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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.
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.
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.
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.
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.
Let’s take a closer look at the oxidation states within the compound. Have a go at working them out yourself.
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.
When people talk about chlorate, they generally mean chlorate(V). This is a different ion with the formula . Roman numerals show the oxidation state of one of the elements in a compound. In chlorate(V), chlorine has an oxidation state of +5.
Struggling to get to grips with oxidation states? Check out Redox for more information.
Chlorine plays many roles in everyday life, but we’re going to focus specifically on one of its main uses: treating wastewater.
When chlorine reacts with water, it forms a mixture of hydrochloric acid, , and chloric acid, . Chloric acid is also known as hypochlorous acid and is based on the chlorate ion, . 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.
Let’s look at the oxidation states of chlorine in the three different species.
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.
In sunlight, a different reaction occurs. The chlorine and water instead break down into oxygen and hydrochloric acid.
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.
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.
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:
This is another example of a disproportionation reaction. Chlorine atoms are both oxidised to make sodium chlorate(I) and reduced to make sodium chloride.
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.
Chlorine can also react with sodium and with other solutions of halide ions.
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.
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.
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.
For example, if you react chlorine with sodium bromide, chlorine will displace the bromide ions to form sodium chloride and bromine:
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.
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.
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:
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.
As with many good things, chlorinating water also has some drawbacks. Let’s look at a few of them now.
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.
Chlorine reacts with potassium bromide to form potassium chloride and bromine. This is an example of a displacement reaction.
Group 1 elements react violently with chlorine.
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.
The free radical substitution of a hydrocarbon by chlorine is exothermic, not endothermic.
Compare and contrast chlorine, chloride, and chlorate.
Define disproportionation reaction.
A reaction in which some atoms of an element are oxidised and some are reduced.
Give one use of sodium chlorate(I).
Chlorinating water kills the bacteria within it. True or false?
Give three advantages of chlorinating water.
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