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Oxidation and Reduction Concepts Tutorial

Key Concepts

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Oxidation Reactions

An oxidation reaction has occurred if a species (atom or ion) has lost one or more electrons.

Electrons carry a negative charge, so, when a species (atom or ion) loses an electron, the product must be more positively charged than the reactant.

  1. Consider the oxidation reaction in which a neutral, highly reactive, sodium atom (Na) loses an electron to form the more stable, much less reactive, sodium cation (Na+):

      reactant products
    Oxidation reaction: Na Na+ + e-

    The sodium atom has been oxidised because it has lost an electron.

    The charge on sodium has been increased from 0 for the neutral atom to 1+ for the sodium ion (Na+).

    Sodium has become more positive.

  2. Consider the oxidation reaction in which the copper(I) ion, Cu+, loses an electron to form the copper(II) ion, Cu2+:

      reactant products
    Oxidation reaction: Cu+ Cu2+ + e-

    A copper ion with a charge of 1+ loses an electron to produce a copper ion with a charge of 2+

    Cu+ has been oxidised because it has lost an electron.

    The charge on copper has been increased from 1+ for the Cu+ ion to 2+ for the Cu2+ ion.

    Copper has become more positive.

  3. Consider the oxidation reaction in which lead(II) ions (Pb2+) lose 2 electrons (2e-) to produce lead(IV) ions (Pb4+):

      reactant products
    Oxidation reaction: Pb2+ Pb4+ + 2e-

    A lead ion with a charge of 2+ loses 2 electrons to produce a lead ion with a charge of 4+

    Pb2+ has been oxidised because it has lost 2 electrons.

    The charge on lead has been increased from 2+ for the Pb2+ ion to 4+ for the Pb4+ ion.

    Lead has become more positive.

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Reduction Reactions

A reduction reaction has occurred if a species (atom or ion) has gained one or more electrons.

Electrons carry a negative charge, so, when a species (atom or ion) gains an electron, the product must be more negatively charged than the reactant.

  1. Consider the reduction reaction in which an atom of highly reactive fluorine (F) gains an electron to produce the much more stable and less reactive fluoride ion (F-):

      reactants product
    Reduction reaction: F + e- F-

    An electrically neutral1 fluorine atom, an atom with no overall charge, gains an electron to form the negatively charged fluoride ion (anion).

    The fluorine atom has been reduced because it has gained an electron.

    The charge on fluorine has decreased from 0 for the neutral atom to 1- for the fluoride ion.

    Fluorine has become more negative.

  2. Consider the reduction reduction in which an atom of oxygen (O) gains two electrons to form the oxide ion (O2-):

      reactants product
    Reduction reaction: O + 2e- O2-

    A neutral oxygen atom, an atom with no overall charge, gains 2 electrons to form the negatively charged oxide ion (O2-).

    The oxygen atom has been reduced because it has gained 2 electrons.

    The charge on oxygen has decreased from 0 for the neutral atom to 2- for the oxide ion (O2-).

    Oxygen has become more negative.

  3. Consider the reduction reaction in which tin(IV) ions, Sn4+, gain 2 electrons to produce tin(II) ions, Sn2+:

      reactants product
    Reduction reaction: Sn4+ + 2e- Sn2+

    The Sn4+ ion gains 2 electrons to form the Sn2+ ion.

    Sn4+ has been reduced because it has gained 2 electrons.

    The charge on tin has decreased from 4+ for Sn4+ to 2+ for Sn2+ .

    Tin has become more negative (or, less positive).

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Worked Example: Determining if a Reaction is an Oxidation or Reduction Reaction

Question 1. Consider the reaction given below:
    Fe3+(aq) + 3e- → Fe(s)
    Is this an oxidation reaction or a reduction reaction?
    Justify your answer.

Solution:

(Based on the StoPGoPS approach to problem solving.)

  1. What is the question asking you to do?

    (i) Determine if the reaction is an oxidation or reduction reaction.
    (ii) Justify your answer.

  2. What data (information) have you been given in the question?

    Extract the data from the question:

    Balanced chemical equation:

    Fe3+(aq) + 3e- → Fe(s)

  3. What is the relationship between what you know and what you need to find out?
    Definition of oxidation: species loses electrons (electrons are a product)

    Definition of reduction: species gains electrons (electrons are a reactant)

  4. Determine whether the reaction is an oxidation or a reduction of iron

    Electrons are a reactant.

    Fe3+ gains 3 electrons to produce Fe(s)

    Therefore this is a reduction reaction.

  5. Is your answer plausible?

    In a reduction reaction, negatively charged electrons are gained by a species, so the species must become more negatively charged (or, less positively charged).
    Fe3+ has a charge of 3+
    Fe(s) has no overall charge, that is, its charge is 0
    The charge on iron has become less positive, +3 is more positive than 0.
    Therefore this is a reduction reaction.

    Since this result agrees with our previous way of deciding the nature of this reaction, we are confident that our answer is plausible.

  6. State your solution to the problem "is this an oxidation or reduction reaction":

    (i) This is a reduction reaction

    (ii) because Fe3+ has gained 3 electrons to form Fe(s) and reduction is defined as a gain of electrons.

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1. Chemists use the word "neutral" to mean different things in different contexts.
Here, "neutral" means "no overall charge", that is, neither positively charged nor negatively charged.
We will reinforce this idea by saying "electrically neutral".

The purists amongst you will note that an atom is also electronically neutral, it contains the same number of positively charged protons in the nucleus as it does negatively charged electrons surrounding the nucleus.

But you have probably also met the term "neutral" when you talk about the acidity of a solution.
In this case, "neutral" means that a substance is neither acidic or basic.
In this sense the solution is neutral (neither acidic nor basic) because [H+] = [OH-]