The Electrolysis of Brine

Question 1

What is brine?

Answer 1

sodium chloride solution (NaCl)

Question 2

Where is sodium chloride found in nature?

Answer 2

in oceans and seas

found in large deposits in the ground - the salt can be extracted from underground deposits by solution mining

Question 3

How does solution mining work?

Answer 3

very hot water under pressure is pumped down into the salt deposits

the salt dissolves and the solution is pumped back to the surface

Question 4

What are the four ions in brine (aqueous sodium chloride)?

Answer 4

hyrodgen, H⁺

chloride, Cl^-

sodium, Na⁺

hydroxide, OH^-

Question 5

Explain the chlor-alkali diaphragm cell

Answer 5

the electrolysis of brine is carried out in a diaphragm cell

the cell is designed to keep the products apart

the porous diaphragm only lets sodium ions go through, not hydroxide ions nor the chloride ions

the cathode is made of steel

the anode is made of titanium because steel would react with the chlorine produced; titanium is less reactive than steel

there is a higher level of brine on the anode side - this makes sure that the flow of liquid is always from the anode to the cathode which prevents spdium hydroxide solution formed finding its way back to where chlorine is being produced

Question 6

Explain why diaphragm cell is designed to keep the products apart?

Answer 6

if chlorine comes into contact with sodium hydorxide solution, it reacts to make bleach - a micture of sodium chloride and sodium chlorate (I) solution (NaClO)

if chlorine comes into contact with hydrogen, it produces a mixture which would explode violently on exposure to sunlight or heat to give hydrogen chloride

Question 7

What are the products of the electrolysis of brine?

Answer 7

Chlorine (Cl^-)

Hydrogen (H⁺)

Sodium Hydroxide (NaOH)

Question 8

What is produced at the titanium anode? What is the half equation for this?

Answer 8

chlorine gas

2Cl^- _(aq) → Cl_{2 (g)} + 2e^-

Question 9

What is produced at the steel cathode? What is the half equation for this?

Answer 9

hydrogen gas (it is too difficult to discharge sodium ions so hydrogen ions from the water are discharged instead)

2H+ _(aq) + 2e^- → H_{2 (g)}

Question 10

What is left in solution?

Answer 10

the sodium ions (Na⁺) and the hydroxide ions (OH^-) from the water are left in solution

sodium hydroxide solution is formed

Question 11

How is the sodium hydroxide solution left in solution concentrated? Why is is concentrated?

Answer 11

It is concentrated because is it contaminated with unchanged sodium chloride

It is concentrated by evaporating it. During this process, most of the sodium chloride crystallises out as as solid salt. This can be separated, redissolved in water and recycled back through the cell again

Question 12

What is the full equation of the electrolysis of brine?

Answer 12

2H⁺ _(aq) + 2Cl^- → Cl_{2 (g)} + H_{2 (g)}

Question 13

What are three uses of chlorine?

Answer 13

sterilising water (chlorination) to make it safe to drink

making hydrochloric acid (by controlled reaction with hydrogen)

making bleach

Question 14

What are uses of sodium hydroxide?

Answer 14

the purification of bauxite to make aluminium oxide (alumina) as a part of the manufacture of aluminium

making paper - the sodium hydroxide helps break the wood down into pulp

making soap - sodium hydroxide reacts with animal and vegetabl fats and oils to make compounds, such as sodium stearate, that are present in soap

making bleach - bleach is formed when sodium hydroxide and chlorine react together in the cold; it is a mixture of sodium chlordie and sodium chlorate (I) solution (NaClO)

Question 15

Sodium hydroxide is a very strong … and is used widely in the …

Answer 15

Sodium hydroxide is a very strong base and is used widely in the chemical industry