Electrolysis

Electrolysis is the process of decomposing the ionic compound into its constituent elements by passing the electric current into the solution of the ionic compound. The concept of electrolysis was first given by the famous scientist of the 19th century Michael Faraday. It is a chemical process that uses electrical energy to bring changes in the chemical reaction. Electrolysis is used to separate components of the ionic compounds.

In this article, we will learn about, electrolysis, its process, faraday’s law of electrolysis and others in detail in this article.

Electrolysis Definition

The process of decomposing the ionic compound into its constituent elements using electric current is called Electrolysis. In electrolysis, the ionic compound is dissolved into the aqueous medium and then electricity is passed through the solution using the electrodes then the ionic compound breaks into its constituent elements and cations are collected at the cathode and anions are collected at the anode.

The cell which converts electrical energy into chemical energy is called an electrolytic cell. 

This redox reaction occurs at electrodes specifically mentioning oxidation occurs at the anode and is a positive plate while reduction occurs at the cathode and is a negative plate. In electrolytic cells, electrical energy is used to perform non-spontaneous chemical reactions and the process that takes place in an electrolytic cell is called Electrolysis.

Electrolytic Process

In the Electrolysis Process, the exchange of ions takes place by the electric current passing through the circuit. By allowing current to pass through the solution we force cations to get attached to the cathode of the electrolytic cell and anions to get attached to the anode of the electrolytic cell. 

We can understand the electrolytic process with the help of NaCl example, 

Electrolysis of NaCl Solution

The aqueous solution of NaCl has Na⁺, Cl^–, H⁺, and OH^– ions present in it. Now when the electrodes are introduced to this solution and the electricity is passed through it that allows Na⁺, and H⁺, ions to move to the negatively charged electrode, (cathode) and the Cl^–, and OH^–  ions move to the positively charged electrode, (anode). 

NaCl ⇌ Na⁺ + Cl^–

H₂O ⇌ H⁺ + OH^–

At Cathode: 

• Na⁺ + e^– → Na

At Anode: 

• 2Cl^– →  Cl₂ + e^–

Now as a result of this electrolysis solution the Sodium metal is deposited on the cathode of the electrolytic cell and the chlorine is released at the anode.

Cell Potential

The least potential required by the electrolytic cell to complete the process of electrolysis depends on the mobility of the ions in the aqueous solution. If the ions have very low mobility then the cell potential of the electrolytic cell must be high to facilitate the movement of the ions in the aqueous solution. 

Whereas if the mobility of the ions is very high in their aqueous solution then an electrolytic cell with low electric potential is used as it can easily facilitate the movements of ions in the electrolysis process.

The cell potential of an electrolytic cell is the sum of the standard oxidation potential and the standard reduction potential of the cell.

Faraday Laws of Electrolysis

The Faraday laws of electrolysis are the basic laws of electrolysis that provide information about the mass of the substance and the charge in the electrolysis process. There are two Faraday Laws of Electrolysis that are,

• Faraday’s First Law of Electrolysis
• Faraday’s Second Law of Electrolysis

Now let’s learn about the Faraday Laws of Electrolysis in detail.

Faraday’s First Law of Electrolysis

Faraday’s First Law of Electrolysis states that “the mass of the substance that undergoes electric current is directly proportional to the charge supplied.”

We know that,

i = Q/t, then Faraday’s First Law of Electrolysis is represented as,

m ∝ Q 

m = ZQ 

m = Zit

where,

• m is the mass of the substance that undergoes electrolytic process
• Q is the charge associated with the electrolytic cell
• i is the current measured in Ampere (A)
• t is the time measured in sec
• Z is the constant of proportionality called Electrochemical Equivalent.

Faraday’s Second Law of Electrolysis

According to the second law of electrolysis, the amount of electrolyte deposited at the electrodes is the directly proportional equivalent weight of the material, if the amount of electricity passing through the solution is constant.

W₁/E₁ = W₂/E₂ = W₃/ E₃ …….

where

• W₁ is the mass of first substance, W₂ is the mass of the second substance and so on…
• E₁ is the Equivalent Weight of the first substance, E₂ is the Equivalent Weight of the second substance and so on…

Product of Electrolysis

The oxidising and the reducing species present in the electrolytic cell are used to find the product of the electrolysis. If in an aqueous solution, we have more than one cation or anion then each ion will not be reduced or oxidised. The reaction with more redox potential will be reduced or oxidised in comparison to others.

