1. Chapter 19.1 Redox Reactions in Acidic or Basic Solutions Dr. Peter Warburton peterw@mun.ca http://www.chem.mun.ca/zcourses/1011.php

2. © Peter Warburton 2008 All media copyright of their respective owners2 Redox reactions are easier to balance if we understand where the electrons are coming from and where they are ending up. Oxidation numbers Oxidation numbers help us figure this out.

3. © Peter Warburton 2008 All media copyright of their respective owners3 The two electrons in a bond are completely assigned to the more electronegative element. UNLESS there are bonds between two atoms of the same element. The electrons are shared equally so that one electron is assigned to each of the atoms. Oxidation numbers

4. © Peter Warburton 2008 All media copyright of their respective owners4 Based on this simple idea, we can follow a set of rules to assign oxidation numbers. We go through the set of rules until we find the FIRST rule that applies to our specific atom in the compound or ion of interest. Oxidation numbers

5. © Peter Warburton 2008 All media copyright of their respective owners5 Atoms of pure elemental compounds (e.g. metals, solid carbon, O 2 gas, Br 2 liquid, I 2 solid, etc.) have an oxidation number of ZERO Oxidation number rules (Rule 1)

6. © Peter Warburton 2008 All media copyright of their respective owners6 Monatomic ions (like Mg 2+, Li +, F -, S 2-, etc.) have an oxidation number equal to the charge Oxidation number rules (Rule 2)

7. © Peter Warburton 2008 All media copyright of their respective owners7 Fluorine, as the most electronegative element, will ALWAYS have an oxidation number of -1 EXCEPT in F 2 where it has an oxidation number of ZERO (Rule 1) Oxidation number rules (Rule 3)

8. © Peter Warburton 2008 All media copyright of their respective owners8 Oxygen, as the second most electronegative element, will usually have an oxidation number of -2 UNLESS it is bonded to another oxygen or fluorine Oxidation number rules (Rule 4)

9. © Peter Warburton 2008 All media copyright of their respective owners9 Hydrogen, will have an oxidation number of +1 unless it is bonded to a metal atom, where it will have an oxidation number of -1 Oxidation number rules (Rule 5)

10. © Peter Warburton 2008 All media copyright of their respective owners10 Halogens (Cl, Br, I, and At), generally have an oxidation number of -1 EXCEPT when bonded to F, O, or halogens of the same type or above it on the periodic table. Oxidation number rules (Rule 6)

11. © Peter Warburton 2008 All media copyright of their respective owners11 The sum of the oxidations numbers for ALL the atoms in a compound or ion MUST ADD UP to match the total charge on the compound (zero) or ion (ion charge). Oxidation number rules (Rule 7)

12. © Peter Warburton 2008 All media copyright of their respective owners12 Apply the rules in the order given. Any atoms not specifically covered in the rules can usually be assigned oxidation numbers by applying Rule 7 and some logic. Applying the rules

13. © Peter Warburton 2008 All media copyright of their respective owners13 Problem Assign oxidation numbers to every atom in the following compounds and ions: S 8 LiH TiO 2 H 2 O H 2 O 2 HSO 4 - Cr 2 O 7 2- CaCO 3

14. © Peter Warburton 2008 All media copyright of their respective owners14 Balancing redox equations 1.Using oxidation numbers, identify what is oxidized (loses electrons) and reduced (gains electrons). 2.What are the products after the oxidation and reduction take place? 3.Is the redox reaction done under acidic or basic conditions?

15. © Peter Warburton 2008 All media copyright of their respective owners15 Balancing redox equations The information from the previous slide results in an unbalanced skeleton equation where we know generally what reactants and products are specifically involved in the electron transfer (redox) process.

16. © Peter Warburton 2008 All media copyright of their respective owners16 Balancing redox equations Examples of skeleton equations with oxidation numbers shown:

17. © Peter Warburton 2008 All media copyright of their respective owners17 Half-reactions We then break the skeleton reaction into two unbalanced half-reactions where the oxidation half-reaction has an atom where the oxidation number becomes more positive and the reduction half reaction has an atom where the oxidation number become more negative.

18. © Peter Warburton 2008 All media copyright of their respective owners18 Half-reactions from skeleton rxn Oxidation half-reaction Reduction half-reaction

19. © Peter Warburton 2008 All media copyright of their respective owners19 Balancing half-reactions in ACIDIC solution 1.Balance all atoms EXCEPT H and O in each half reaction: 2.Balance O atoms by adding water to the side missing O atoms:

20. © Peter Warburton 2008 All media copyright of their respective owners20 Balancing half-reactions in ACIDIC solution 3.Balance H atoms by adding H + to the side missing H atoms: Oxidation half-reaction Reduction half-reaction

21. © Peter Warburton 2008 All media copyright of their respective owners21 Oxidation half-reaction Reduction half-reaction Balancing half-reactions in ACIDIC solution 4.Balance charge by adding electrons to the side with more total positive charge:

22. © Peter Warburton 2008 All media copyright of their respective owners22 Oxidation half-reaction Reduction half-reaction Balancing half-reactions in ACIDIC solution 5.Make the number of electrons the same in both half-reactions by multiplication, while avoiding a fractional number of electrons:

23. © Peter Warburton 2008 All media copyright of their respective owners23 Added together Simplified (should have NO electrons!) Balancing half-reactions in ACIDIC solution 6.Add the half reactions together and then simplify by cancelling out species that show up on both sides:

24. © Peter Warburton 2008 All media copyright of their respective owners24 Balanced reaction in ACIDIC solution 7.Confirm that the reaction is balanced in number of atoms and total charge on both sides of the arrow. If the reaction stoichiometry can be simplified by division without giving fractional coefficients, you can simplify further:

25. © Peter Warburton 2008 All media copyright of their respective owners25 Balanced reaction in BASIC solution First follow steps one to seven as seen in acidic solution. 8.Add the same number of OH - groups as there are H + present to BOTH sides of the equation:

26. © Peter Warburton 2008 All media copyright of their respective owners26 Balanced reaction in BASIC solution 9.One side of the reaction has BOTH OH - and H + present in equal amounts. Combine these together to make an equal amount of water:

27. © Peter Warburton 2008 All media copyright of their respective owners27 Balanced reaction in BASIC solution 10.Simplify by cancelling out an equal number of water from each side until one side has no water and confirm that the reaction is balanced in number of atoms and total charge on both sides of the arrow:

28. © Peter Warburton 2008 All media copyright of their respective owners28 Problem Balance the following unbalanced redox skeleton equation in BASIC solution