MCAT General Chemistry Chapter 11: Oxidation Reduction Reactions

Oxidation and Reduction

• Oxidation: loss of electrons 

• Reduction: gain of electrons 

• Oxidizing agent: causes another atom in a redox reaction to undergo oxidation

  • Is reduced

• Reducing agent: causes another atom in a redox reaction to be reduced 

  

Law of Conservation Governs Redox Reactions

• Law of conservation of charge states that electrical charge can be neither created or destroyed 

• Biochemical redox reagents act as both oxidizing and reducing agents at different times during metabolic pathways 

Assigning Oxidation Numbers

• are assigned to atoms in order to keep track of the redistribution of electrons during chemical reactions 

  • Based on the oxidation numbers of the reactants and products 

Determining Oxidation Numbers

1. Oxidation number of a free element is zero

2. Oxidation number of a monatomic ion is equal to the charge of the ion 

3. Oxidation number of each group 1A element is +1 

4. Oxidation number of each group 2A element is +2

5. Oxidation number of each group 7A element is -1 

   1. Except when combined with an element of higher electronegativity 

6. Oxidation number of hydrogen is usually +1 

   1. Is -1 in compounds with less electronegative elements 

7. In most compounds, oxidation number of oxygen is -2

8. Sum of the oxidation numbers of all the atoms present in a neutral compound is zero 

   1. Sum of oxidation numbers of atoms present in a polyatomic ion is equal to charge of the ion 

Balancing Oxidation Reduction Reactions

• Assigning oxidation numbers to reactants and products can determine how many moles of each species are required for conservation of charge and mass 

• Requires that net charge and number of atoms is equal on both sides 

• Half-reaction method (ion-electron method)  

  • The equation is separated into two half-reactions, the oxidation and the reduction 

  • Each reaction is balanced separately and added to give a balanced half reaction 

Net Ionic Equations

• Show reaction species that actually participate in the reaction, eliminating spectator ions 

Net Ionic Equation of Metathesis (Double Dispalcement) Reactions

Involve the switching of counterions and are not usually oxidation-reduction reactions because all atoms retain oxidation state

Disproportionation REactions

• A specific type of redox reaction in which an element undergoes both oxidation and reduction in producing its products 

  • Ex.: catalysis of peroxides by catalase 

    • 2H₂O₂ → 2H₂O (l) + O₂ (g) 

    • +1   -1    +1  -2         0

• Usually orchestrated by enzymes who have metals like Cu and Zu in active sites to act as reducing agents 

Oxidation-reduction TItrations

• Follow the transfer of charge to reach the equivalence point 

• Can utilize indicators that change color at a particular voltage value 

Potentiometric Titration

redox titration where no indicator is used and electrical potential difference is measured using a voltmeter