1. OXIDATION-REDUCTION REACTIONS (REDOX) PART ONE

2. Oxidation-reduction (redox) reactions involve transfer of electrons Oxidation – loss of electrons Reduction – gain of electrons “OIL-RIG” or “LEO says GER” Both half-reactions must happen at the same time Redox reactions can be identified by understanding and assigning oxidation states/numbers

3. OXIDATION STATES Oxidation number is assigned to each element in a molecule based on the distribution of electrons in molecule There are rules for assigning oxidation numbers (Please refer to handout: Rules For Assigning Oxidation States)

5. Examples of different oxidation numbers using chromium Different oxidation states of chromium have different colors Chromium (II) chloride = blue (chromium has + 2 oxidation state) Chromium (III) chloride = green (chromium has + 3 oxidation state) Potassium chromate = yellow (chromium has + 6 oxidation state) Potassium dichromate = orange (chromium has + 6 oxidation state)

6. OXIDATION Oxidation: reactions in which the atoms or ions of an element experience an increase in oxidation state due to the loss of electrons Example: Reaction of metallic sodium with chlorine gas

7. Formation of sodium ions shows oxidation because each sodium atom loses an electron to become sodium ion Oxidation state is represented by putting the oxidation number above the symbol of each atom and ion The oxidation state of sodium is changed from 0 (elemental state) to +1 (state of the ion) A species whose oxidation number increases is oxidized The sodium atom is oxidized to sodium ion

8. REDUCTION Reduction: reactions in which the oxidation state of an element decreases Example: Chlorine in the reaction with sodium Each chlorine atom accepts an electron, becoming a chloride ion The oxidation state of chlorine decreases from 0 (elemental state) to - 1 (state of the ion) A species that undergoes a decrease in oxidation state is reduced The chlorine atom is reduced to the chloride ion

9. OXIDATION AND REDUCTION AS A PROCESS Electrons are made in oxidation and acquired in reduction For oxidation to happen during chemical reaction, reduction must happen as well Number of electrons made in oxidation must equal number of electrons acquired in reduction due to the law of conservation of mass

10. Transfer of electrons causes changes in oxidation states of one or more elements Redox reaction: any chemical process in which elements undergo changes in oxidation number Example: When copper is oxidized and NO 3 - from nitric acid is reduced

11. Part of the reaction involving oxidation or reduction alone can be written as a half-reaction Overall equation is sum of two half-reactions Number of electrons are the same in oxidation and reduction, they cancel and don’t appear in overall equation

12. Electrons lost in oxidation appear on product side of oxidation half-reaction Electrons gained in reduction appear as reactants in reduction half-reaction

13. When copper reacts in nitric acid 3 copper atoms are oxidized to Cu +2 ions as 2 nitrogen atoms are reduced from a + 5 oxidation state to a + 2 oxidation state

14. If no atoms in reaction change oxidation state, it is NOT a redox reaction Example: Sulfur dioxide gas dissolves in water to form acidic solution of sulfurous acid

15. Is this a redox reaction? When solution of NaCl is added to solution of AgNO 3, an ion- exchange reaction occurs and white AgCl precipitates

16. REDOX REACTIONS AND COVALENT BONDS Substances with covalent bonds also undergo redox reactions Unlike ionic charge, oxidation number has no physical meaning Oxidation number based on electronegativity relative to other atoms to which it is bonded in given molecule Oxidation states in covalent compounds are NOT based on charge!

17. Example: An ionic charge of 1 - results from a complete gain of one electron by atom (the number of electrons an atom has exceeds the number of protons by one) An oxidation state of - 1 means an increase in attraction for a bonding electron Change in oxidation number does not require change in actual charge

18. When elemental hydrogen reacts with elemental chlorine a covalent bond forms from sharing of two electrons The two bonding electrons in hydrogen chloride are not shared equally Chlorine’s higher electronegativity results in a stronger attraction for the pair of electrons causing them to be closer to and spend more time around the chlorine atom

19. As specified by Rule #3, chlorine in HCl is assigned oxidation number of - 1 Oxidation number for chlorine atoms changes from 0 to - 1 So, chlorine atoms are reduced

20. From Rule #1, oxidation number of each hydrogen atom in elemental hydrogen is 0 By Rule #6, oxidation state of hydrogen atom in HCl is + 1 Hydrogen atoms are oxidized

21. No electrons are totally lost or gained in this process Hydrogen has donated a share of its bonding electron to chlorine Hydrogen has NOT completely transferred that electron Assignment of oxidation numbers allows determination of partial transfer of electrons in compounds that are not ionic Increases/decreases in oxidation number can be viewed in terms of complete loss or gain of electrons (as in ionic compounds) OR partial loss or gain of electrons (as in covalent molecules)

22. Reactants and products in redox reactions are not limited to monatomic ions and uncombined elements Elements in molecular compounds or polyatomic ions can also undergo redox if they have more than one oxidation state Example: copper and nitric acid

23. Nitrate ion, NO 3 -, is converted to nitrogen monoxide, NO Nitrogen is reduced in this reaction Instead of saying the “nitrogen atom” is reduced, we say the “nitrate ion” is reduced to nitrogen monoxide