Chapter 17 - Electrochemistry

What is electrochemistry?

the study of the movement of electrons (e⁻) in chemical reactions

Electric charge =

Charge on an electron

Measure for electric charge =

Coulomb (C), 1 e⁻ = 1.602 × 10⁻¹⁹ C

Electric current =

movement of charge

Measure for Electric current =

Ampere (A) = 1 C/s

Electric potential =

force pushing charge

Measure for electric potential =

Volt (V) = 1 Joule/Coulomb

Electric field =

Force acting on nearby charges

Electrochemical reactions involve electron transfer, which means they are:

redox reactions

For oxidation number, the elemental form is:

zero

The oxidation number of a monatomic ion =

charge of the monatomic ion

ex. Oxidation number of S–2 = –2

The oxidation number of all Group 1A metals =

+1

The oxidation number of all Group 2A metals =

+2

What is the oxidation number of hydrogen if bonded to a non-metal?

+1

What is the oxidation number of hydrogen if bonded to a metal?

-1

What is the oxidation number of oxygen in peroxides (O2 2-)?

-1

What is the oxidation number of oxygen in all other compounds?

-2

The oxidation number of fluorine (F) is always:

-1

The sum of the oxidation numbers of all atoms (or ions) in a neutral compound =

0

The sum of the oxidation numbers of all atoms in a polyatomic ion =

charge on the polyatomic ion

What is the saying for redox reactions?

LEO the lion goes GER

What does LEO stand for?

loss of e- is oxidation

What does GER stand for?

gain of e- is reduction

The reducing agent is:

the substance that gets oxidized

The oxidizing agent is:

the substance that gets reduced

What method is used to balance redox reaxtions?

the half-reaction method

Steps to the half reaction method:

• break into half reactions; mass balance all but H & O

• balance O by adding H2O

• balance H by adding H+

• equalize e- and add half reactions

• if basic: turn H+ into H2O by adding OH- to both sides

Galvanic cells are also called:

voltaic cells

Galvanic cells produce:

electricity by spontaneous redox reactions

What direction do the electrons move?

they leave anode and go to cathode

What is a battery?

it’s strictly a series of linked galvanic cells

What is the saying for the galvanic cells?

AnOx the RedCat

What does AnOx mean?

Anode is Oxidation

What does RedCat mean?

Reduction at Cathode

Charge of Anode =

negative

Charge of Cathode =

positive

What side of the half-equation has the electrons in for oxidation?

right side

What side of the half-equation has the electrons in for reduction?

left side

Why is a salt bridge needed?

• contains a salt paste

• allows ions to pass into cells

• stops charge buildup

More reactive metals are:

more likely to oxidize and act an an anode

What drives electric current in galvanic cells?

the difference in electrical potential energy between the electrodes

A galvanic cell can be described in compact notation called:

a cell diagram

When is an inert electrode used?

when a reactant is a poor conductor (nonmetal, liquid, gas…)

What is used for inert electrode?

platinum wire — this allows electrons from the left beaker to move into the right beaker

Which electrode gains and which electrode loses mass?

anode loses mass and cathode gains mass (gains electrons)

An electrode that gains mass has to:

form a solid

An electrode that loses mass must:

go to solution

Standard voltage (Eocell) occurs whenever:

• All [solute] = 1 M (or saturated if solubility < 1 M)

  All gases : P = 1 bar

  All solids : pure

What is SHE?

Standard Hydrogen Electrode

What is the standard electrode potential of SHE in standard conditions?

exactly 0.00 V

What does a positive cell potential tell you about a galvanic cell?

The galvanic cell reaction is spontaneous.

For a problem, if you are given the electrode but don’t know which is which, what do you do?

choose which is at the anode and which is at the cathode so that you get a positive cell potential

Ecell predicts if:

a reaction will occur between chemicals mixed in a single container

A reaction occurs if:

Ecell is positive

What happens if the Ecell is negative?

the reverse reaction is predicted

On the half reaction chart, what is the trend for power of oxidizing agent?

the power increases from bottom to top of the chart

On the half reaction chart, what is the trend for power of reducing agent?

the power increases from top to bottom of the chart

Given a chart of potential values, which is the strongest oxidizing agent?

most positive standard reduction potential

Given a chart of potential values, which is the weakest oxidizing agent?

most negative standard reduction potential

Given a chart of potential values, which is the strongest reducing agent?

most negative standard reduction potential

Given a chart of potential values, which is the weakest reducing agent?

most positive standard reduction potential

When is the Nernst equation used?

if the temperature is 25°C or 298K

E° values apply ONLY if:

[solute] = 1 M (or saturated)

Electrolytic cells:

drive non–spontaneous redox reactions using supplied electric current through a process called electrolysis

In an electrolytic cell, what are the charges of the anode and the cathode?

anode = +

cathode = -

Electrolytic Cell: Spontaneous =

(once ignited) – produces electrical current

Electrolytic Cell: Nonspontaneous =

consumes electrical

current

Example of electrolysis:

the splitting of water to produce H2

• Power source provides e– at cathode

• Water absorbs e–; splits into H2 and OH–

• Anode absorbs e– from water; water splits into O2 and H+

What are the two types of batteries?

primary and secondary

Primary batteries:

• single use

• dry cell; no fluid component

Secondary batteries:

rechargeable

• lead acid battery

• nickel-cadium (NiCd) battery

• lithium ion battery

Lead acid battery:

• high capacity, high current

• rechargeable — reaction can be reversed

• toxic

• designed to replace zinc-carbon battery

• Ecell = 2.04V

Nickel-Cadmium Battery:

• NiCd battery

• Rechargeable (~1000x)

• Toxic Cd metal

• Ecell = 1.2 – 1.25 V

Lithium Ion Battery:

• Can provide a large amount of current and are lighter than comparable batteries

• Rechargeable (~1000x)

• Toxic Cd metal

• Ecell = 3.7 V

Corrosion:

deterioration of a metal by an electrochemical process (ex. iron rusts)