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Chapter 18: Problem 53

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

Short Answer

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(a) Trihalomethanes (THMs) are organic compounds containing one carbon atom, one hydrogen atom, and three halogen atoms (such as chlorine, bromine, or iodine). They form as a byproduct when disinfectants like chlorine react with organic matter in water and are common water contaminants with potential negative health effects. (b) Two example THMs are Chloroform (CHCl鈧�) and Bromoform (CHBr鈧�): Chloroform Lewis Structure: Cl \ C - H / Cl Bromoform Lewis Structure: Br \ C - H / Br

Step by step solution

01

Definition of Trihalomethanes (THMs)

Trihalomethanes (THMs) are a group of organic chemical compounds that consist of one carbon atom, one hydrogen atom, and three halogen atoms (such as chlorine, bromine, or iodine). THMs are formed as a byproduct when chlorine or other disinfectants are used to treat water, which reacts with naturally occurring organic matter in water. They are common water contaminants and can have negative health effects if present at high levels.
02

Drawing the Lewis Structures of Two Example THMs

In order to draw the Lewis structures of two example THMs, we should first choose two examples of THMs and then draw their respective Lewis structures. Example 1: Chloroform (CHCl鈧�) 1. Count the total number of valence electrons: \(6 (from C) + 1 (from H) + 3 \times 7 (from Cl) = 28\). 2. Place the carbon atom in the center since it is less electronegative than chlorine, and connect all other atoms to the central atom with single bonds. 3. Complete the octets of the outer atoms (chlorine) by adding lone pairs of electrons. 4. Calculate the formal charge if necessary and adjust the bonding patterns as needed (in this case, no adjustment is needed). Lewis structure of Chloroform: Cl \ C - H / Cl Example 2: Bromoform (CHBr鈧�) 1. Count the total number of valence electrons: \(6 (from C) + 1 (from H) + 3 \times 7 (from Br) = 28\). 2. Place the carbon atom in the center since it is less electronegative than bromine, and connect all other atoms to the central atom with single bonds. 3. Complete the octets of the outer atoms (bromine) by adding lone pairs of electrons. 4. Calculate the formal charge if necessary and adjust the bonding patterns if needed (in this case, no adjustment is needed). Lewis structure of Bromoform: Br \ C - H / Br

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Organic Chemical Compounds
Organic chemical compounds are any compounds that contain carbon atoms bonded to other elements like hydrogen, oxygen, or halogens. They form the basis of all known life and have a wide variety of structures and purposes. In organic chemistry, carbon鈥檚 versatility allows it to form stable chains and frameworks, facilitating the creation of diverse molecules.
Trihalomethanes (THMs) are a subset of organic compounds distinguished by their composition of a single carbon atom bonded to one hydrogen atom and three halogen atoms. This category of compounds is significant because it highlights the vast arrangements possible even with a simple carbon framework.
  • They can be represented by the formula CHX鈧�, where "X" represents a halogen such as chlorine, bromine, or iodine.
  • Understanding the basic structure of organic compounds helps us to predict how they will react in different environments and why certain compounds like THMs can become problematic as water contaminants.
Recognizing these compounds' nature and behavior is crucial to comprehend their role in biological systems and environmental impact.
Lewis Structures
Lewis structures, also known as electron dot structures, are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They provide a simple and effective way to visualize the electrons involved in bond formation. When drawing Lewis structures, the total number of valence electrons is determined first, then distributed to fulfill the octet rule for each atom.
For trihalomethanes like chloroform (CHCl鈧�) and bromoform (CHBr鈧�), the Lewis structure process involves:
  • Placing the carbon atom at the center due to its lower electronegativity.
  • Connecting hydrogen and halogen atoms to the carbon through single bonds.
  • Adding lone pairs to the halogen atoms to satisfy the octet rule.
If drawn correctly, these structures help to predict molecular shape, bond angles, and potential chemical reactivity. They are fundamental in understanding the structural basis of organic compounds.
Water Contaminants
Water contaminants are substances that taint the purity of water, making it unsuitable for consumption or its intended use. Contaminants include a wide range of chemicals, microorganisms, and physical substances, which can have detrimental health and environmental effects.
Among chemical contaminants are organic molecules like trihalomethanes, which can form during the treatment of water. THMs are notable due to their potential health risks if consumed over long periods at high levels. When water is disinfected using chlorine, it reacts with organic matter naturally present in the water, leading to the formation of these compounds.
  • Regulations often control permissible levels of THMs to safeguard public health.
  • Continuous monitoring is essential to ensure water safety and reduce the presence of such contaminants.
This understanding reinforces the necessity of effective water treatment processes and the importance of ongoing scientific research in identifying and mitigating water contamination.
Disinfection Byproducts
Disinfection byproducts (DBPs) are chemicals formed unintentionally during the water disinfection processes. When disinfectants like chlorine interact with organic and inorganic matter in water, a variety of byproducts can result.
Trihalomethanes are among the most common and studied DBPs. Their formation is particularly tied to chlorination and presents a challenge in balancing the benefits of effective disinfection against the potential risks from DBPs.
  • Water treatment facilities continuously work to optimize disinfection processes to minimize THMs and other DBPs.
  • Alternative disinfection methods, like using ozone or ultraviolet light, are explored to reduce DBP formation.
Understanding the chemical processes leading to DBPs, like THMs, is essential to improve water treatment technologies, ensuring the safety and health of communities.

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Most popular questions from this chapter

Natural gas consists primarily of methane, \(\mathrm{CH}_{4}(g)\) . (a) Write a balanced chemical equation for the complete combustion of methane to produce \(\mathrm{CO}_{2}(g)\) as the only carbon-containing product. (b) Write a balanced chemical equation for the incomplete combustion of methane to produce CO(g) as the only carbon-containing product. (c) At \(25^{\circ} \mathrm{C}\) and 1.0 atm pressure, what is the minimum quantity of dry air needed to combust 1.0 \(\mathrm{L}\) of \(\mathrm{CH}_{4}(\mathrm{g})\) completely to \(\mathrm{CO}_{2}(\mathrm{g}) ?\)

The ultraviolet spectrum can be divided into three regions based on wavelength: UV-A \((315-400 \mathrm{nm}), \mathrm{UV}-\mathrm{B}(280-315\) \(\mathrm{nm} ),\) and \(\mathrm{UV}-\mathrm{C}(100-280 \mathrm{nm}) .\) (a) Photons from which region have the highest energy and therefore are the most harmful to living tissue? ( b) In the absence of ozone, which of these three regions, if any, are absorbed by the atmosphere? (c) When appropriate concentrations of ozone are present in the stratosphere, is all of the UV light absorbed before reaching the Earth's surface? If not, which region or regions are not filtered out?

(a) Write a chemical equation that describes the attack of acid rain on limestone, CaCO_. (b) If a limestone sculpture were treated to form a surface layer of calcium sulfate, would this help to slow down the effects of acid rain? Explain.

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