Electrolysis of Sodium Chloride

Two commonly used methods of electrolysis is that of molten sodium chloride and aqueous sodium chloride which give different products.

Key Points

• Electrolysis of molten sodium chloride yields sodium metal and chloride gas.

• Electrolysis of aqueous sodium chloride yields hydrogen and chlorine (with aqueous sodium hydroxide remaining in solution).

• The reason for the difference is that the reduction of Na+ (E° = –2.7 v) is energetically more difficult than the reduction of water (–1.23 v).

Terms

• The electrode of an electrochemical cell at which reduction occurs.

• The electrode of an electrochemical cell at which oxidation occurs.

Examples

• How much electric power is required to produce 1 metric ton (1000 kg) of chlorine from brine, assuming the cells operate at 2.0 volts and assuming 100% efficiency? Solution: moles of Cl2 produced: (106 g) ÷ 70 g mol–1 = 14300 mo; Cl2 faradays of charge: (2 F/mol) × (14300 mol) = 28600 F; charge in coulombs: (96500 c/F) × (28600 F) = 2.76 × 109 c; duration of electrolysis: (3600 s/h) x (24 h) = 86400 s; current (rate of charge delivery): (2.76 × 109 amp-sec) ÷ (86400 sec) = 32300; amps power (volt-amps): (2.0 v) × (32300 a) = 64.6 kw;  energy in kW-h: (64.6 kw) × (24 h) = 1550 kw-h; energy in joules: (1550 kw-h) × (3.6Mj/kw-h) = 5580 Mj (megajoules)  (In the last step, recall that 1 w = 1 j/s, so 1 kw-h = 3.6 Mj).

Figures

1. Electrolysis Cell for Molten Sodium Chloride

A commercial electrolysis cell for the production of metallic sodium and chlorine gas from molten NaCl. Liquid sodium floats to the top of the melt above the cathode and is drained off into a storage tank. Chlorine gas bubbles out of the melt above the anode.

2. Electrolysis of Aqueous Sodium Chloride

Electrolysis of aqueous NaCl results in hydrogen and chloride gas.

3. Deduce the products of the electrolysis of a molten salt

Electrolysis of a molten salt produces the elements from the salt. So the electrolysis of WCl4 produces W and Cl2. Metals ions receive electrons at the negative electrode electrode, the non-metals lose them at the positive electrode.

As we have covered, electrolysis is the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of materials. Two commonly used methods of electrolysis is that of molten sodium chloride and aqueous sodium chloride. You might think that both methods would give you the same products, but, in fact, this not the case. Let's go through each of the methods to understand the different processes.

If sodium chloride is melted (above 801°C) and two electrodes are inserted into the melt as shown in Figure 1 and an electric current is passed through the molten salt, then chemical reactions take place at the electrodes.

Sodium ions migrate to the cathode, where electrons enter the melt and are reduced to sodium metal:

${Na}^{+} + {e}^{-} \rightarrow Na$

Chloride ions migrate the other way, toward the anode, give up their electrons to the anode, and are oxidized to chlorine gas:

${Cl}^{-} \rightarrow \frac{1}{2}{Cl}_{2} + {e}^{-}$

The overall reaction is the breakdown of sodium chloride into its elements:

$2NaCl \rightarrow Na(s) + {Cl}_{2}(g)$

Now what happens when we have an aqueous solution of sodium chloride as shown here Figure 2? Well, we can't forget that we now have water to factor into the equation. Since water can be oxidized and reduced it competes with the solute. Rather than producing sodium, hydrogen is produced. The reaction at the cathode is:

${H}_{2}O (l) + 2 {e}^{–} \rightarrow {H}_{2}(g) + 2{ OH}^{–}$

and at the anode:

${Cl}^{–} \rightarrow \frac{1}{2} {Cl}_{2}(g)$

The overall reaction is as follows:

$NaCl(aq) + {H}_{2}O(l) \rightarrow {Na}^{+}(aq) + {OH}^{-}(aq) + {H}_{2}(g) + \frac{1}{2}{Cl}_{2}(g)$

Reduction of Na+ (E° = –2.7 v) is energetically more difficult than the reduction of water (–1.23 v), so in aqueous solution the latter will prevail. Electrolysis of salt ("brine") is carried out on a huge scale and is the basis of the chloralkali industry.

Figure 3

Key Term Glossary

anode
The electrode of an electrochemical cell at which oxidation occurs.
Appears in these related concepts:
aqueous
Consisting mostly of water.
Appears in these related concepts:
cathode
The electrode of an electrochemical cell at which reduction occurs.
Appears in these related concepts:
charge
An electric charge.
Appears in these related concepts:
chemical reaction
A process involving the breaking or making of interatomic bonds, in which one or more substances are changed into others.
Appears in these related concepts:
coulomb
In the International System of Units, the derived unit of electric charge; the amount of electric charge carried by a current of 1 ampere flowing for 1 second. Symbol: C
Appears in these related concepts:
coulombs
In the International System of Units, the derived unit of electric charge; the amount of electric charge carried by a current of 1 ampere flowing for 1 second. Symbol: C
Appears in these related concepts:
electrode
the terminal through which electric current passes between metallic and nonmetallic parts of an electric circuit. In electrolysis, the electrodes (cathode and anode) are placed in the solution separately.
Appears in these related concepts:
electrolysis
the chemical change produced by passing an electric current through a conducting solution or a molten salt
Appears in these related concepts:
electron
The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.
Appears in these related concepts:
element
Any one of the simplest chemical substances that cannot be decomposed in a chemical reaction or by any chemical means, and are made up of atoms all having the same number of protons.
Appears in these related concepts:
energy
a quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent
Appears in these related concepts:
The quantity of electricity required to deposit or liberate 1 gram equivalent weight of a substance during electrolysis: approximately 96,487 coulombs.
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gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
Appears in these related concepts:
ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
Appears in these related concepts:
ionic
of, relating to, or containing ions
Appears in these related concepts:
joule
in the International System of Units, the derived unit of energy, work and heat; the work required to exert a force of one newton for a distance of one metre; also equal to the energy of one watt of power for a duration of one second; symbol: J
Appears in these related concepts:
metal
Any of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity
Appears in these related concepts:
metric
a measurement of the "distance" between two points in some metric space: it is a real-valued function d(x,y) between points x and y satisfying the following properties: (1) "positive definiteness": d(x,y) \ge 0 and d(x,y) = 0 \mbox{ iff } x=y , (2) "symmetry": d(x,y) = d(y,x) , and (3) "triangle inequality": d(x,y) \le d(x,z) + d(z,y)
Appears in these related concepts:
mole
In the International System of Units, the base unit of the amount of substance; the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. Symbol: mol. The number of atoms in a mole is known as Avogadro’s number.
Appears in these related concepts:
product
a chemical substance formed as a result of a chemical reaction
Appears in these related concepts:
reduction
a reaction in which electrons are gained and valence is reduced, often by the removal of oxygen or the addition of hydrogen
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salt
An ionic compound that is composed of cations and anions The product is electrically neutral. The two ions are held together by ionic bonds, not covalent bonds.
Appears in these related concepts:
solute
Any substance that is dissolved in a liquid solvent to create a solution.
Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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solvent
a substance that dissolves a solute, resulting in a solution
Appears in these related concepts:
substance
Physical matter; material.