Batteries and Electrolysis

25min Part 1 / Ch2 / Lesson 7

Prerequisites: 1-2-6

Objectives

Explain how galvanic cells (Daniell cell) work
Understand the principles of electrolysis
Perform calculations using Faraday's law

How Batteries Work

Battery (galvanic cell): converts chemical energy from redox reactions into electrical energy.

Anode (−): oxidation occurs (electrons released)
Cathode (+): reduction occurs (electrons accepted)
Electrons flow through the external circuit from anode → cathode

The Daniell Cell

Structure of a Daniell cell

Electrode	Reaction
Anode (Zn)	$\mathrm{Zn \rightarrow Zn^{2+} + 2e^-}$ (oxidation)
Cathode (Cu)	$\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$ (reduction)

Activity series: $\mathrm{Zn > Cu}$ , so Zn dissolves while Cu deposits.

Electrolysis

Electrolysis: using external electrical energy to drive a non-spontaneous redox reaction. The reverse of a battery.

Cathode (−): reduction (cations gain electrons)
Anode (+): oxidation (anions lose electrons)

	Battery	Electrolysis
Energy conversion	Chemical → Electrical	Electrical → Chemical
Reaction	Spontaneous	Non-spontaneous (forced)
Anode/Cathode (−)	Oxidation	Reduction
Cathode/Anode (+)	Reduction	Oxidation

Battery vs. electrolysis

Faraday’s Law

Faraday’s law: the amount of substance produced in electrolysis is proportional to the charge passed.

$Q = It$

$Q$ : charge (C), $I$ : current (A), $t$ : time (s)

Faraday constant: $F = 9.65 \times 10^4\ \mathrm{C/mol}$ (charge of 1 mol of electrons)

Worked Example

Example: Electrolysis calculation

A copper(II) sulfate solution is electrolyzed at $2.0\ \mathrm{A}$ for $965\ \mathrm{s}$ . How many grams of copper deposit at the cathode? ( $\mathrm{Cu} = 64$ , $F = 9.65 \times 10^4\ \mathrm{C/mol}$ )

$Q = It = 2.0 \times 965 = 1930\ \mathrm{C}$

$n_{e^-} = \frac{Q}{F} = \frac{1930}{9.65 \times 10^4} = 0.020\ \mathrm{mol}$

$\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$ , so Cu : e⁻ = 1 : 2

$n_{\mathrm{Cu}} = \frac{0.020}{2} = 0.010\ \mathrm{mol}$

$m = 0.010 \times 64 = 0.64\ \mathrm{g}$

Check Your Understanding

Q1 In a battery, electrons flow through the external circuit from:

Q2 In a Daniell cell, which metal is the anode?

Q3 What reaction occurs at the cathode during electrolysis?

Exercises

Q1. When an aqueous sodium chloride solution is electrolyzed, what is produced at each electrode? Write the half-reactions.

Solution

Anode (oxidation): Chlorine gas $\mathrm{Cl_2}$ $2\mathrm{Cl^- \rightarrow Cl_2 + 2e^-}$

Cathode (reduction): Hydrogen gas $\mathrm{H_2}$ $2\mathrm{H_2O + 2e^- \rightarrow H_2 + 2OH^-}$

Na⁺ is too high in the activity series to be deposited, so water is reduced instead.