Let's break down each part of the question: --- ## QUESTION 2 ### a) Importance of Applying Sunscreens (5 marks) Sunscreens are important because they protect the skin from the harmful effects of ultraviolet (UV) radiation from the sun. UV radiation can cause sunburn, premature skin aging, and increase the risk of skin cancer by damaging the DNA in skin cells. Sunscreens contain active ingredients that either absorb or reflect UV rays, reducing their penetration into the skin. Regular use of sunscreen not only prevents sunburn but also helps in maintaining overall skin health and reduces the risk of developing melanoma and other types of skin cancer. --- ### b) Environmental Effects of Ultraviolet Light (5 marks) Ultraviolet light, especially UV-B and UV-C, can have significant environmental impacts. High levels of UV radiation can harm living organisms, particularly those in aquatic ecosystems like plankton, which form the base of the food chain. Increased UV exposure, due to ozone layer depletion, can also damage terrestrial plant life, affecting photosynthesis and crop yields. In animals, it can cause DNA damage, leading to mutations and affecting reproduction. UV radiation also degrades materials like plastics and paints, reducing their lifespan. --- ### c) Diatomic Chlorine Dissociation #### i. **Calculate the Maximum Wavelength** Given: ΔH = +121.7 kJ/mol Energy of 1 photon: - \( E = \frac{hc}{\lambda} \) - For 1 mole: \( E_{mol} = \frac{N_A hc}{\lambda} \) Where: - \( h = 6.626 \times 10^{-34} \) J·s (Planck’s constant) - \( c = 3.00 \times 10^8 \) m/s (speed of light) - \( N_A = 6.022 \times 10^{23} \) mol⁻¹ (Avogadro’s number) - ΔH = 121.7 kJ/mol = 121,700 J/mol \[ \lambda_{max} = \frac{N_Ahc}{\Delta H} \] \[ \lambda_{max} = \frac{(6.022 \times 10^{23})(6.626 \times 10^{-34})(3.00 \times 10^8)}{121,700} \] \[ \lambda_{max} = \frac{1.197 \times 10^{-1}}{121,700} \] \[ \lambda_{max} \approx 9.84 \times 10^{-7} \text{ m} = 984 \text{ nm} \] --- #### ii. **Does this Wavelength Correspond to Visible, UV-A, or UV-B?** - The visible spectrum: 400–700 nm - UV-A: 320–400 nm - UV-B: 280–320 nm - 984 nm is in the **infrared region**, not visible, UV-A, or UV-B. --- ### d) Energy (kJ/mol) for Given Wavelengths Use the formula: \[ E_{mol} = \frac{N_Ahc}{\lambda} \] where: - \( \lambda \) in meters #### i. 275 nm = \( 2.75 \times 10^{-7} \) m \[ E_{mol} = \frac{(6.022 \times 10^{23})(6.626 \times 10^{-34})(3.00 \times 10^8)}{2.75 \times 10^{-7}} \] \[ E_{mol} = \frac{1.197 \times 10^{-1}}{2.75 \times 10^{-7}} \] \[ E_{mol} = 4.35 \times 10^{5} \text{ J/mol} = 435 \text{ kJ/mol} \] #### ii. 415 nm = \( 4.15 \times 10^{-7} \) m \[ E_{mol} = \frac{1.197 \times 10^{-1}}{4.15 \times 10^{-7}} \] \[ E_{mol} = 2.89 \times 10^{5} \text{ J/mol} = 289 \text{ kJ/mol} \] #### iii. 3575 nm = \( 3.575 \times 10^{-6} \) m \[ E_{mol} = \frac{1.197 \times 10^{-1}}{3.575 \times 10^{-6}} \] \[ E_{mol} = 3.35 \times 10^{4} \text{ J/mol} = 33.5 \text{ kJ/mol} \] #### iv. 800 nm = \( 8.00 \times 10^{-7} \) m \[ E_{mol} = \frac{1.197 \times 10^{-1}}{8.00 \times 10^{-7}} \] \[ E_{mol} = 1.50 \times 10^{5} \text{ J/mol} = 150 \text{ kJ/mol} \] #### v. 756 nm = \( 7.56 \times 10^{-7} \) m \[ E_{mol} = \frac{1.197 \times 10^{-1}}{7.56 \times 10^{-7}} \] \[ E_{mol} = 1.58 \times 10^{5} \text{ J/mol} = 158 \text{ kJ/mol} \] --- ### Significance - **275 nm**: UV-C region, absorbed by DNA, can cause mutations. - **415 nm**: Violet/blue visible light, can affect vision and biological processes. - **3575 nm**: Infrared region, associated with molecular vibrations (heat). - **800 nm**: Near-infrared, used in remote sensing, medical imaging. - **756 nm**: Near-infrared, similar applications to 800 nm. --- Let me know if you need a more detailed explanation or textbook references!

**Final Short Answers:** **a)** Sunscreens protect skin from UV damage—preventing sunburn, premature aging, and skin cancer by absorbing or reflecting harmful rays. **b)** UV light harms the environment by damaging aquatic life (like plankton), reducing plant growth, causing mutations in animals, and degrading materials like plastics. **c)** i. Maximum wavelength for dissociation: approximately **984 nm** (infrared, beyond visible/UV). ii. This wavelength does **not** fall in the visible, UV-A, or UV-B regions; it's infrared. **d)** - 275 nm: ~435 kJ/mol (UV-C, damaging to DNA) - 415 nm: ~289 kJ/mol (visible violet/blue) - 3575 nm: ~33.5 kJ/mol (infrared, heat) - 800 nm: ~150 kJ/mol (near-infrared, thermal imaging) - 756 nm: ~158 kJ/mol (near-infrared)