Chapter 16: Green Chemistry & Nanochemistry
Highly Detailed Mark-wise Solutions for Board Exam Preparation
1 Mark Questions (Very Short Answer)
Answer: Green Chemistry is defined as the design of chemical products and processes that reduce or entirely eliminate the use and generation of hazardous substances, promoting pollution prevention at the molecular level.
Answer: The size range of nanomaterials is from 1 nm to 100 nm in at least one dimension.
Answer: Carbon Nanotubes (CNTs) or Nanowires. (They are nanoscale in two dimensions, but macroscopic in the third dimension, i.e., length).
Answer: Silver nanoparticles possess highly effective antibacterial and antimicrobial properties, which help in killing bacteria and purifying the water.
2 Mark Questions (Short Answer-I)
Answer:
- Prevention of Waste: It is always better to prevent the formation of waste during a chemical process than to treat or clean up the waste after it has been formed (Zero Waste Technology).
- Atom Economy: Synthetic chemical methods should be designed to maximize the incorporation of all starting materials (reactants) into the final desired product, minimizing by-products.
Answer:
| Top-down Approach | Bottom-up Approach |
|---|---|
| Involves breaking down a large, macroscopic (bulk) material into progressively smaller pieces until they reach the nanoscale. | Involves assembling or combining individual atoms or molecules together to build up a nanomaterial. |
| Physical methods are primarily used. | Chemical and biological methods are primarily used. |
| Example: High-energy ball milling, Lithography. | Example: Sol-gel process, Chemical Vapor Deposition (CVD). |
Answer:
Formula:
$\% \text{ Atom Economy} = \frac{\text{Formula weight of the desired product}}{\text{Sum of formula weights of all reactants used}} \times 100$
Reason:
An atom economy of 100% means that every single atom of the reactants ends up exclusively in the desired product. This means absolutely zero waste or toxic by-products are generated, making the chemical process highly efficient, environmentally friendly, and economical.
3 Mark Questions (Short Answer-II / Numericals)
Answer:
The sol-gel process is a wet-chemical technique (a bottom-up approach) primarily used for the fabrication of metal oxides. It involves the following steps:
- Hydrolysis: Precursors (usually metal alkoxides) are dissolved in water or alcohol and undergo hydrolysis to form a colloidal solution consisting of solid particles suspended in a liquid. This is called a Sol.
- Polycondensation: The particles in the sol connect and link together to form a continuous 3D network that encloses the liquid phase. This semi-solid state is called a Gel.
- Aging and Drying: The gel is aged to allow further network formation. It is then dried (by evaporation or supercritical drying) to remove the liquid phase, forming a porous solid network.
- Calcination (Heating): The dried solid is heated at high temperatures to decompose organic residues and form the final dense nanomaterial (e.g., $TiO_2$ or $ZnO$ nanoparticles).
$C_4H_9OH + NaBr + H_2SO_4 \rightarrow C_4H_9Br + NaHSO_4 + H_2O$
Given atomic masses: C=12, H=1, O=16, Na=23, Br=80, S=32. March 2018, Oct 2020
Answer:
Step 1: Identify the desired product
The desired product is Butyl bromide ($C_4H_9Br$).
Step 2: Calculate mass of desired product
Mass of $C_4H_9Br = (4 \times 12) + (9 \times 1) + 80 = 48 + 9 + 80 = 137 \text{ u}$
Step 3: Calculate sum of masses of ALL reactants
Mass of $C_4H_9OH = (4 \times 12) + (10 \times 1) + 16 = 48 + 10 + 16 = 74 \text{ u}$
Mass of $NaBr = 23 + 80 = 103 \text{ u}$
Mass of $H_2SO_4 = (2 \times 1) + 32 + (4 \times 16) = 2 + 32 + 64 = 98 \text{ u}$
Total mass of reactants $= 74 + 103 + 98 = 275 \text{ u}$
Step 4: Calculate % Atom Economy
$$\% \text{ Atom Economy} = \frac{\text{Mass of desired product}}{\text{Total mass of reactants}} \times 100$$
$$\% \text{ Atom Economy} = \frac{137}{275} \times 100$$
$$\% \text{ Atom Economy} = 0.4981 \times 100 = 49.81\%$$
The percentage atom economy of the reaction is 49.81%.
4 Mark Questions (Long Answer / Combined)
Note: In the HSC Board Exam, 4-mark questions in Chemistry are frequently split into sub-questions (e.g., a 3-mark properties question combined with a 1-mark theory question).
(b) What is meant by 'Design for Degradation' in Green Chemistry? [1 Mark] Oct 2017, March 2021
Answer (a): Properties of Nanomaterials
Optical Properties:
The optical properties of nanomaterials are fundamentally different from their bulk counterparts. For example, bulk gold appears yellow, but nano-sized gold particles can appear red, purple, or blue depending strictly on their exact size and shape. This is due to a phenomenon called Surface Plasmon Resonance (SPR), where the electrons on the surface of the nanoparticles resonate with the frequency of the incident visible light, absorbing specific wavelengths and reflecting others.
Catalytic Properties:
Nanomaterials possess an extraordinarily high surface-area-to-volume ratio. This means a vast majority of the atoms are located on the surface of the particle and possess free valencies. This allows them to easily adsorb reactant molecules, lowering the activation energy and making them highly active, efficient, and selective catalysts compared to bulk materials. (E.g., Pd and Pt nanoparticles in catalytic converters).
Answer (b): Design for Degradation
Design for Degradation is the 10th principle of Green Chemistry. It states that chemical products (like plastics, pesticides) should be designed so that at the end of their functional life, they naturally break down into innocuous (harmless) degradation products and do not persist in the environment.
(b) Give one disadvantage of Nanotechnology. [1 Mark] March 2016, March 2019, Oct 2022
Answer (a): Applications of Nanochemistry
- In Medicine: Nanoparticles are used for targeted drug delivery. They can be engineered to deliver drugs directly to diseased cells (like cancer cells) without harming healthy surrounding cells, thereby reducing side effects.
- In Electronics: Carbon nanotubes and quantum dots are used to manufacture smaller, faster, and highly efficient microchips, transparent conducting films, and LED displays.
- In Cosmetics: Zinc oxide ($ZnO$) and Titanium dioxide ($TiO_2$) nanoparticles are extensively used in sunscreens because they effectively absorb harmful UV radiation while remaining transparent on the skin.
Answer (b): Disadvantage of Nanotechnology
Nanotoxicology: Due to their extremely small size, nanoparticles can easily be inhaled into the lungs, pass through cell membranes, and even cross the blood-brain barrier. They may accumulate in the body and cause severe respiratory issues, DNA damage, or unforeseen cellular toxicity.
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