Chlorine is a chemical element classified as a halogen, belonging to Group 17 of the periodic table. In its elemental form, it is recognizable as a pale yellow-green gas. Chlorine is characterized by its high chemical reactivity, acting as a powerful oxidizing agent in most reactions. It has the highest electron affinity of all elements, signifying its strong tendency to readily acquire an electron to achieve a stable outer shell. Due to this drive to form compounds, chlorine is rarely found in nature in its elemental gaseous form, instead existing combined with other substances like common salts.
How Chlorine Interacts with Water
When chlorine gas (\(\text{Cl}_2\)) is introduced into water (\(\text{H}_2\text{O}\)), it rapidly undergoes hydrolysis, forming two distinct acids. This reaction produces both hypochlorous acid (\(\text{HClO}\)) and hydrochloric acid (\(\text{HCl}\)). Hypochlorous acid is the primary active agent responsible for destroying pathogens, accounting for chlorine’s ability to disinfect.
Hypochlorous acid is a weak acid that exists in a dynamic equilibrium with the hypochlorite ion (\(\text{ClO}^-\)) and a hydrogen ion (\(\text{H}^+\)). This balance is dependent on the water’s pH level. The effectiveness of chlorine as a disinfectant is tied directly to the proportion of hypochlorous acid present in the solution.
At a neutral pH of 7.0, approximately 76% of the free available chlorine is hypochlorous acid, which is highly efficient at oxidizing microorganisms. As the water’s pH increases, the equilibrium shifts to favor the formation of the hypochlorite ion. This ion is a significantly poorer disinfectant, being up to 300 times less effective than the acid form.
The reduced disinfection power at higher pH levels is due to the ion’s negative electrical charge. This charge makes it difficult for the hypochlorite ion to pass through the negatively charged cell walls of bacteria and other microbes. Consequently, at a pH above 8.5 or 9, the vast majority of chlorine exists as the less germicidal hypochlorite ion, resulting in a substantial drop in sanitization efficacy.
Fundamental Reactions with Elements
Chlorine’s status as a strong oxidizing agent dictates its foundational reactions with almost every other element. In reactions with metals, chlorine readily accepts electrons, driving the metal atom into a positive oxidation state to form an ionic compound. These reactions typically yield metal chlorides, which are salts formed through the complete transfer of electrons.
For instance, chlorine gas reacts vigorously with metals like sodium (\(\text{Na}\)) to produce sodium chloride (\(\text{NaCl}\)), commonly known as table salt. Iron (\(\text{Fe}\)) also reacts with chlorine to form iron chlorides, a process that is often highly exothermic once initiated. When a metal has several possible oxidation states, chlorine tends to force the formation of the compound in which the metal exhibits its higher oxidation state.
In contrast, chlorine reacts with non-metals to form covalent compounds, where electrons are shared rather than fully transferred. These reactions result in the creation of molecular chlorides, which often exist as liquids or gases at room temperature. Examples include its reaction with phosphorus (\(\text{P}\)) to create various phosphorus chlorides or with sulfur (\(\text{S}\)) to form sulfur chlorides.
Chlorine also reacts with other halogens to form a distinct class of substances known as interhalogen compounds. For example, chlorine gas reacts with solid iodine (\(\text{I}_2\)) to form iodine monochloride (\(\text{ICl}\)), or with gaseous bromine (\(\text{Br}_2\)) to form bromine monochloride (\(\text{BrCl}\)). These reactions illustrate chlorine’s ability to form stable bonds with elements that are chemically similar to itself.
Hazardous Mixing and Toxic Byproducts
A major safety concern involves the reaction of chlorine bleach (sodium hypochlorite) with nitrogen-containing compounds such as ammonia (\(\text{NH}_3\)). Mixing these substances, which can occur accidentally with certain household cleaners or urine, immediately produces toxic gases called chloramines. Inhaling these chloramine fumes can cause severe respiratory distress, leading to symptoms like coughing, nausea, and wheezing, and may even cause chemical pneumonitis.
An equally dangerous reaction occurs when chlorine bleach is combined with strong acids, such as hydrochloric acid or high concentrations of vinegar. This interaction releases highly concentrated elemental chlorine gas (\(\text{Cl}_2\)), which is a potent respiratory poison. Even short-term exposure can cause burning and irritation of the eyes and throat, significant breathing difficulties, and in high concentrations, it can be fatal.
Chlorine also reacts with organic materials in water, leading to the formation of unintended byproducts that are a concern for public health. During the disinfection of drinking water, chlorine reacts with naturally occurring organic matter, such as decaying vegetation. This process produces a complex mixture of chlorinated organic compounds, most notably trihalomethanes (THMs), including chloroform.
The formation of these disinfection byproducts is influenced by factors such as water temperature and the duration of contact between chlorine and organic matter. Higher water temperatures and longer reaction times increase the concentration of trihalomethanes in the water supply. Water treatment facilities must carefully manage the chlorination process to maximize pathogen destruction while minimizing the creation of these potentially harmful byproducts.