A2 Units 3.1 & 3.2 Redox Reactions & Electrochemistry

Redox

Oxidation numbers

Deciding whether reduction or oxidation occurred

Ion-Electron Half Equations

Rules for working out:

1. Elements - check they are balanced

2. Oxygens - balance by adding H20

3. Hydrogens - balance by adding H+

4. Charge - balance by adding e-

(Every One Hates Charlie)

practice questions on pg. 6

Using half-reactions to obtain the equation for a redox reaction - Example

(practice questions on pg. 8)

n = v x c questioned based on redox

(practice questions on pg. 17)

Redox reaction between Cu2+ and I- and determination of the liberated iodine with S2O3(2-)

Example

Standard Electrode Potentials

The electrode potential values range from +2.88 volts (F2/F-) to -3.04 volts (Li+/Li). Therefore, E* has a sign.

Strong oxidising systems have a large positive values of E°. (accepts / attracts e-)

Strong reducing systems have large negative values of E°. (donates / repels e-)

You do not need to remember E° values. See sheet which contains most systems used in the exam.

The convention is to write the symbol equation as reductions and use the convention that:

Measuring Standard Electrode Potentials

Why is Platinum Foil used?

E* Measurements & The Salt Bridge

• Reading on the voltmeter gives the VALUE of E° for the measured system.

• The direction of movement of electrons through the voltmeter gives the sign of E and indicates its polarity

• If the measured system is MORE positive than the SHE, then the sign of E° is Positive (i.e. electrons flow from the SHE to the measured system)

• If the measured system is MORE negative than the SHE, then the sign of E° is negative (i.e. electrons flow into the SHE from the measured system through the voltmeter).

• The more positive the value of E°, the more energetically favourable the reaction.

(Questions on pg. 25)

Putting half cells together + Standard cell potential

Calculate the emf (voltage) of the cell between Cr2072- /Cr and a Zn2+ / Zn electrode:

Electron Flow

Examples

Formula: EMF = Red - Ox

Red = EMF + Ox

Ox = EMF - Red

Daniel Cell

SHE

If E* is positive, the reaction is feasible.

Predicting the redox reactions and their feasibility

By studying standard electrode potentials, predictions can be made about the likely feasibility of a reaction. In general, a process is feasible if the overall cell potential is positive. i.e. the equilibrium will lie to the RIGHT

Standard Electrode Potential Sheet

Displacement & Questions

Electrochemical Series

(practice questions on pg. 34)

Hydrogen Fuel Cell

A fuel cell is an electrochemical cell. A fuel cell converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity.

The other electrochemical device that we are all familiar with is the battery. A battery has all of its chemicals stored inside, and it converts those chemicals into electricity too. This means that a battery eventually "goes dead" and you either throw it away or recharge it.

With a fuel cell, chemicals constantly flow into the cell so it never goes dead -- as long as there is a flow of chemicals into the cell, the electricity flows out of the cell.

Electrode Equation

Benefits & Drawbacks of using Hydrogen as an energy source