Problem 55 - Chemistry The Central Science

Question

(a) What are trihalomethanes (THMs)? (b) Draw the Lewis structures of two example THMs.

Step-by-Step Solution

Answer

(a) Trihalomethanes (THMs) are chemical compounds comprising a central carbon atom bonded to three halogen atoms (chlorine, bromine, iodine, or fluorine) and one hydrogen atom. They are often formed as byproducts in the process of disinfecting drinking water and are considered environmental pollutants with potential adverse health effects. (b) Two example THMs are Trichloromethane (CHCl3) and Bromodichloromethane (CHBrCl2). The Lewis structure for Trichloromethane (CHCl3) is: H | Cl - C - Cl | Cl The Lewis structure for Bromodichloromethane (CHBrCl2) is: H | Br - C - Cl | Cl

Step 1: (a) Definition of Trihalomethanes (THMs)

Trihalomethanes (THMs) are a group of chemical compounds that consist of a central carbon atom bonded to three halogen atoms (chlorine, bromine, iodine, or fluorine) and one hydrogen atom. They are usually formed as byproducts in the process of disinfecting drinking water with chemicals such as chlorine. THMs are considered environmental pollutants and can have adverse health effects, so their presence in drinking water is monitored and regulated.

Step 2: (b) Choosing Two Example THMs

Let's choose two example THMs for which we will draw the Lewis structures. We can choose Trichloromethane (CHCl3) and Bromodichloromethane (CHBrCl2).

Step 3: (b1) Drawing the Lewis Structure for Trichloromethane (CHCl3)

To draw the Lewis structure for Trichloromethane (CHCl3), follow these steps: 1. Count the total number of valence electrons: Carbon (C) has 4, Hydrogen (H) has 1, and each Chlorine (Cl) has 7. The total number of valence electrons is 4 + 1 + 3 * 7 = 26. 2. Place the least electronegative atom (C) in the center, and arrange the other atoms around it (H and Cl). 3. Add single bonds between the central atom and the surrounding atoms, using 2 valence electrons for each bond: 26 - 4 * 2 = 18 valence electrons remain. 4. Distribute the remaining valence electrons as lone pairs around the surrounding atoms (H and Cl), starting with the most electronegative ones (each Cl should complete an octet, while H only needs a duet): 18 - 3 * 2 * 2 = 6 valence electrons remain after completing the octets of the Cl atoms. 5. Distribute the leftover valence electrons as lone pairs on the central atom (C): 6 - 2 = 4 valence electrons remain on the C atom, completing its octet.

Step 4: (b2) Drawing the Lewis Structure for Bromodichloromethane (CHBrCl2)

To draw the Lewis structure for Bromodichloromethane (CHBrCl2), follow these steps: 1. Count the total number of valence electrons: Carbon (C) has 4, Hydrogen (H) has 1, Bromine (Br) has 7, and each Chlorine (Cl) has 7. The total number of valence electrons is 4 + 1 + 7 + 2 * 7 = 26. 2. Place the least electronegative atom (C) in the center, and arrange the other atoms around it (H, Br, and Cl). 3. Add single bonds between the central atom and the surrounding atoms, using 2 valence electrons for each bond: 26 - 4 * 2 = 18 valence electrons remain. 4. Distribute the remaining valence electrons as lone pairs around the surrounding atoms (H, Br, and Cl), starting with the most electronegative ones (each Cl and Br should complete an octet, while H only needs a duet): 18 - 3 * 2 * 2 = 6 valence electrons remain after completing the octets of the Cl and Br atoms. 5. Distribute the leftover valence electrons as lone pairs on the central atom (C): 6 - 2 = 4 valence electrons remain on the C atom, completing its octet.