Chemistry - Electrochemistry

Standard conditions

Temperature = 298K (25 deg C)

Concentration = 1.00 moldm-3

Gas pressure = 1 atm (100kPa)

Standard electrode potential

The e.m.f. of a half cell compared with a standard hydrogen half-cell at standard conditions.

Inert electrode used

Platinum - electrical conductor and will not interfere with equilibrium.

E* value of hydrogen half cell

0.00 V

An acid that can be used to create the hydrogen half cell

Strong monoprotic acid eg HCl or HNO3

How do electrons and ions move in cell?

Electrons move through wire and ions through salt bridge from + to - electrode

Feasibility of a reaction

The species undergoing reduction must have the more positive E* value

Changes in conditions resulting in shift to the right

More reduction will occur and cause electrode potential to become more positive. Standard cell potential increases.

Changes in conditions resulting in shift to the left

More oxidation will occur and cause electrode potential to become more negative. Standard cell potential increases.

Fuel cell definition

Uses the energy from the reaction of a fuel with oxygen to create a voltage.

Why may a feasible reaction not take place?

Activation energy may be too high or conditions not standard

Explain in terms of equilibrium why cell potential increases when water is added (Cu²⁺/Cu is +0.34V and Ag⁺/Ag is +0.80V)

• Cu2+ + 2OH- → Cu(OH)₂ so [Cu²⁺] decreases

• equilibrium position shifts to the left to minimise the decrease in [Cu²⁺]

• the E value for Cu²⁺/Cu half cell becomes less positive

• cell potential increases due to bigger difference between electrode potentials

What happens to E value if equilibrium position shifts to the right?

E value becomes more positive (less electrons released)

What happens to E value if equilibrium position shifts to the left?

E value becomes less positive (more electrons released)