Problem 53 - Chemistry: The Central Science in SI Units, Global Edition

Question

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

Step-by-Step Solution

Answer

(a) Trihalomethanes (THMs) are a group of organic compounds derived from the reaction between chlorine disinfectants and natural organic matter found in water. They have the general formula CHX_3, where X is a halogen such as chlorine (Cl), bromine (Br), fluorine (F), or iodine (I). (b) Here are the Lewis structures of two example THMs: 1. Chloroform (CHCl_3): Cl | H - C - Cl | Cl 2. Bromodichloromethane (CHBrCl_2): Cl | H - C - Cl | Br

Step 1: (a) Definition of Trihalomethanes

Trihalomethanes (THMs) are a group of organic compounds derived from the reaction between chlorine disinfectants and natural organic matter found in water. They have the general formula CHX_3, where X is a halogen such as chlorine (Cl), bromine (Br), fluorine (F), or iodine (I).

Step 2: (b) Examples of Trihalomethanes

We will now draw two examples of THMs.

Step 3: Example 1: Chloroform (CHCl_3)

Chloroform is one of the most common trihalomethanes, containing three chlorine atoms (Cl). The Lewis structure for CHCl_3 is as follows: C is the central atom, surrounded by 3 Cl atoms and 1 H atom. C forms single bonds with the 3 Cl atoms and the H atom. Cl | H - C - Cl | Cl

Step 4: Example 2: Bromodichloromethane (CHBrCl_2)

Bromodichloromethane contains one bromine atom (Br) and two chlorine atoms (Cl). The Lewis structure for CHBrCl_2 is as follows: C is the central atom, surrounded by 1 Br atom, 2 Cl atoms, and 1 H atom. C forms single bonds with the 1 Br atom, the 2 Cl atoms, and the H atom. Cl | H - C - Cl | Br

Key Concepts

Organic Chemistry

Organic chemistry is a branch of chemistry focusing on compounds primarily made of carbon and hydrogen. These compounds, known as organic compounds, often include other elements like oxygen, nitrogen, sulfur, and halogens (fluorine, chlorine, bromine, iodine). Trihalomethanes (THMs) are a perfect example of organic chemistry in action, as they are organic compounds formed by a carbon atom bonded to three halogen atoms and one hydrogen atom. Understanding organic chemistry is essential because carbon's ability to form four covalent bonds allows for an incredibly diverse range of molecular structures. This is what leads to the complexity and variety we see in organic molecules. In THMs, specifically, the halogen atoms attached to the carbon significantly influence the properties and reactivity of these compounds.

Lewis Structures

Drawing Lewis structures is a fundamental skill in chemistry. Lewis structures reveal how atoms in a molecule are bonded to each other and can help predict the shape, behavior, and reactivity of the molecule. For trihalomethanes, drawing a Lewis structure starts by arranging the carbon atom in the center, since it forms four bonds—three with halogen atoms and one with a hydrogen atom. When constructing a Lewis structure:

• Count the total number of valence electrons available for bonding.
• Place the least electronegative atom (often carbon) at the center.
• Arrange other atoms around the central atom and connect them using single bonds.
• Distribute remaining electrons to complete the octets where necessary.

For chloroform (CHCl₃), the carbon atom is bonded to three chlorine atoms, each contributing seven valence electrons, and one hydrogen atom, contributing one. For another example, bromodichloromethane (CHBrCl₂), the structure involves a carbon atom making bonds with one bromine atom, two chlorine atoms, and one hydrogen atom, following similar steps for electron counting and bond formation.

Chlorine Disinfection Byproducts

Chlorine disinfection is a critical step in water treatment, ensuring safe drinking water free of harmful pathogens. However, this process often results in the formation of byproducts, particularly when chlorine reacts with natural organic matter in water. One significant group of these byproducts is trihalomethanes (THMs). Common THMs include chloroform and bromodichloromethane. This reaction is important to monitor because THMs have been identified as potential health hazards, particularly in relation to long-term exposure risks like cancer. Water treatment facilities must balance effective disinfection with minimizing THM formation. Regulations require constant monitoring of THM levels in public water supplies to ensure they remain within safe limits.