Chlorine Reactions Physical Chemistry

Chlorine Reactions Physical Chemistry

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Chlorine is a type of element that belongs to group 17 on the periodic table. Its atomic number is 17. In this article, we will be discussing the different reactions of chlorine and its compounds. First, we will explain the differences between chlorine, chloride, and chlorate(I). Then, we will talk about how chlorine and chlorate(I) are used to clean wastewater and the reaction between chlorine and sodium hydroxide. We will also look at how chlorine reacts with oxygen, sodium, and other halide solutions. Lastly, we will discuss the advantages and disadvantages of using chlorine in treating wastewater. Learn about the physical chemistry of chlorine reactions in this article.

What are chlorine, chloride, and chlorate(I)?

Before we go any further, we need to look at some of the species related to chlorine: chloride and chlorate(I). Despite their similar names, they do have their differences and you shouldn’t get them mixed up.


Chlorine is a type of halogen that can be found in group 17 of the periodic table. At room temperature, it appears as a yellow gas made up of two chlorine atoms that are covalently bonded together to form a single molecule. Each atom within the molecule has an oxidation state of +0, and the molecule itself is neutral.

A chlorine molecule
A chlorine molecule


If you take a chlorine atom and add on an electron, you’ll end up with chloride. Chloride is a negative chlorine ion with a charge of -1 and oxidation state of -1. In fact, in most - but not all - compounds containing chlorine, you’ll find it with this oxidation state.

A chloride ion


When chlorine bonds to oxygen and picks up an extra electron, it forms the chlorate(I) ion, . This is a negative ion with a charge of -1. It is also known as hypochlorite.

A chlorate(I) ion
A chlorate(I) ion

Let's examine the oxidation states within the compound. In an ion, the sum of all the oxidation states equals the charge on the ion. In this case, all the oxidation states should add up to -1. Within the compound, the element with the highest electronegativity takes the lowest oxidation state. Oxygen is the most electronegative element and therefore has an oxidation state of -2 (except in some cases). In order for all the oxidation states in the compound to equal -1, chlorine has an oxidation state of +1. This may seem unusual because chlorine typically has an oxidation state of -1, but in this compound, it takes a higher oxidation state because it is less electronegative than oxygen. When people refer to chlorate, they usually mean chlorate(V), which is a different ion with the formula. The Roman numeral indicates the oxidation state of one of the elements in a compound. In chlorate(V), chlorine has an oxidation state of +5. If you're having trouble understanding oxidation states, head over to Redox for more information.

How does chlorine react with water?

Chlorine plays many roles in everyday life, but we’re going to focus specifically on one of its main uses: treating wastewater.

Reaction of chlorine with water

When chlorine reacts with water, it produces a mixture of hydrochloric acid (HCl) and chloric acid (HClO). Chloric acid is also known as hypochlorous acid and is based on the chlorate ion, . It is a potent oxidizing agent that can kill various bacteria and viruses, ranging from the common cold to cholera. This technique is used to disinfect water for swimming pools and drinking purposes. The equation for the reaction is as follows:

Cl2 + H2O → HCl + HClO

Let's examine the oxidation states of chlorine in the three different species. In Cl2, chlorine has an oxidation state of +0. In HCl, chlorine has an oxidation state of -1, and in HClO, chlorine has an oxidation state of +1. This indicates that chlorine has undergone both oxidation and reduction in this reaction, making it an example of a disproportionation reaction. In a disproportionation reaction, some atoms of an element are oxidized, while others are reduced.

Reaction of chlorine with water in sunlight

In sunlight, a different reaction occurs. The chlorine and water instead break down into oxygen and hydrochloric acid.

Reaction of chlorate(I) with water

The reaction between chlorine and an alkali such as sodium hydroxide doesn’t give us chloric acid, but instead sodium chlorate(I), which is used to treat wastewater and is also the active ingredient in household bleach. Solutions containing sodium chlorate(I) are particularly useful for removing stains on cutlery caused by tannins in tea. The equation for this reaction is:
2Cl2 + 2NaOH → NaClO3 + NaCl + H2O[^1]

This is another example of a disproportionation reaction. Chlorine atoms are both oxidised to make sodium chlorate(I) and reduced to make sodium chloride.

The reaction between chlorine and sodium hydroxide
The reaction between chlorine and sodium hydroxide

Ignaz Semmelweis was a pioneer in modern hygiene practices in hospitals and surgeries. He discovered that a high percentage of babies delivered by certain doctors were dying soon after birth, compared to those delivered by midwives. These doctors often came straight from dissecting rooms where they were working on cadavers. Semmelweis hypothesized that these doctors were carrying 'cadaveric particles,' which could transmit decay from dead corpses to newborns. To prevent the spread of disease, Semmelweis found that a simple solution of chlorine dissolved in water was an effective solution.

Semmelweis's discovery was groundbreaking because it led to the widespread adoption of antiseptic practices. His use of chlorine to disinfect surgical instruments and wounds helped to reduce the spread of disease and infection in hospitals, saving countless lives. Today, chlorine is still widely used as a disinfectant in hospitals, water treatment plants, and households worldwide.

How else does chlorine react?

Chlorine can also react with sodium and with other solutions of halide ions.

