Ch. 20: Electrochemistry

Question 1

oxidation-reduction (redox) reactions

Answer 1

The reactions that create the flow of electric charge within a battery.

Question 2

oxidation

Answer 2

Loss of electrons

Corresponds to an increase in oxidation state.

Question 3

reduction

Answer 3

Gain of electrons

Corresponds to a decrease in oxidation state.

Question 4

Rules for assigning oxidation states

Answer 4

(These rules are hierarchical. If any two rules conflict, follow the rule that is higher on the list)

1. The oxidation state of atom in a free element is 0.
2. The oxidation state of a monoatomic ion is equal to its charge.
3. The sum of the oxidation states of all atoms in:
   
   • a neutral molecule or formula unit is 0.
   • an ion is equal to the charge of the ion.
4. In their compounds, metals have positive oxidation states.
   
   • Group 1A metals always have an oxidation state of +1.
   • Group 2A metals always have an oxidation state of +2.
5. In their compounds, we assign nonmetals oxidation states according to the table below. Entries at the top of the table take precedence over entries at the bottom of the table.
   
   • Flourine: -1
   • Hydrogen: +1
   • Oxygen: -2
   • Group 7A: -1
   • Group 6A: -2
   • Group 5A: -3

Question 5

half-reaction method of balancing

Answer 5

A special procedure where we can balance redox reactions occurring in aqueous solutions. In this procedure, we break down the overall eqution into two half-reactions: one for oxidation and one for reduction. We then balance the half-reactions individually and add them together.

The steps differ slightly for reactions occurring in acidic and in basic solution. Basic solution, you add OH^- twice in order to combine the H⁺ and OH^- to form H₂O.

Question 6

electrical current

Answer 6

The flow of electric charge.

Question 7

electrochemical cell

Answer 7

A device in which a chemical reaction either produces or is carried out by an electrical current.

Question 8

voltaic (or galvanic) cell

Answer 8

An electrochemical cell that produces electrical current from a spontaneous chemcial reaction.

Question 9

electrolytic cell

Answer 9

A second type of electrochemical cell that consumes electrical current to drive a nonspontaneous chemical reaction.

Question 10

half-cell

Answer 10

One half of an electrochemical cell in which either oxidation or reduction occurs.

Question 11

electrodes

Answer 11

Conductive surfaces through which electrons can enter or leave the half-cells.

Question 12

amperes (A)

Answer 12

To measure electrical current. Also called amps. One amperes represents the flow of one coulomb (a measure of electrical charge) per second.

1 A = 1 C/s

Question 13

potential difference

Answer 13

It is a measure of the diffeence in potential energy (usually in joules) per unit of charge (coulombs).

Question 14

volt (V)

Answer 14

1 V = 1 J/C

The SI unit of potential difference.

Question 15

electromotive force (emf)

Answer 15

The force that results in the motion of electrons due to a difference in potential.

Question 16

Cell potential (E_cell)

cell emf

Answer 16

The potential difference between the cathode and the anode in an electrochemical cell.

A measure of the overall tendency of the redox reaction to occur spontaneously – the lower the cell potential, the lower the tendency to occur. A negative cell potential indicates that the forward reaction is not spontaneous.

Question 17

standard cell potential (E°_cell)

standard emf

Answer 17

Cell potential when the reactants and products are in their standard states (1 M concentration for substances in solution and 1 atm pressure for gaseous substances).

Question 18

anode

Answer 18

The electrode where oxidation occurs.

Question 19

cathode

Answer 19

The electrode where reduction occurs.

Question 20

salt bridge

Answer 20

An inverted, U-shaped tube that contains a strong electrolyte such as KNO₃ and connects the two half-cells. Allows electrical current to flow.

Question 21

standard electrode potential

Answer 21

A measure of the potential energy experienced by charged particles at an electrode in an electrochemical cell; the standard cell potential is the difference between the standard electrode potentials of the anode and cathode.

When the cells are connected, electrons flow from the electrode with greater potential energy (more negatively charged) to the electrode with less potential energy (more positively charged).

Question 22

standard hydrogen electrode (SHE)

Answer 22

A half-cell consisting of an inert platinum electrode immersed in 1 M HCl with hydrogen gas at 1 atm bubbling through the solution; used as the standard of a cell potential of zero.

Question 23

Faraday’s constant (F)

Answer 23

F = 96,485 C / mol e^-

Question 24

Nernst equation

Answer 24

E_cell = E°_cell - [0.0592 V/n][log Q]

Question 25

dry-cell batteries

Answer 25

Common batteries, such as the kind you find in a flashlight. They do not contain large amounts of liquid water.

Question 26

alkaline batteries

Answer 26

More common. Employ slightly different half-reactions in a basic medium. The zinc is oxidized in a basic environment. It has a longer working life and a longer shelf life than their nonalkaline counterparts.

Question 27

lead-acid storage batteries

Answer 27

Batteries in most automobiles. Consist of six electrochemical cells wired in series.