Spontaneous Redox Reactions and Non-spontaneous Redox Reactions Chemistry Tutorial

Key Concepts

For standard electrochemical cells¹:

• A redox reaction is spontaneous if the standard electrode potential for the redox reaction, E^o_{(redox reaction)}, is positive.

  E^o_{(redox reaction)} = E^o_{(reduction reaction)} + E^o_{(oxidation reaction)}
  and E^o_{(redox reaction)} > 0
  that is, E^o_{(redox reaction)} is positive

• If E^o_{(redox reaction)} is positive, the reaction will proceed in the forward direction (spontaneous).

  Spontaneous redox reactions can be used to produce electricity
  (see galvanic cells (voltaic cells)).

• If E^o_{(redox reaction)} is negative (E^o_{(redox reaction)} < 0), the reaction will not proceed in the forward direction (non-spontaneous).

  Note: The reaction will be spontaneous in the reverse direction!
  And non-spontaneous redox reactions cannot be used to produce electricity.

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Using Electrode Potentials to Determine if a Redox Reaction is Spontaneous or Not

Revision (see Standard Electrode Potentials for Redox Reactions Tutorial for fuller discussion):

• Redox reactions are made up of two reactions:

  (a) Reduction reaction: a species gains electrons

  (b) Oxidation reaction: a species loses electrons

• For each reaction there is an associated electrode potential measured in volts.
  Electrode potentials are usually tabulated for the reduction reaction in which the species are present in their standard states, and these tables are referred to as tables of "Standard Reduction Potentials".
  A table of standard reduction potentials is given below this discussion.

  The electrode potential for a reduction reaction can be read straight from the table.

  The electrode potential for an oxidation reaction will have the opposite sign to the one given in the table,

  that is if E^o for the reduction reaction is positive then it will be negative for the oxidation reaction.

• The electrode potential for a redox reaction is the sum of the electrode potential for the reduction reaction and the electrode potential for the oxidation reaction:

  E^o_(redox) = E^o_(reduction) + E^o_(oxidation)

How to decide if a redox reaction as written is spontaneous:

• If E^o for the redox reaction as written is positive, the reaction proceeds in the forward direction and is said to be spontaneous.
• If E^o for the redox reaction as written is negative, the reaction DOES NOT proceed in the forward direction and is said to be non-spontaneous.

You will need to refer to a table of Standard Reduction Potentials in order to determine the standard electrode potential for a given redox reaction.
A table of Standard Reduction Potentials is given below:

Worked Example: Given E^o for the Redox Reaction

Question For the reaction:

Ca²⁺_(aq) + Zn_(s) → Ca_(s) + Zn²⁺_(aq)     E^o = -2.11 V

Is this redox reaction as written spontaneous or non-spontaneous?

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

   Determine if the redox reaction as written is spontaneous.

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

   Extract the data from the question:
   E^o_{(redox reaction)} = -2.11V

3. What is the relationship between what you know and what you need to find out?

   (a) If E^o_{(redox reaction)} is positive, redox reaction is spontaneous

   spontaneous reaction if E^o_{(redox reaction)} > 0

   (b) If E^o_{(redox reaction)} is negative, redox reaction is NOT spontaneous (non-spontaneous)

   NOT a spontaneous reaction if E^o_{(redox reaction)} < 0

4. Decide if redox reaction as written is spontaneous or not:

   E^o_{(redox reaction)} = -2.11V
   E^o_{(redox reaction)} is negative.
   -2.11 < 0
   Redox reaction as written is not spontaneous
   (The reaction as written is non-spontaneous).

5. Is your answer plausible?

   Consider the activity (reactivity) of the metals involved:
   (a) Calcium (Ca), a Group 2 metal, is a very active (reactive) metal
   (b) Zinc (Zn), is a transition metal and is much less active (reactive) than calcium metal.
   We therefore expect calcium to exist as ions in solution.
   This means we expect zinc to exist as the metal.
   The spontaneous reaction would be for calcium to oxidise to calcium ions, and for zinc ions to accept these electrons to produce zinc metal:
   that is, the spontaneous reaction would be:
   Zn²⁺_(aq) + Ca_(s) → Ca²⁺_(aq) + Zn_(s)
   And this reaction is the reverse of the reaction we were given, so the reaction we were given must be non-spontaneous!

6. State your solution to the problem "is the redox reaction as written spontaneous or not":

   The reaction Ca²⁺_(aq) + Zn_(s) → Ca_(s) + Zn²⁺_(aq) will not proceed spontaneously in the forward direction.

Worked Example: E^o for the Redox Reaction NOT Given

Question: A strip of magnesium metal is placed in an aqueous 1 mol L^-1 copper(II) sulfate solution.
Will a spontaneous redox reaction occur?

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

   Determine if the redox reaction as given is spontaneous.

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

   magnesium in its standard state, that is as the metal: Mg_(s)

   standard aqueous solution of copper(II) sulfate: CuSO_4(aq)
   contains copper(II) ions, Cu²⁺_(aq), and sulfate ions, SO₄^2-_(aq)

3. What is the relationship between what you know and what you need to find out?

   (a) Write the half-equations for the reaction as given in the question:

   Oxidation of magnesium metal: Mg_(s) → Mg²⁺_(aq) + 2e^-
   Reduction of copper(II) ions: Cu²⁺_(aq) + 2e^- → Cu_(s)

   (b) Use a table of standard reduction potentials to find the value of E^o for each half-reaction:

   Mg(s)	→	Mg2+(aq) + 2e-		Eo = +2.36 V
   Cu2+(aq) + 2e-	→	Cu(s)		Eo = +0.34 V

   (c) Add the equations together, and add the E^o value for each half-reaction together

   Mg(s)	→	Mg2+(aq) + 2e-		Eo = +2.36 V
   Cu2+(aq) + 2e-	→	Cu(s)		Eo = +0.34 V
   Mg(s) + Cu2+(aq)	→	Mg2+(aq) + Cu(s)		Eo= +2.36 + +0.34 Eo = +2.70 V

4. Decide if the redox reaction as written is spontaneous:

   (a) Spontaneous reaction if E^o_(redox) > 0

   (b) Not a spontaneous reaction if E^o_(redox) < 0

   E^o is positive (+2.70 V)
   +2.70 > 0
   the redox reaction is spontaneous

5. Is your answer plausible?

   Consider the activity (reactivity) of the metals involved:
   (a) Magnesium (Mg), a Group 2 metal, is a very active (reactive) metal
   (b) Copper (Cu), is a transition metal and is much less active (reactive) than magnesium metal.
   We therefore expect magnesium to exist as ions in solution.
   This means we expect copper to exist as the metal.
   The spontaneous reaction would be for magnesium to oxidise to magnesium ions, and for copper ions to accept these electrons to produce copper metal:
   that is, the spontaneous reaction would be:
   Cu²⁺_(aq) + Mg_(s) → Mg²⁺_(aq) + Cu_(s)
   And this reaction is the same as the reaction we were given, that is, we were given a piece of magnesium metal, Mg_(s), to place in an aqueous solution of copper(II) ions, Cu²⁺_(aq), so we expect a reaction between these two substances to occur spontaneously!

6. State your solution to the problem "is the redox reaction as given spontaneous or not":

   The reaction Mg_(s) + Cu²⁺_(aq) → Cu_(s) + Mg²⁺_(aq) will proceed spontaneously in the forward direction.