Lecture 20.3- Balancing Redox
Upcoming SlideShare
Loading in...5
×
 

Lecture 20.3- Balancing Redox

on

  • 2,554 views

Section 20.3 lecture for Honors & Prep Chemistry

Section 20.3 lecture for Honors & Prep Chemistry

Statistics

Views

Total Views
2,554
Views on SlideShare
2,530
Embed Views
24

Actions

Likes
0
Downloads
31
Comments
0

1 Embed 24

http://www.slideshare.net 24

Accessibility

Upload Details

Uploaded via as Microsoft PowerPoint

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

Lecture 20.3- Balancing Redox Lecture 20.3- Balancing Redox Presentation Transcript

  • Bellwork
  • Balancing redox reactions Read the 5 steps for balancing redox equations using the half-reaction method. You will get the steps on the test, but you must know what they mean in order to use them.
  • Balance the equation MnO 4 - + Fe 2+  Fe 3+ + Mn 2+
    • STEP 1 – split the complete equation into oxidation and reduction half-reactions
    • MnO 4 -  Mn 2+ reduction ½ reaction
    • Fe 2+  Fe 3+ oxidation ½ reaction
    Each ½ reaction contains a reactant and a product that “look the same”
  • STEP 2- balance each ½ reaction using steps 2a-2d
    • MnO 4 -  Mn 2 +
    • 2a- balance all elements except H and O Mn is already balanced with 1 on each side.
    • 2b- balance oxygen by adding H 2 O There are 4 O on the left, add 4 H 2 O to the right.
    + 4 H 2 O 2c- balance hydrogen using H + There are 8 H on the right, add 8H + on the left 2d- balance the charge using electrons The total charge is +7 on the left and +2 on the right, add 5e - to the left giving both sides a charge of +2. 5e - + 8H + +
    • Fe 2+  Fe 3+
    • 2a- Fe is already balanced
    • 2b- there is no O
    • 2c- there is no H
    • 2d- the left side is +2 the right side is +3
    • Adding one negative e - to the right side will give both sides an equal charge of +2
    STEP 2- balance each ½ reaction using steps 2a-2d + e - +2 = +3 + (-1)
  • STEP 3- The 1/2 reactions must have the same number of e - If necessary, multiply one or both 1/2 reactions to make the number of electrons even.
    • 5 e - + 8 H + + MnO 4 -  Mn 2 + + 4 H 2 O
    • Fe 2+  Fe 3+ + e -
    • The reduction ½ reaction shows a gain of 5e -
    • The oxidation ½ reaction shows a loss of 1 e -
    • Multiply the Fe reaction by 5 to equalize the number of e -
    5 5 5
  • STEP 4- add the ½ reactions together and cancel out anything that appears on both sides
    • 5 e - + 8 H + + MnO 4 -  Mn 2 + + 4 H 2 O
    • 5Fe 2 +  5Fe 3 + + 5e -
    • 5e - + 8H + + MnO 4 - + 5Fe 2 +  Mn 2 + + 4H 2 O + 5Fe 3 + + 5e -
    The electrons will always cancel out 8H + + MnO 4 - + 5Fe 2+  Mn 2+ + 4H 2 O + 5Fe 3+
  • STEP 5 – check that elements and charges balance
    • 8H + + MnO 4 - + 5Fe 2+  Mn 2+ + 4H 2 O + 5Fe 3+
    • Left side right side
    • 8H 8H 1Mn 1Mn 4 O 4 O 5 Fe 5 Fe (+1 x 8) + (-1) + (+2 x 5) = +17 (+2) + (+3x5) = +17
  • Two Ways to Balance Redox Equations
      • Using Oxidation-Number Changes
        • In the oxidation-number-change method , you balance a redox equation by comparing the increases and decreases in oxidation numbers.
    20.3
  • Two Ways to Balance Redox Equations
        • Step 1 Assign oxidation numbers to all the atoms in the equation.
    20.3
  • Two Ways to Balance Redox Equations
        • Step 2 Identify which atoms are oxidized and which are reduced.
        • Step 3 Use one bracketing line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction.
    20.3
  • Two Ways to Balance Redox Equations
      • In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balanced by the total decrease in the oxidation number of the species reduced.
    20.3
  • Two Ways to Balance Redox Equations
        • Step 4 Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients.
    20.3 2
        • Step 5 Finally, make sure that the equation is balanced for both atoms and charge.