In an Oxidation Reduction (Redox) Reaction we have the transferring of an electron(s) between reactants. The reactant that gains electrons is reduced and represents the oxidizing agent, while the reactant that loses electrons is oxidized and represents the reducing agent.
Oxidation Reduction (Redox) Reactions
Redox reactions are characterized by a gain or loss of electrons and to remember the difference between oxidation and reduction just say “LEO the lion goes GER.”
Calculating Oxidation Numbers
In oxidation vs reduction reactions, examining how the oxidation numbers of the reactants change is key to determining which one has been oxidized or reduced. When calculating the oxidation number of an element or compound we can use the following rules.
Rule 1: When an element is in its standard state its oxidation number is equal to zero.
Na (s) Br2 (l) S8 (s) F2 (g) Ne (g) Ca (s)
Rule 2: The charge of an ion or polyatomic ion is equal to its oxidation number.
Ca2+ Oxidation Number = +2 PO43– Oxidation Number = – 3
Rule 3: For metals from Group 1A, their oxidation number is equal to +1 when connected to other elements.
Rule 4: For metals from Group 2A, their oxidation number is equal to +2 when connected to other elements.
Rule 5: For hydrogen, the oxidation number is +1 when connected to non-metals and -1 when connected to boron or a metal.
Rule 6: For fluorine, the oxidation number is always -1 when connected to other elements.
Rule 7: For oxygen, the oxidation number is -1 when it’s a peroxide, -1/2 when it’s a superoxide and -2 in most other cases.
A peroxide represents a compound containing two elements from Group 1A bonded to two oxygen atoms.
A superoxide represents a compound containing one element from Group 1A bonded to two oxygen atoms.
Rule 8: For the halogens of Cl, Br, and I their oxidation numbers are -1 unless they are connected to oxygen.
So now let’s take a look at redox reaction to determine which reactant has been oxidized and which reactant has been reduced.
PRACTICE: Identify the reducing agent and oxidizing agent from the following redox reaction.
STEP 1: Identify the monoatomic ions because their charges equal their oxidation numbers.
H+ Oxidation Number = +1 Cr3+ Oxidation Number = + 3
STEP 2: Assign oxidation numbers to the elements in compounds with known values based on the rules we’ve just reviewed.
STEP 3: If an element doesn’t have a rule to calculate its oxidation number then assigned it an “x” and then solve.
Oxidation numbers in dichromate ion, Cr2O72-.
Oxidation numbers in ethanol, C2H5OH.
Oxidation numbers in carbon dioxide, CO2.
STEP 4: Look to see which element or compound was reduced.
Chromium (Cr) is transformed from Cr2O72– to Cr3+ and has its oxidation number go from +6 to +3. Its oxidation number has decreased and because Cr is a part of Cr2O72– we would say that the entire compound has been reduced and represents the oxidizing agent.
STEP 5: Look to see which element or compound was oxidized.
Carbon (C) is transformed from C2H5OH to CO2 and has its oxidation number go from +2 to +4. Its oxidation number has increased and because C is a part of C2H5OH we would say that the entire compound has been oxidized and represents the reducing agent.
Types of Redox Reactions
Different types of chemical reactions will also fall under the classification of being a redox reaction.
Single Displacement or Replacement Reactions
In a single displacement reaction we have one element replacing another element within a compound. To determine if an element can replace another element we use an activity series chart.
A very common example is the displacement of a hydrogen atom by magnesium when reacting with hydrochloric acid.
When following the traditional defintion this involves O2 reacting with a compound made of (C & H) or (C, H & O) to produce CO2 and H2O.
A disproportionation reaction is one where the reactant compound is both oxidized and reduced.
Redox Reactions play an important role in many reactions you will see in your chemistry and biology courses. In addition to recognizing the oxidizing agent and reducing agent you will learn about balancing redox reactions, the connections to electrochemical cells, spontaneity, stoichiometry and solution chemistry.