for decomposing to produce phosphine gas when heated, it's quite reactive.
• Metaphosphoric Acid (HPO₃): Typically formed from the condensation of phosphoric acid molecules.
• Peroxymonophosphoric Acid (H₅P₃O₁₀): This is less common and includes additional oxygen atoms, existing in a less stable form.

Understanding these acids is crucial, as they can undergo various chemical reactions, including decomposition, especially under heating conditions.

Decomposition Reaction

Decomposition reactions are chemical processes where a compound breaks into two or more simpler substances. This type of reaction is important in many industrial and laboratory processes.
For phosphorus-containing compounds, we have a specific decomposition reaction when heated:

• Phosphorous acid decomposes through a reaction to produce phosphine and phosphoric acid.
• The equation for this reaction is:\[ 4 \text{H}_3\text{PO}_3 \rightarrow 3 \text{H}_3\text{PO}_4 + \text{PH}_3 \]This tells us that four molecules of phosphorous acid will yield three molecules of phosphoric acid and one molecule of phosphine gas.

Understanding this concept helps you predict the behavior of certain chemicals under specific conditions. These reactions are crucial in generating products like phosphine, which have significant applications.

Phosphorous Acid

Phosphorous acid, scientifically named orthophosphorous acid, stands out among phosphorus-containing acids due to its unique chemical properties and reactions. Its formula is H₃PO₃, and it plays a significant role in the production of phosphine gas.
Upon heating, phosphorous acid undergoes a decomposition reaction. This process is vital in generating phosphine, a colorless, flammable, and highly toxic gas.
The reaction of phosphorous acid breaking down can be represented as:\[ 4 \text{H}_3\text{PO}_3 \rightarrow 3 \text{H}_3\text{PO}_4 + \text{PH}_3 \]Here are a few noteworthy points about phosphorous acid:

• It is less stable than phosphoric acid, and its tendency to decompose when heated is a key property.
• This acid is used in chemical synthesis as a reducing agent and has roles in agriculture.
• Despite its utility, it's essential to handle it with care due to its reactive nature.

Understanding phosphorous acid is crucial for students, especially those studying chemistry, as it exemplifies how compounds can transform under certain conditions.