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Balancing Simple Half-Equations For Ions Tutorial

Key Concepts

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Writing Oxidation and Reduction Equations

Writing a balanced half-equation for an oxidation reaction or a reduction reaction is a 3 step process:

  1. Write a skeletal equation for the oxidation or reduction equation based on the information provided.
  2. Balance the number of atoms of each element present
    (refer to balancing chemical equations)
    .
  3. Balance the charge by adding electrons (e- or e) to the side deficient in negative charge.

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Worked Example: Write a Balanced Half-Equation for an Oxidation Reaction

Question: Write a balanced half equation for the oxidation of Fe2+ to Fe3+.

Solution:

(Based on the StoPGoPS approach to problem solving.)

  1. What is the question asking you to do?

    Write a balanced half-equation for the oxidation of Fe2+

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

    Extract the data from the question:

    (i) Fe2+ is the reactant to be oxidised.

    (ii) Fe3+ is the product of the reaction.

  3. What is the relationship between what you know and what you need to find out?
    Step 1: Write a skeletal equation for the equation.

      reactant product
    skeletal
    equation:
    Fe2+ Fe3+

    Step 2: Balance the number of atoms of each element present.

      reactant product  
    skeletal
    equation:
    Fe2+ Fe3+  
    No. Fe atoms: 1 = 1 balanced

    Step 3: Balance the charge by adding electrons (e-) to the side deficient in negative charge.

    skeletal equation: Fe2+ Fe3+    
    Find the charge on each side of the equation: 2+   3+    
    Which side is deficient in negative charge? less positive   more positive    
    (that is, deficient in negative charge)
    Add electrons, e-, to
    the more positive side
    Fe2+ Fe3+ + e-
    Add together the charges on each side of the equation: 2+   3+ + 1-
    2+   3+ + 1- = 2+
    Compare left hand side total charge and right hand side total charge 2+ = 2+
    Charges are balanced, so the equation is balanced ! Fe2+ Fe3+ + e-

  4. Write the balanced half-equation:

    Fe2+ → Fe3+ + e-

  5. Is your answer plausible?

    First, check that you have written an equation for the oxidation of Fe2+
    Oxidation means the Fe2+ will lose electrons, so electrons will be a product of the reaction.
    An electron, e-, is written on the right hand side (product side) of our equation so, yes, we have written an equation for the oxidation reaction.

    Next, check that the charges are balanced:
    + 2 on left hand side
    +3 -1 = +2 on right hand side
    charges are balanced.

    Then, check that number of iron "atoms" are balanced:
    1 Fe on left hand side
    1 Fe on right hand side
    Iron "atoms" are balanced.

    So we are reasonably confident that our answer is plausible.

  6. State your solution to the problem "write a balanced half-equation for the oxidation of Fe2+":

    Fe2+ → Fe3+ + e-

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Worked Example: Write a Balanced Half-Equation for a Reduction Reaction

Question: Write a balanced half equation for the reduction of I2 to I-

Solution:

(Based on the StoPGoPS approach to problem solving.)

  1. What is the question asking you to do?

    Write a balanced half-equation for the reduction of I2

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

    Extract the data from the question:

    (i) I2 is the reactant to the reduced.

    (ii) I- is the product of the reaction.

  3. What is the relationship between what you know and what you need to find out?
    Step 1: Write a skeletal equation for the equation.

      reactant product
    skeletal
    equation:
    I2 I-

    Step 2: Balance the number of atoms of each element present.

      reactant product  
    skeletal
    equation:
    I2 I-  
    No. I atoms: 2 1 NOT balanced
    Multiply I- in chemical equation by 2: I2 2I-  
    No. I atoms: 2 = 2 Balanced

    Balanced skeletal equation is: I2 → 2I-

    Step 3: Balance the charge by adding electrons (e-) to the side deficient in negative charge.

    skeletal equation: I2 2I-
    Find the charge on each side of the equation: 0   2 × 1- = 2-
    Which side is deficient in negative charge? more positive   less positive
    (that is, deficient in negative charge)  
    Add electrons, e-, to
    the more positive side
    e- + I2 2I-
    Add together the charges on each side of the equation: 1- + 0   2-
    1-   2-
    Compare left hand side total charge and right hand side total charge 1- 2-
    Charges are NOT balanced, so multiply number of electrons by 2 2e- + I2 2I-
    Check to see if the charges are now balanced: 2 × 1- = 2- + 0   2-
    2- = 2-
    Charges are balanced, so equation is balanced: 2e- + I2 2I-

  4. Write the balanced half-equation:

    I2 + 2e- → 2I-

  5. Is your answer plausible?

    First, check that you have written an equation for the reduction of I2
    In a reduction reaction a species gains electrons, electrons must be a reactant.
    Electrons are present on the left hand side (reactant side) of our equation, so yes we have written an equation for a reduction reaction.

    Next, chaeck that the charges are balanced:
    Charge on left hand side = 0 + (2 × 1-) = 0 + 2- = 2-
    Charge on right hand side = 2 × 1- = 2-
    Charged is balanced.

    Then check that the number of "atoms" of I is balanced:
    No. I atoms on left hand side = 2
    No. I atoms on right hand side = 2
    Iodine "atoms" are balanced.

    We are now reasonably confident that our balanced equation is plausible.

  6. State your solution to the problem "write a balanced half-equation for the reduction of I2":

    I2 + 2e- → 2I-

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