What is Lead Iodide?
Plumbous Iodide or commonly known as Lead Iodide is a salt which has a widespread application as a solvent, in the medical industry and the photography industry. Essentially, Lead Iodide is derived from the reaction of potassium nitrate and Lead Iodide through the process of double displacement. Universally, the Lead Iodide formula is PbI2. It’s physical as well as chemical properties make it ideal for a wide array of applications in the printing, medical and industrial solvents realm. It is often interchangeably referred to as lead diiodide due to its structure.
What are the Physical Properties of Lead Iodide?
PbI2 colour is yellow crystalline solid.
It is odourless.
Lead Iodide has a density of 6.16 g/cm3.
Lead Iodide is denser than water and is insoluble in most liquids, barring boiling water and potassium iodine.
The melting point of Lead iodide is 402 °C.
The boiling point is 953 °C.
The molecular weight of Lead Iodide is 461.01 g/mol
The thermal conductivity is 26.864.3 10-3 W cm-1 K
Lead Iodide is non-combustible
Lead Iodide Structure
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When looked through X-ray powder diffraction, Lead Iodide resembles a hexagonal close-packed system. It alternates between layers of iodine and lead atoms. It is mostly ionic bonds with weak Van der Waals interactions. Lead iodide solid can also take a rhombohedral structure.
What are the Chemical Properties of Lead Iodide?
Lead Iodide belongs to the Lead-14 and the Iodine-17 group and is a part of the P-3mI symmetrical group. As mentioned earlier the chemical formula of Lead Iodide is PbI2. Lead Iodide is highly resistant to a large spectrum of temperature and portrays extreme chemical stability. This makes it an ideal ingredient for film development and also as an oxidizing agent.
In ambient air (oxygen), thin films of Lead Iodide, however, is quite unstable and forms iodine.
2 PbI2 + O2 → 2 PbO + 2 I2↑
It has a molar mass of 461.01 g/mol.
Lead(II) iodide reacts with concentrated sulfuric acid to produce lead(II) sulfate, hydrogen sulfide, iodine and water.
4PbI2 + 5H2SO4 (conc.) → 4PbSO4 + 4I2 + H2S +4H2O
Now that we know the properties of Lead Iodide, let us now understand the process of the formation of Lead Iodide.
How is Lead Iodide prepared?
The chemical formula Lead Iodide involves the double displacement of soluble lead nitrate and potassium iodine. The product of this chemical reaction is a yellow, colourless solid Lead Iodide and the by-product is a potassium nitrate solution. This follows the chemical principle that when two soluble salts of the metal group react with each other, one soluble salt is formed along with an insoluble metal salt. In our case, the later is PbI2 or Lead Iodide. This method is known as the Bridgman-Stockbarger method.
Pb(NO3)2 + 2 KI → PbI2 + 2 KNO3
This insoluble metal salt precipitates as yellow crystals and is recovered through the process of filtration. This is referred to as the “golden rain”. Typically, this filtration process involves washing the Lead Iodide precipitate in cold water. However, if they are dissolved in hot water and then allowed to recrystallise, they form fascinating golden crystals of Lead Iodide. Care must be taken that any amount of soluble Lead Iodide is removed, to get unadulterated PbI2. This may be done by allowing it to react with a hot composition of any bicarbonate. This will result in a cloudy lead carbonate precipitate which then can be easily segregated from Lead Iodide using filtration and we will be left with the golden crystals of Lead Iodide.
Alternatively, one may also produce Lead Iodide by getting iodine vapour to react with molten lead, at a controlled temperature ranging between 500 degrees to 700 degrees celsius.
No matter how spectacular the formation of Lead Iodide may be, it entails several health hazards. This is because any lead-bearing compounds can be highly toxic, especially when heated. Hence, adequate precautions must be taken while chemically preparing Lead Iodide.
FAQs on Lead Iodide
1. What is the chemical formula and molar mass of lead iodide?
The chemical formula for lead(II) iodide is PbI₂. Its molar mass is calculated by adding the atomic mass of one lead atom (approximately 207.2 u) and two iodine atoms (2 × 126.9 u), which totals to 461.0 g/mol.
2. What is the colour of lead iodide and in what state does it exist?
At room temperature, lead iodide is a bright yellow, dense crystalline solid. This distinctive colour is a key identifier in many chemical reactions, particularly in qualitative analysis for identifying the presence of lead(II) ions.
3. How is the yellow precipitate of lead iodide formed in a reaction?
The yellow precipitate of lead iodide is typically formed through a double displacement reaction, also known as a precipitation reaction. When a solution of a soluble lead salt, such as lead(II) nitrate (Pb(NO₃)₂), is mixed with a solution of a soluble iodide salt, like potassium iodide (KI), the insoluble lead iodide immediately precipitates out. The net ionic equation is: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s).
4. Why does lead iodide dissolve in hot water but reappears as golden flakes upon cooling?
This phenomenon, often called the 'golden rain' experiment, highlights the effect of temperature on solubility. Lead iodide is only sparingly soluble in cold water but becomes significantly more soluble in hot water. When a hot, saturated solution cools, its capacity to hold dissolved PbI₂ decreases. This causes the compound to recrystallise and precipitate out as shimmering, golden-yellow flakes that slowly fall through the solution.
5. What are some important real-world applications of lead iodide?
Due to its unique semiconductor and photoelectric properties, lead iodide has several important applications:
- X-ray and Gamma-ray Detectors: It is used as a detector material in medical imaging and high-energy physics because of its high atomic number and wide bandgap.
- Solar Cells: It is a critical precursor in the synthesis of perovskite materials used in next-generation, high-efficiency solar cells.
- Cloud Seeding: In the past, it has been explored as a potential agent for artificial rain-making.
6. What makes lead iodide a hazardous substance and what precautions are necessary?
Lead iodide is hazardous primarily because it contains lead, a heavy metal that is toxic and can accumulate in the body. Ingestion or inhalation of lead compounds can damage the nervous system, kidneys, and reproductive system. Therefore, when handling lead iodide, it is essential to:
- Wear appropriate personal protective equipment (PPE) such as gloves and safety goggles.
- Work in a well-ventilated area or under a fume hood to avoid inhaling dust.
- Wash hands thoroughly after any contact.
7. Although lead iodide is an ionic compound, why does it exhibit some covalent character?
Lead iodide is primarily an ionic compound, formed by the electrostatic attraction between the Pb²⁺ cation and I⁻ anions. However, it exhibits significant covalent character due to electronic effects explained by Fajans' rules. The highly polarising power of the lead(II) cation (Pb²⁺) distorts the large, easily polarisable electron cloud of the iodide anion (I⁻). This distortion leads to a degree of electron sharing between the ions, introducing a covalent nature to the bond.