For example, the electrolysis of aqueous sodium chloride gives different products such as,

• Hydrogen and Chlorine
• Hydrogen and Oxygen
• Hydrogen, Oxygen and Chlorine

Electrolysis of Sodium Chloride (NaCl) solution also called Brine is discussed below,

• At Anode: Chlorine Gas (Cl₂) is produced
• At Cathode: Hydrogen Gas (H₂) is produced

In the solution, Sodium Hydroxide (NaOH) is formed.

Factors Affecting Electrolysis

Various factors affecting electrolysis are discussed below,

• Nature of the Electrolyte
• Nature of the Electrode
• Voltage at the Electrodes

Now let’s learn about them in brief.

Nature of the Electrolyte

In the electrolysis process, the charged particles called ions dissolved in the aqueous solution move under the influence of the electric potential, and so the nature of the electrolyte plays important in the electrolysis process. If we take an electrolyte that dissolves easily in the aqueous solution then it facilitates the process of electrolysis.

Nature of the Electrode

Nature of the electrode plays an important role in the electrolysis process taking different electrodes in the same electrolyte solution gives different outputs.

The aqueous solution of copper sulphate solution on electrolysis gives the following results,

At cathode: (Reduction)

• Cu²⁺ (aq) + 2e^– →Cu (s)    (E° = 0.34V)
• 2H₂O + 2e^–→H₂ + 2OH^–    (E° = -1.02V

At anode: (Oxidation)

• Cu(s) →Cu²⁺ (aq) + 2e^–         (E° = – 0.34V)
• 2H₂O → O₂(g) + 4H⁺ + 4e^–    (E° = +1.4 V)

Here, we see that if copper is taken at the cathode we get a different reaction and if copper is taken at the anode we get a different reaction.

Voltage at Electrodes

The redox potential of the electrolyte plays an important role in the electrolysis process. If the redox potential of the electrolysis reaction is more than the thermodynamic potential of the electrolysis reaction the reaction becomes unfavourable and the product of electrolysis changes.

If we perform the hydrolysis of aqueous sodium chloride, at the anode there are two oxidation reactions possible. The reduction potential of water and chloride is +0.82V and -1.36V, respectively in both these reactions and they are represented as,

• 2H₂O→O₂(g) + 4H⁺ + 4e^–    (E° = -0.82 V)
• 2Cl^– → Cl₂ + 2e^–                 (E = -1.36V)

Here we observe that the oxidation of water is more positive and hence is more feasible, so oxygen is evolved at the anode. But the evolution of the oxygen resulted in an overvoltage of -0.6 V making the voltage of oxidation of water to be -1.42

Applications of Electrolysis

Electrolysis is one of the most important processes used in electrochemistry. It is used for various purposes and some of its applications are,

• Caustic soda is prefabricated by electrolysis of sodium chloride solution
• Manufacture of O₂ and H₂ by Electrolysis of Water
• Determination of Equivalent Weight of Substances
• Purification of Metals
• Electroplating for Corrosion Resistance, Ornaments etc.

Learn more about Refining of Metals

What is Electrolytic Cell?

Electrolysis is the procedure of decomposition of an electrolyte by the passage of electricity throughout its aqueous solution or the molten state of an electric current. This cell is utilized to perform electrolysis which is electrolyte cells.

Water, for example, can be electrolyzed (with the help of an electrolytic cell) to produce gaseous oxygen and hydrogen. This is accomplished by utilising the flow of electrons (into the reaction environment) to overcome the non-spontaneous redox reaction’s activation energy barrier.

The following are the three major components of electrolytic cells:

• Cathode: It is negatively charged for electrolytic cells
• Anode: It is a type of electrode that is (which is positively charged for electrolytic cells)
• Electrolyte: The electrolyte serves as a conduit for electrons to flow between the cathode and the anode. Water (containing dissolved ions) and molten sodium chloride are common electrolytes in electrolytic cells.

