go to the AUS-e-TUTE homepage

Spontaneous Redox Reactions and Non-spontaneous Redox Reactions Tutorial

Key Concepts

For standard electrochemical cells1:

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

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):

How to decide if a redox reaction as written is 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:

Some Standard Reduction Potentials

Oxidants

 

Reductants

E0
(volts)

 
Weakest
Oxidant
K+ + e- K(s) -2.94 Strongest
Reductant
Ba2+ + 2e- Ba(s) -2.91
Ca2+ + 2e- Ca(s) -2.87
Na+ + e- Na(s) -2.71
Mg2+ + 2e- Mg(s) -2.36
Al3+ + 3e- Al(s) -1.68
Mn2+ + 2e- Mn(s) -1.18
H2O + e- ½H2(g) + OH- -0.83
Zn2+ + 2e- Zn(s) -0.76
Fe2+ + 2e- Fe(s) -0.44
Ni2+ + 2e- Ni(s) -0.24
Sn2+ + 2e- Sn(s) -0.14
Pb2+ + 2e- Pb(s) -0.13
H+ + e- ½H2(g) 0.00
SO42- + 4H+ + 2e- SO2(aq) + 2H2O 0.16
Cu2+ + 2e- Cu(s) 0.34
½O2(g) + H2O + 2e- 2OH- 0.40
Cu+ + e- Cu(s) 0.52
½I2(s) + e- I- 0.54
½I2(aq) + e- I- 0.62
Fe3+ + e- Fe2+ 0.77
Ag+ + e- Ag(s) 0.80
½Br2(l) + e- Br- 1.08
½Br2(aq) + e- Br- 1.10
½O2(g) + 2H+ + 2e- H2O 1.23
½Cl2(g) + e- Cl- 1.36
½Cl2(aq) + e- Cl- 1.40
MnO4- + 8H+ + 5e- Mn2+ + 4H2O 1.51
Strongest
Oxidant
½F2(g) + e- F- 2.89 Weakest
Reductant

Do you know this?

Join AUS-e-TUTE!

Play the game now!

Worked Example: Given Eo for the Redox Reaction

Question For the reaction:

Ca2+(aq) + Zn(s) → Ca(s) + Zn2+(aq)     Eo = -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:
    Eo(redox reaction) = -2.11V

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

    (a) If Eo(redox reaction) is positive, redox reaction is spontaneous

    spontaneous reaction if Eo(redox reaction) > 0

    (b) If Eo(redox reaction) is negative, redox reaction is NOT spontaneous (non-spontaneous)

    NOT a spontaneous reaction if Eo(redox reaction) < 0

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

    Eo(redox reaction) = -2.11V
    Eo(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.

Do you understand this?

Join AUS-e-TUTE!

Take the test now!

Worked Example: Eo 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: CuSO4(aq)
    contains copper(II) ions, Cu2+(aq), and sulfate ions, SO42-(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 Eo 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 Eo 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 Eo(redox) > 0

    (b) Not a spontaneous reaction if Eo(redox) < 0

    Eo 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.

Can you apply this?

Join AUS-e-TUTE!

Take the exam now!


1. Reactants and products must be present in their standard states:
(a) For elements, the standard state is the state of the element at 100 kPa
(b) For ions in solution, the standard state is approximated by a concentration of 1 mol L-1
If solutions are not in their standard states, you can use the nernst equation to calculate the value of the electrode potential.