Reaction of chlorine with sodium

Chlorine and sodium react to form a staple ingredient that is essential to our health and wellbeing - sodium chloride, commonly known as salt. This reaction is an example of a redox reaction that forms an ionic compound. The equation for this reaction is:

Cl2 + 2Na → 2NaCl

While salt is often blamed for raising blood pressure and causing heart disease, the truth is that our cells need salt to function properly. Without salt, cells wouldn't be able to regulate the movement of water in or out of themselves by osmosis. Nerve cells in particular rely on salts - they use the movement of ions to send impulses from sensory organs through to the central nervous system and out to muscles. While too much salt can be harmful, too little salt can also cause health issues. Therefore, it is important to consume salt in moderation to maintain a healthy balance in our bodies.

Reaction of chlorine with halide ions

Chlorine is a halogen. Halogens take part in displacement reactions - a more reactive halogen will displace a less reactive halide ion from an aqueous solution. Halogens get less reactive as you go down the group in the periodic table. This means that chlorine can displace bromide and iodide ions. However, it can’t displace fluoride ions. For example, if you react chlorine with sodium bromide, chlorine will displace the bromide ions to form sodium chloride and bromine:

Reaction of chlorine with oxygen

Chlorine doesn’t normally react with oxygen. However, in the presence of UV light, chlorine can react with oxygen or ozone molecules to form the chlorine monoxide free radical, : Like all radicals, this species is extremely reactive and can break down ozone in the ozone layer. To find out more about how chlorine destroys ozone, check out Ozone Depletion.

Pros and cons of chlorinating water

In 1897, Maidstone, England, became the first town to have its entire water supply treated directly with chlorine. This marked the beginning of a new era in water treatment, aimed at preventing waterborne diseases. Widespread permanent chlorination began in 1905, following a serious typhoid epidemic in the city of Lincoln. Since then, chlorine has played a major role in preventing disease and keeping our drinking and washing water clean. However, there are both positives and negatives associated with chlorinating water.

On the positive side, chlorinating water is a cheap and effective way to disinfect water and kill harmful bacteria and viruses that can cause illness. Chlorine is also easy to use and can be added to water at any point in the distribution system, ensuring that water remains safe throughout the journey from treatment plant to tap.

However, there are also some downsides to using chlorine. While it is effective in killing harmful bacteria and viruses, it can also react with organic matter in water to form byproducts that can be harmful to human health. These byproducts, such as trihalomethanes (THMs), have been linked to an increased risk of cancer and other health problems. Chlorine can also have an unpleasant taste and odor, which may discourage some people from drinking tap water.

Despite these drawbacks, chlorination remains the most popular method of water treatment worldwide, due to its cost-effectiveness and ease of use. Water treatment plants are constantly working to find ways to minimize the formation of harmful byproducts and improve the taste and odor of chlorinated water.


In addition to the advantages mentioned earlier, chlorinating water is also a very economical method of water treatment. As chlorine is widely available and can be used on both large and small scales, it is a popular choice for water treatment plants around the world.

But how exactly does chlorine kill pathogens? Chlorine and chloric acid are uncharged molecules that can penetrate the cell wall of bacteria and other pathogens. Within the cell, they oxidize proteins and enzymes, causing damage to the cell and rendering it unable to function properly. This process effectively kills the pathogen, making the water safe for consumption.

It should be noted that while chlorination is effective at killing many types of pathogens, it may not be effective against all types. For example, some parasites and viruses may be resistant to chlorine and may require additional treatment methods. It is also important to maintain proper levels of chlorine in the water, as too little may not effectively kill pathogens, while too much can create harmful byproducts.

In conclusion, while there are some drawbacks to chlorinating water, such as the formation of harmful byproducts, it remains a popular and effective method of water treatment that has saved countless lives. Water treatment plants continue to work towards minimizing the formation of byproducts and improving the taste and odor of chlorinated water, while also exploring alternative methods of water treatment.


While chlorinating water is a widely used and effective method of disinfecting water, it is important to note its potential drawbacks. Chlorine can have negative effects on skin and hair, and some people may dislike the taste and odor of chlorinated water. Additionally, chlorine can react with organic compounds in water to form disinfectant byproducts that may have negative health effects if levels are too high.

It is important to note that these negative effects are rare and occur only when chlorine levels are not carefully monitored and controlled within safe limits. Chlorine remains a widely used and effective method of water treatment, and water treatment plants are constantly working to improve its effectiveness and minimize its potential negative effects. Alternatively, some people choose to use ozone to sterilize water as an alternative to chlorination.

Chlorine Reactions

What happens when chlorine reacts with potassium bromide solution?

Chlorine reacts with potassium bromide to form potassium chloride and bromine. This is an example of a displacement reaction.

Which elements react violently with chlorine? 

Group 1 elements react violently with chlorine.

What is the reaction of chlorine with water? 

Chlorine reacts with water to form a mixture of hydrochloric acid and chloric acid. In sunlight, a different reaction occurs - chlorine and water react to form oxygen and hydrochloric acid.

Is reacting a hydrocarbon with chlorine an endothermic reaction?

The free radical substitution of a hydrocarbon by chlorine is exothermic, not endothermic.

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