Read More,

• Electrochemical Cells
• Conductance of Electrolytic Solutions
• Galvanic Cells

Solved Examples on Electrolysis

Example 1: How many coulombs are needed for 40.5 g of aluminium to react when the electrode is:

Al³⁺ + 3e^– ⇒ Al

Solution:

1 mol of Al requires 3 mol of electrons or 3 × 96500 C

1 mol of Al = 27g

27g of Al require =3 × 96500 C

40.5g of Al require =(3*96500C × 40.5)/27 = 434,250 C

Example 2: In the electrolysis of acidic water, it is desired to obtain hydrogen at 1cc sec at the STP position. What should be the current pass?

Solution:

2H⁺ + 2e^– ⇒ H₂

1 mol of H₂ or 22400 cc of H₂ at STP requires = 2 × 96500 C

1cc of H₂ at NTP requires = (2×96500)/22400 = 8.616 C

Now, Q = I × t

I = Q/t 

I = 8.616/1

I = 8.616 ampere

Example 3: How many moles of mercury will be produced by galvanic isolation 1.0 M Hg(NO₃)₂ solutions with a current of 2.00 A for 3 hours?

Solution:

Hg²⁺ + 2e^– ⇒ Hg

Charged Passed(Q) = I × t(sec)

Q  = 2.0 A × 3.0 × 60 × 60 = 21600 C

2 × 96500 C of Charge produce = 1 mol of Mercury

21600 C of Charge will produce = 1× 21600 / (2×96500) = 0.112mol of Mercury

Example 4: A solution of CuSO₄ is electrolyzed for 10 minutes with a current of 1.5 amperes. What is the mass of copper deposited at the cathode?

Solution:

Current (I) = 1.5 A

Time (t) = 10 min = 10 × 60 = 600s 

amount of electricity passed = I × t = (1.5 A) × (600 s) = 900 C (A s = C)

Copper is Deposited: Cu⁺² + 2e^– → Cu(s)

2 mol of Electrons or 2 × 96500 C of current deposit copper = 63.56 g

900 C of current will deposit copper = 63.56/(2 ×96500) = 0.296 g

Example 5: Calculate how long it will take to deposit 1.0 g of chromium when a current of 1.25 A flows through a solution of chromium (III) sulphate. (Molar mass of Cr=62).

Solution:

Cr³⁺+3e^– → Cr(s) 

3 mol of electricity are needed to deposit 1 mol of Cr,

52 g of Cr require current = 3 × 96500 C

1g of Cr will require current = (3 × 96500)/52  = 5567.3 C

Number of Coulombs = Current × t

Time (s) required = No. of Coulombs / Current 

Time (s) required = 5567.3 C / 1.25 (Ampere)

                            = 4453.8s or 1.24hr

Example 6: How many hours does it take to reduce 3 mol of Fe with 2.0 A current? (F= 96500 C) Solution: Reduction of Fe³⁺ to Fe²⁺.

Solution:

Fe³⁺ + e^– → Fe²⁺

Reduction of 1 mol of Fe³⁺ requires = 96500 C

Reduction of 3 mol of Fe³⁺ require = 3 × 96500 C = 2.895 × 10°C

Quantity of electricity = Current × Time

2.895 × 10 = 2 × Time

Time = 2.895 × 10⁵ 

= 14475 × 10⁵ s

Time = (14475 × 10⁵ )/(60 × 60) 

        = 40.21 hours

FAQs on Electrolysis

Q1: What is Electrolysis?

Answer:

The chemical process that happens when the electric current is passed through an ionic solution is called the electrolysis. In this process, ions move in their aqueous solution forming the compounds on the cathode and anode.

Q2: What is an Electrolytic Cell?

Answer:

A chemical cell with two electrodes a positive electrode called an anode and the negative called the cathode is dipped in the aqueous solution of an ion and electricity is passed through it, which is called an electrolytic cell.

Q3: Where is the Process of Electrolysis used?

Answer:

Electrolysis process is a highly used process in electrochemistry. It is used in the purification of molten metals, breaking an ionic compound into its constituent elements,  electrolysis of water, etc.

Q4: What is Cathode?

Answer:

The electrode of the electrolytic cell which is negatively charged is called the cathode. It attracts the positive cation and the reduction takes place at the cathode.

Q5: What is Anode?

Answer:

The electrode of the electrolytic cell which is positively charged is called the anode. It attracts the positive anion and the oxidation takes place at the anode.