Observation 4: Heat Change Associated with a reaction

Many chemical reactions occur with large amounts of heat being consumed or

produced. We can sense these changes by determining if the temperatures of the

materials change when they are mixed together. Simply holding a beaker in your

hands will allow for the detection of these changes.

Oxidation-Reduction Reactions: Movement of

electrons

Oxidation-reduction reactions involve the transfer of electrons from one atom to

a different atom. Many types of chemical reactions are oxidation-reduction

reactions; combination reactions, many decomposition reactions, combustion

reactions, and single-displacement reactions are all classified as oxidation-

reduction reactions. The key feature to identify is the changing oxidation

numbers of different atoms when these reactions occur. When transition metals

are involved, we will typically see color changes occurring when the transition

metal undergoes a change in oxidation state.

Red-Ox Reaction 1: Oxidation States of Manganese

Here we will look at the reaction of potassium permanganate (KMnO₄) with

iron(II) chloride (FeCl₂) under acidic conditions.

In a medium test-tube, mix about 3 mL of the potassium permanganate solution

with 2 mL of HCl solution and 1 mL of iron(II) chloride solution. Record your

observations. Manganese compounds in aqueous solutions tend to have different

colors based on the oxidation state of the manganese. When manganese is in the

+2 oxidation state, aqueous solutions are a pale pink color. When manganese is

in the +4 oxidation state, it tends to form insoluble brown compounds. When

manganese is in the +7 oxidation state, aqueous solutions have a rich purple

color.

Oxidation-reduction reactions involve the transfer of electrons. Is

manganese losing or gaining electrons in this process? Some other atom

in the reaction must have the opposite behavior with its oxidation state.

What other atom do you think is changing its oxidation state? Is it being

oxidized or reduced?

Write the reduction half-reaction, oxidation half-reaction, and the

balanced Red-Ox reaction for this process.

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