Redox Processes (IB)

Electron Transfer Theory

All reactions consist of two parts (half-reactions) where electrons are transferred between species.

Oxidation

Process where a species loses electrons, leading to an increase in its oxidation number.

Reduction

Process where a species gains electrons, resulting in a decrease in its oxidation number.

Reducing Agent

Substance that donates or loses electrons during a redox reaction.

Oxidizing Agent

Substance that accepts or gains electrons during a redox reaction.

Oxidation Number

The charge an atom would have if the shared electrons in a compound belonged solely to the more electronegative atom.

LEO says GER

Acronym indicating Loss of Electrons is Oxidation, Gain of Electrons is Reduction.

Balancing Redox Reactions

Process involving assigning oxidation numbers, balancing atoms, charges, and electrons in half-reactions, and combining them.

Reactivity Series

Ranking substances based on their ability to oxidize or reduce other substances.

Standard Cell Potentials

Maximum electric potential difference of a cell under standard conditions, calculated as E°(cathode) - E°(anode).

Spontaneous reaction

oxidizing agent (A) is below the reducing agent (RA)

Voltaic Cell

• Spontaneous reaction

• Converts chemical energy to electrical energy

• Anode: 

  • Negative charge

  • Oxidation half-reaction

• Cathode:

  • Positive charge

  • Reduction half-reaction

Oxidation Number Rules

1. The oxidation number for any atom in an element is zero.

2. The oxidation number of a monatomic ion is equal to the charge on the ion.

3. The oxidation number of each hydrogen atom in most of its compounds is +1, except hydrides (which are -1).

4. The oxidation number of each oxygen atom in most of its compounds is - 2.

5. Peroxides are an exception (they are -1). In OF2 oxygen is + 2.

6. In compounds, the elements of group 1, group 2, and aluminum have positive oxidation numbers of +1, +2, and +3, respectively.

Electrolytic Cell

• Non-spontaneous reaction

• Converts electrical energy to chemical energy

• Anode: 

  • Positive charge

  • Oxidation half-reaction

• Cathode:

  • Negative charge

  • Reduction half-reaction

E° cell

• maximum electric potential difference (voltage) of the cell operating under standard conditions (SATP: 25°C, IM)

Standard Hydrogen Electrode

Can act as both an ANODE as well as cathode in an electrochemical cell

Half-reactions

All reactions are a combination of 2 parts

How to balance half-reaction equations

both equations are balanced by mass and by charge (the number electrons lost by one atom is gained by the other)

Oxidation Number Main Rule

The sum of the oxidation numbers of all the atoms must equal the apparent charge of that particle

What are the 3 steps to balancing redox reactions in acidic conditions?

1. Writing half-reactions

2. balance equation

3. combine half-reactions

What is the writing half-reaction step?

1. Assign oxidation numbers

2. Separate into the two half-equations

What is the balancing half-reaction step?

1. Balance all atoms other than oxygen and hydrogen

2. Balance oxygens by adding H20(1) (if needed)

3. Balance hydrogens by adding H+(aq) ions (if needed)

4. Balance charges by adding electrons to the more positive side

5. Double check each half-equation is balanced in terms of atoms and charge

What is the combining half-reaction step?

1. Make sure the number of electrons in the two-half equations are equal (If not, multiply each half reaction equation by simple whole numbers to balance the electrons gained/lost)

2. Add the two half-reaction equations, canceling out anything that is the same on both sides of the reaction

Electron Tug-of-War

• A redox reaction can be viewed as a competition for electrons between substances

• Example - Zinc and Copper(Il) sulfate

Development of the reactivity series

• Determined by performing various single displacement reactions and examining whether they occur spontaneously or not

• More reactive metals are stronger reducing agents than reactive metals; more reactive non-metals are stronger oxidizing agents than less reactive non-metals

How to Predict Spontaneity of Redox Reactions

• A spontaneous reaction occurs only if the oxidizing agent (A) is below the reducing agent (RA) in a table of relative strengths of oxidizing and reducing agents

What type of reaction happens in a voltaic cell?

spontaneous reaction

What type of reaction happens in an electrolytic cell?

non-spontanous reaction

How is energy converted in a voltaic cell?

chemical energy to electrical energy

How is energy converted in an electrolytic cell?

electrical energy to chemical energy

What occurs at the anode in a voltaic cell?

oxidation

What occurs at the cathode in a voltaic cell?

reduction

What occurs at the anode in an electrolytic cell?

oxidation

What occurs at the cathode in an electrolytic cell?

reduction

Uses of electrolysis

• Electrolysis to produce pure elements from compounds

• Electroplating: plate or coat some object with a metal (ex. gold or silver-plated jewelry)

What is standard cell potentials

• E° cell is the maximum electric potential difference (voltage) of the cell operating under standard conditions (SATP: 25°C, IM)

• It represents the energy difference (per unit charge) between the cathode and the anode