Electrochemistry and Electroplating: Study Notes

Electrochemistry and Electroplating

Introduction to Electroplating

Electroplating is the process of coating one metal onto another using electrolysis. This technique is commonly used for decorative purposes, corrosion protection, and improving surface properties. For example, gold plating is used in jewelry manufacturing.

• Electrolysis: A chemical process that uses electrical energy to drive a non-spontaneous reaction.

• Application: Gold plating, silver plating, nickel plating, etc.

Electrochemical Cells

Electrochemical cells convert chemical energy into electrical energy or vice versa. They consist of two half-cells connected by a salt bridge, allowing ions to flow and maintain charge balance.

• Voltaic (Galvanic) Cell: Generates electricity from spontaneous redox reactions.

• Electrolytic Cell: Uses electrical energy to drive non-spontaneous reactions (e.g., electroplating).

Example: Voltaic Cell (Sn/Al)

• Cathode Reaction: $Sn^{4+} + 2e^- \rightarrow Sn^{2+}$

• Anode Reaction: $Al_{(s)} \rightarrow Al^{3+} + 3e^-$

• Cell Potential ($E_{cell}$): $1.52\,V$

• Spontaneity: The reaction is spontaneous if $E_{cell} > 0$.

• Electrodes: The cathode is where reduction occurs; the anode is where oxidation occurs.

• Effect of Electrode Size: Doubling the size of electrodes does not double the $E_{cell}$ value. (False)

• Electron Flow: Electrons flow from anode to cathode; the tin electrode gains mass.

• Electrode Polarity: In a voltaic cell, the tin electrode does not become reduced. (False)

Example: Electrolytic Cell (KBr/H2O)

• Cell Potential ($E_{cell}$): $-1.90\,V$ (non-spontaneous)

• Cathode Reaction: $2H_2O_{(l)} + 2e^- \rightarrow H_2_{(g)} + 2OH^-_{(aq)}$

• Anode Reaction: $2Br^-_{(aq)} \rightarrow Br_2_{(g)} + 2e^-$

Faraday's Laws of Electrolysis

Faraday's laws relate the amount of substance deposited or dissolved at an electrode to the quantity of electricity passed through the electrolyte.

• First Law: The mass ($m$) of a substance altered at an electrode is proportional to the quantity of electricity ($Q$) passed.

• Formula: $n_e = \frac{It}{F}$, where $n_e$ is the number of moles of electrons, $I$ is current (A), $t$ is time (s), and $F$ is Faraday's constant ($9.65 \times 10^4\,C/mol$).

• Mass Calculation: $m = n \times M$, where $n$ is moles and $M$ is molar mass.

Example: Silver Deposition

• Reaction: $Ag^+_{(aq)} + e^- \rightarrow Ag_{(s)}$

• Given: $I = 15.6\,A$, $t = 10,008\,s$

• Calculation:

  • $n_e = \frac{It}{F} = \frac{(15.6)(10,008)}{9.65 \times 10^4} = 1.62\,mol$

  • $m = n \times M = 1.62\,mol \times 26.98\,g/mol = 14.6\,g$ Additional info: The molar mass for Ag is 107.87 g/mol; the calculation uses Al as an example, but the process is the same.

Example: Nickel Plating

• Reaction: $Ni^{2+}_{(aq)} + 2e^- \rightarrow Ni_{(s)}$

• Given: $m = 2.21\,g$, $M = 58.69\,g/mol$, $I = 3.20\,A$

• Calculation:

  • $n = \frac{2.21}{58.69} = 0.0376\,mol$

  • $n_e = 2 \times 0.0376 = 0.0753\,mol$

  • $t = \frac{n_e \times F}{I} = \frac{0.0753 \times 9.65 \times 10^4}{3.20} = 2270\,s = 37.8\,min$

Summary Table: Key Electrochemical Concepts

Concept	Definition	Example
Electroplating	Coating one metal onto another using electrolysis	Gold plating for jewelry
Voltaic Cell	Spontaneous redox reaction generates electricity	Zn/Cu cell
Electrolytic Cell	Non-spontaneous reaction driven by electricity	Electroplating silver onto copper
Faraday's Law	Relates mass deposited to charge passed	Calculating mass of Ag deposited

Key Equations

• $n_e = \frac{It}{F}$

• $m = n \times M$

• $t = \frac{n_e \times F}{I}$

• $E_{cell} = E_{cathode} - E_{anode}$

Additional info:

• Electroplating is widely used in industry for both functional and decorative purposes.

• Faraday's constant ($F$) is approximately $9.65 \times 10^4\,C/mol$.

• Electrochemical cell notation: Anode | Electrolyte || Electrolyte | Cathode.