Ch. 21 - Electrochemistry

Slide 20

How do we use spontaneous reactions to generate electrical energy?

Voltaic/Galvanic cells

Which electrode goes on the right?

Cathode (ex. Cu)

Which electrode goes on the left?

Anode (ex. Zn)

Reduction occurs at the ___.

cathode (red cat)

Oxidation occurs at the ___.

anode (an ox)

Reduction goes on the __.

right.

Oxidation goes on the __.

left.

Voltaic or Galvanic cell

Uses a spontaneous reaction to produce electricity; does work on the surroundings

Electrolytic cells

Uses electrical energy to drive a nonspontaneous reaction; the surroundings do work on the system

Why and how long does a voltaic cell work?

Why? Because the reaction is spontaneous

How long? Until the reaction is no longer spontaneous (G = 0)

A positive cell potential means delta G is…

negative → spontaneous

At equilibrium, Ecell (cell potential) is…

0

You must only put ___ potentials into the Eknot cell formula.

reduction → negative sign flips it to oxidation potential

Oxidizing agents are all on the ____ of the reaction arrow.

Left

If the reduction potential is negative, does it want to be reduced?

No!

SHE

Standard Hydrogen Electrode → Set arbitrary standard = 0.00V

Reactions with strong reducing agents and strong oxidizing agents can result in…

explosive reactions due to their difference in electrochemical potential.

If E° is bigger, is the reduction easier or harder?

Easier

Are these reducing agents?

No, they have already been oxidized! Instead, Zn, H2, Cu

The general rule of relative strengths of oxidizing and reducing agents

• Strength of reducing agent increases as E°half‑rxn becomes smaller or more negative.

• Strength of oxidizing agent increases as E°half‑rxn becomes bigger (more positive).

What makes precious metals “precious”?

They are not easily oxidized (bad reducing agents) → They avoid tarnishing.

Which is a better oxidant? Cl2 or I2

Cl2 because it has higher reduction potential.

When E° = -1.10, is the reaction spontaneous?

No → Using the equation, a negative cell potential gives a positive Gibbs free energy.

Any metal below the standard hydrogen electrode, will get ____.

oxidized by the hydrogen → will make bubbles, metal will corrode

What three quantities are always thermodynamically related?

Gibbs free energy, K, cell potential

The anode is ____ in an electrolytic cell because….

positive; the external power supply is forcibly removing electrons from the anode

Electrolysis

The splitting of a substance by the input of electrical energy; often used to decompose a compound into its elements

For pure molten salts, the cation is ___, and the anion is ___.

reduced; oxidized

What periodic trends do we use to decide which elements will be oxidized or reduced in electrolysis?

Ionization energy and electronegativity

Ampere (A) times volt (V) =

Watts (W)

Faraday’s Law of Electrolysis

The amount of substance produced at each electrode is directly proportional to the amount of charge (electrons) flowing through the cell.

Current =

amount of charge moving per unit time → Usually measured in amps

Potential for the electrolysis of water

-1.23 V

The most ___ reduction potential still gets reduced in electrolysis.

positive

Why are there kinetic barriers for the electrolysis of water?

It requires an overpotential.

“Discharge” is used when cell is __.

voltaic

“Recharge” is used when cell is ___.

electrolytic

When electrochemical cells are connected in series…

their voltages sum.

Rechargeable battery =

Secondary battery; ex. Lead acid/lead storage battery; nickel-metal hydride battery

A more stable form of Li is…

LiC6

Fuel cell

Generating electricity through controlled combustion (oxidation) of the fuel

Iron is both ____ and, later, ____ to form ___.

an anode; a cathode; to form rust

2 steps of iron corrosion

1. Loss of iron

2. Formation of rust

Sacrificial anodes

Get oxidized in place of the metal you do not want to corrode