Water, Alkalis & Other Reactions (2023)

Chlorine is a halogen found in group 17 in the periodic table. It has the atomic number 17. We'll be studying chlorine reactions here.

• In this article, we’ll look at the reactions of chlorine and its related compounds.
• Firstly, we’ll explain the differences between chlorine, chloride, and chlorate(I).
• We’ll then investigate how chlorine and chlorate(I) are used to treat wastewater and look at the reaction between chlorine and sodium hydroxide.
• We’ll also look at the reaction of chlorine with oxygen, sodium, and other halide solutions.
• Finally, we’ll explore the pros and cons of using chlorine in wastewater treatment.

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

As we mentioned before, chlorine is a halogen in group 17 on the periodic table. At room temperature, it is found as a yellow diatomic gas. This means that each molecule of chlorine is made up of two chlorine atoms covalently bonded together. The molecule is neutral, and each chlorine atom within the molecule has an oxidation state of +0.

Fig. 1 - A chlorine molecule

Chloride

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.

Fig. 2 - A chloride ion

Chlorate(I)

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

Fig. 3 - A chlorate(I) ion

Let’s take a closer look at the oxidation states within the compound. Have a go at working them out yourself.

• In an ion, the sum of all the oxidation states equals the charge on the ion. In this case, they should all total -1.
• In a compound, the most electronegative element takes the lowest oxidation state. Here it is oxygen.
• You should know that oxygen always has an oxidation state of -2 (except in a few special cases).
• In order for all the oxidation states in the compound to equal -1, chlorine must have an oxidation state of +1.

Not what you would expect? Chlorine normally has an oxidation state of -1. But here it takes the higher oxidation state in the compound because it is less electronegative than oxygen.

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 forms 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, ClO^-. It is a powerful oxidising agent that kills all sorts of bacteria and viruses, from common colds to cholera. We use this technique to disinfect water for both swimming pools and for drinking purposes. The equation is given below.

\[Cl_2 (g) + H_2O(l) \rightarrow HClO (aq) + HCl (aq)\]

Let’s look at the oxidation states of chlorine in the three different species.

• In Cl₂, it has an oxidation state of +0.
• In HCl, it has an oxidation state of -1.
• In HClO, it has an oxidation state of +1.

This means that chlorine has been both oxidised and reduced. This reaction is therefore an example of a disproportionation reaction.

In a disproportionation reaction, some atoms of an element are oxidised and some 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.

\[2Cl_2 (g) + 2H_2O(l) \rightarrow 4HCl (aq) + O_2(g)\]

Reaction of chlorate(I) with water

Instead of adding chlorine directly, we can also treat water using chlorate(I) ions. We get these from solid sodium or calcium chlorate(I), another compound based on chlorate(I). They react with water to produce sodium ions, hydroxide ions, and chloric acid.\[NaClO (s) + H_2O \leftrightharpoons Na^+(aq) + OH^-(aq) + HClO(aq)\]

This is a reversible reaction - it doesn’t go to completion. To keep the equilibrium on the right, we keep the solution slightly acidic. However, the pH is always monitored closely to ensure that the water is safe for our use, be it for washing, swimming, or drinking.

How does chlorine react with alkalis?

The reaction between chlorine and an alkali such as sodium hydroxide doesn’t give us chloric acid, but instead sodium chlorate(I), which we met above. Again, this is used to treat wastewater, but it 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 is given below:

\[Cl_2 (g) + 2NaOH (aq) \rightarrow NaClO (aq) + NaCl (aq) + H_2O (l)\]

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

Fig. 4 - The reaction between chlorine and sodium hydroxide. Anna Brewer, StudySmarter Originals

You might have heard of Ignaz Semmelweis, one of the founders of modern hygiene practices in hospitals and surgeries. Semmelweis noticed that a strangely high proportion 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 proposed that they were carrying 'cadaveric particles' that transmitted decay from dead corpse to newborn child. He found that a simple solution of chlorine dissolved in water was an effective way of stopping the spread of disease.

How else does chlorine react?

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

Reaction of chlorine with sodium

Chlorine reacts with sodium to form a staple ingredient that is vital to our health and wellbeing - sodium chloride, often referred to simply as salt. This is an example of a redox reaction that forms an ionic compound. The equation is shown below.

\[Cl_2 + 2Na \rightarrow 2NaCl\]

Salt frequently gets a bad reputation. We’re told that it raises blood pressure and leads to heart disease and sometimes even early death. However, our cells need salt to function. 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. Too much salt may be bad for us - but too little salt is definitely an issue as well.

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:

\[Cl_2 (aq) + 2NaBr (aq) \rightarrow 2NaCl (aq) + Br_2 (aq)\]

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, \(ClO \cdot\):

\[Cl_2 \rightarrow 2Cl \cdot \qquad Cl \cdot + O_3 \rightarrow ClO \cdot + O_2\]

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. Widespread permanent chlorination started in 1905 when the city of Lincoln suffered a serious typhoid epidemic. Since then, chlorine has played a major part in preventing disease and keeping our drinking and washing water clean. However, there are both positives and negatives associated with chlorinating water.

Pros of chlorinating water

As we’ve already mentioned, chlorinating water is first and foremost an effective way of killing off all manner of pathogens. It has helped prevent millions of deaths worldwide and this shouldn’t be underestimated.

Other advantages include:

• It is cheap and economical.
• It is widely available.
• It can be used on both large and small scales.

How does chlorine kill pathogens? Both chlorine and chloric acid are uncharged molecules that are able to disrupt the cell wall in bacteria and other pathogens. Once within the cell, they oxidise proteins and enzymes, damaging them so the cell can’t function.

Cons of chlorinating water

As with many good things, chlorinating water also has some drawbacks. Let’s look at a few of them now.

• Chlorine can dry out skin and hair.
• Some people dislike the taste of chlorinated water. One alternative is to use ozone to sterilise water.
• Chlorine can react with organic compounds in water to produce disinfectant byproducts, or DBPs. These can have negative effects on health and have been linked to problems such as liver and kidney cancer, heart disease, and the slowing of brain activity.

It is important to remember that these are worst-case scenarios and very rarely occur. Chlorine levels in water are always carefully monitored to ensure that they stay within safe limits.