Chemistry of Salt

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The Chemical Composition of Salt

Sir Humphrey Davy (1778-1829) was the first person to separate salt into its constituent parts of sodium and chlorine. He did this in 1807 but at that time no one could think of anything useful to do with them! Since then however both sodium and chlorine have become the bedrock for many manufacturing industries.

Today salt and the constituent parts keep our industries alive. The properties of chlorine and sodium, plus the principal compounds from them, make salt one of the most important raw materials used by industry.

Chlorine compounds of commercial importance include hydrochloric acid (used to make PVC), chlorinated hydrocarbons (used in dry cleaning) and bleaching powder (used to make water safe).

Important sodium compounds include sodium carbonate (used to soften water), sodium sulphate (used in washing powders), baking soda (used by bakers to help lighten and soften their bread & cakes), sodium phosphate (also used by bakers) and sodium hydroxide (used in the pulping of wood for making paper).

Electrolysis of Sodium Chloride Solution

The electrolysis of concentrated sodium chloride solution is an important industrial process and produced three useful products:

• Chlorine (Cl2)
• Sodium hydroxide (NaOH)
• Hydrogen (H2)

An electric current is passed through the solution between electrodes made of inert materials so they do not react with these useful products made during the electrolysis reaction.

During the electrolysis of sodium chloride solution:

Hydrogen ions are reduced to hydrogen molecules
Chloride ions are oxidised to chlorine molecules.

Reduction and oxidation must always occur together so they are sometimes referred to as “redox” reactions.

How Electrolysis of Sodium Chloride Solution Works

During electrolysis pairs of hydrogen ions are attracted to the negative electrode (the cathode) where they pick up electrons to form hydrogen molecules.

Hydrogen ions + Electrons → Hydrogen Molecules

Pairs of chloride ions are attracted to the positive electrodes (the anode) where they deposit electrons to form chlorine molecules

Chloride Ions → Chlorine Molecules + Electrons

A solution of sodium hydroxide (NaOH) is also produced.

Chlorine

Among the most abundant of nature’s 118 identified elements, chlorine is at work all around us, combining with other elements to sustain life and the natural processes of the environment. Chlorine is found in the Earth itself and, as salt, in the seas that cover seven-tenths of the planet’s surface.

It is fundamental to the life of plants and animals. In our bodies, chlorine, as hydrochloric acid, helps break down food for digestion. It is also part of the immune system that protects us from infection.

Nature and chlorine can do some remarkable things like making a pain killer 200 times more powerful than morphine but with no side-effects – all part of the natural defences of an Ecuadorian tree frog!

A Power of Good

Perhaps it is not surprising that so many man-made chlorine products come from naturally good ideas. The very first were disinfectants to combat the spread of disease.

Chlorine was first introduced into drinking water late in the 19th century to control the spread of waterborne diseases like cholera and typhoid – disease that have killed more people than all the wars in human history.

Continuing to fight diseases is just as important today. Up to fifteen million people still die every year from the effects of drinking untreated water.

Chlorine is used in many other ways to make a huge range of products. The secret is reactivity – chlorine’s ability to bond easily with other chemical elements. This makes chlorine one of the most successful chemical building blocks in industry and in nature. Over 2000 natural organic chlorine compounds are at work in the environment.

Essentials for Life

In the modern world, as in nature, chlorine helps to sustain life and protect our health.

Chlorine based water treatments are among the most effective available and the only ones that keep working right up to the tap. The same powerful disinfecting properties are found in hygiene products used in hospitals and homes everywhere. Preventing the spread of infection is particularly important in hospitals and chlorine is used to produce sterile packaging, disposable equipment and even saline drips and blood bags.

Modern medicine relies on chlorine in other ways too. 85% of pharmaceuticals contain chlorine or are made with it including treatments for heart disease, leukaemia, arthritis and allergies.

Chlorine is also at work in agriculture – but without it, people still live in near famine in many parts of the world. 96% of the products used to control pests, crop diseases and weeds are based on chlorine. The result is high quality, high yield crops and lower food prices.

Keeping Pace… Keeping in Touch

Far from being the thing of the past, many industries have turned to chlorine chemistry to find new ways of doing things. For example, high performance plastics and polymers made with chlorine have replaced scarce natural materials like hardwoods in the modern school, office and home. It is used to make computer equipment (including, most likely, the device you’re using to read this page), televisions, CDs, tennis racquets, shoes, skis, car parts, telephones, satellites… the list is almost endless.

Chlorine is an asset in the modern world of fashion, used in the production of modern fabrics as well as in the dry-cleaning process.

Why is Electrolysis Useful? Some Common Uses.

All three products, products: chlorine (Cl2), sodium hydroxide (NaOH), and hydrogen (H2) are useful individually but can also be combined to make new important compounds.

Sodium hydroxide and chlorine can be made into sodium chlorate (l). This is a strong oxidising agent and is excellent at killing bacteria – but is can be explosive when mixed with the wrong chemicals. Sodium chlorate is used as the active ingredient in a wide variety of commercial herbicides. The main commercial use for sodium chlorate is for making chlorine dioxide (ClO2) used for the bleaching of pulp.

Hydrogen and chlorine react together to form hydrogen chloride. This is made into hydrochloric acid by dissolving it in water. Hydrochloric acid made in this way is very pure and can be used safely in the food and pharmaceutical industries.

An important downstream product which helps protect us in our daily lives is sodium hypochlorite (NaClO) which is commonly used as household bleach, used as a disinfectant and in water chlorination. Today the preferred method for the large scale production of sodium hypochlorite is known as the Hooker process (named after Hooker Chemicals, now Occidental Petroleum). In the process, sodium hypochlorite (NaClO) and sodium chloride (NaCl) are formed when chlorine is passed into cold and dilute sodium hydroxide solution. It is prepared industrially by electrolysis with minimal separation between the anode and the cathode. The solution must be kept below 40 °C (by cooling coils) to prevent the undesired formation of sodium chlorate.

Cl2 + 2NaOH →NaCl + NaClO + H2O

Hence, chlorine is simultaneously reduced and oxidized; this process is known as disproportionation.

Hydrolysis

Hydrolysis is a reaction involving the breaking of a bond in a molecule using water. The reaction mainly occurs between an ion and water molecules and often changes the pH of a solution. In chemistry, there are three main types of hydrolysis: salt hydrolysis, acid hydrolysis, and base hydrolysis. http://en.wikipedia.org/wiki/Hydrolysis

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