Oxidation and Reduction

What is Electron Transfer Theory?

• all reactions are a combination of 2 parts (half-reactions)

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

• Reduction: # of electron is increased

• Oxidation: # of electron is lost

• Reducing Agent: gives or lose electrons

• Oxidizing Agent: gains or accepts electrons

• Oxidation number: the charge that an atom in a compound would have if the electron pair in the bond belonged solely to the more electronegative atom

  • Oxidation numbers always refer to single atoms

Why does LEO say GER?

• loss of electrons = oxidation

• gain of electrons = reduction

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.

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

• An increase in the oxidation number indicates that an atom has lost electrons and therefore oxidized

• A decrease in the oxidation number indicates that an atom has gained electrons and therefore reduced

Balancing Redox Reactions in Acidic Conditions: The Half-Reaction Method

Writing half-reactions:

1. Assign oxidation numbers

2. Separate into the two half-equations

For each equation:

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

Combining half-reactions:

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

Predicting Redox Reactions

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

• Some substances are better at oxidizing than others

• Can be placed in order of their oxidizing ability

• 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

Predicting 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

Electrochemical Cells

Voltaic and Electrolytic Cells

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

• All reduction potentials are measured with reference to the standard hydrogen electrode

To Calculate

• E° cell = E° (cathode) - E° (anode)

• Where E° represents the "standard electrode potential"- the ability of the half-cell to attract electrons (under standard conditions), thus undergoing a reduction

• If it is a positive value (according to chart in data booklet), then it is more likely to undergo reduction compared to a negative value

How can you use Standard Cell Potential to determine the spontaneity of a chemical reaction?

Uses:

• Electrolysis to produce pure elements from compounds

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

Standard Hydrogen Electrode

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

• Used to measure all the other electrodes