2 - Thermodynamics and Kinetics of Corrosion

Question 1

usable energy in a thermodynamic system that can be used to perform work

Answer 1

Free energy

Question 2

This is the maximum amount of work a system can perform at constant temperature and pressure.

Answer 2

Gibbs free energy

Question 3

a property of the system that depends only on its current state and not on how it reached that state.

(temperature, pressure, composition)

Answer 3

state function

Question 4

is a quantitative measure of a chemical reaction’s tendency to start, which includes the reaction of a metallic material with its environment (corrosion is an example).

Answer 4

Gibbs free energy change (∆G)

Question 5

Indicates a spontaneous reaction, where the system releases usable energy and moves towards a more stable state (lower energy state).

Answer 5

Negative ΔG

Question 6

Indicates a non-spontaneous reaction that requires an external energy input to occur.

Answer 6

Positive ΔG

Question 7

It’s defined as the product of the charge moved (Q) and the potential difference (E) across which it moves.

Answer 7

electrical work (w)

Question 8

Faraday’s constant

Answer 8

96,485 C/mol

Question 9

Equation of free energy change (and electrical work)

Answer 9

w=∆G=-nFE
where:
∆G – free energy change
n – no. of electrons involved in the reaction
F – Faraday’s constant (96,485 C/mol)
E – cell potential

Question 10

This refers to the potential difference (voltage) between the two electrodes in an electrochemical cell.

Answer 10

Cell Potential (E)

Question 11

This is the change in free energy under specific standard conditions (1 atm pressure, 1 M concentration for solutions, and 25°C temperature).

Answer 11

Standard Free Energy Change (ΔG°)

Question 12

a higher cell potential (E) corresponds to a more negative standard free energy change (ΔG° ) under standard conditions. This relationship is expressed by the equation:

Answer 12

ΔG° = -nF E°

ΔG°: Standard free energy change (J/mol)
n: Number of electrons transferred in the reaction (mol)
F: Faraday’s constant (conversion factor, 96485 C/mol)
E°: Standard cell potential (V)

Question 13

A more (—–) value of ∆G results in a (—–) tendency to make the reaction proceed.

Answer 13

more, larger

Question 14

When ∆G=0, it is said that the reaction is in an

Answer 14

equilibrium state

Question 15

accounts for these non-idealities and quantifies how the concentration of a dissolved species deviates from its ideal behavior.

Answer 15

Activity coefficient (γ)

Question 16

considers deviations from ideal gas behavior. It reflects how the actual “escaping tendency” of a gas molecule differs from a perfect gas at the same pressure.

Answer 16

fugacity coefficient (φ)

Question 17

an instrument used to determine equilibrium potential

Answer 17

Danielle cell

Question 18

Solids, Liquids, Gases, and Elements: Their standard state is the pure substance at a pressure and temperature of —–

Answer 18

1 atm (101.3 kPa) and a temperature of 25°C (298.15 K)

This implies the substance is in its most stable form @ these conditions

Question 19

The standard concentration for solutes in solutions is

Answer 19

1M

Question 20

The standard concentration for solutes in solutions is

Answer 20

E_cell=E_cathode-E_anode

Question 21

a single electrode in contact with an electrolyte solution containing ions of the same element or compound as the electrode. It represents one half of a complete electrochemical reaction (oxidation or reduction).

Answer 21

half-cell

Question 22

Nerst Equation

Answer 22

E = E° - (RT / nF) * ln (Q)

Question 23

allows us to calculate the cell potential (E) of a galvanic cell under non-standard conditions, considering the actual concentrations/activities of reactants and products.

Answer 23

Nernst equation

Question 24

The study of reaction rates at the interface between an electrode and a liquid

Answer 24

Electrode Kinetics

Question 25

an electrode at which a net oxidation process occurs

Answer 25

Anode

Question 26

an electrode at which a net reduction reaction occurs

Answer 26

Cathode

Question 27

synonymous with oxidation reaction (loses electron)

Answer 27

Anodic reaction

Question 28

synonymous with reduction reaction (gaining electron)

Answer 28

Cathodic reaction

Question 29

the deviation from equilibrium potential

Answer 29

Polarization

Question 30

measurement of magnitude of polarization with respect to the equilibrium potential of an electrode.

Answer 30

Overvoltage

Question 31

Types of polarization

Answer 31

1.Activation polarization
2.Concentration polarization

Question 32

• Refers to an electrochemical process that is controlled by the reaction sequence at the metal-electrolyte interface.
• Usually is the controlling factor during corrosion in media containing a high concentration of active species (e.g. concentrated solids)
• This type of polarization refers to the slowing down of the rate-determining step in an electrochemical reaction at the electrode surface. It arises due to the activation energy barrier that needs to be overcome for the reaction to proceed.

Answer 32

Activation polarization

Question 33

• It refers to electrochemical reactions that are controlled by the diffusion in the electrolyte.
• This type of polarization occurs when the concentration of reactants near the electrode surface is depleted or the concentration of products accumulates due to the ongoing reaction.

Answer 33

Concentration polarization

Question 34

polarization can lead to the formation of protective oxide layer on the metal’s surface which acts as a barrier, slowing down further corrosion.

Answer 34

Passivation

Question 35

simply the amount of current (I) over a given area (A).

Answer 35

Current Density, i

Question 36

Total charge (Q) through the cell when N moles of the metal M reacts

Answer 36

Q=zFn

Question 37

• The current density at equilibrium
• When the system isn’t polarized/overpotential

Answer 37

Exchange Current Density, i¬0

Question 38

A model for the current density of an electrode when the only significant limiting factor is activation.

Answer 38

Tafel equation

Question 39

Tafel equation assumptions

Answer 39

• Reaction is slow, therefore: n_diffusion = n_ion resistance = 0
• Surface of a metal is at a potential E

Question 40

• When no polarization

Answer 40

i_for=i_rev

Question 41

• When potential is higher at anodic site

Answer 41

i_net=i_for-i_rev

Question 42

simplified tafel equation

Answer 42

Butler-Volmer Equation