Chapter 18.2 - Balancing redox equations Redox reactions: reduction – oxidaton reactions.

1. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Chapter 18.2 - Balancing redox equations Redox reactions: reduction – oxidaton reactions. Reduction: lowering of positive charge (increase of negative charge) Oxidation: increase of positive charge (lowering of negative charge) If an electron is transferred, then the negative charge must decrease somewhere (where the electron comes from), and it must increase somewhere else (where the electron goes), hence reduction and oxidation must occur together. There is a balance in such changes, and reduction – oxidaton reactions provide good examples for balancing chemical equations. Redox Reactions, Acid, Base, Paul G. Mezey

2. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Chapter 18: Electrochemistry • Chapter 18: Electrochemistry • 18.2 Balancing Oxidation-Reduction Equations [19.1] • (Note: This section is usually covered with a very brief review of oxidation numbers and oxidizing/reducing agents, as covered in Chemistry 1010, which is found in section 4.9 of the Tro text).

3. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Oxidation numbers Redox reactions are easier to balance if we understand where the electrons are coming from and where they are ending up. Oxidation numbers help us figure this out. Redox Reactions, Acid, Base , Paul G. Mezey

4. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] In a very simplistic way, the basis of oxidation numbersis the following idea: Each (single) chemical bond between any two atoms X and Y is formed by two electrons. There are three possibilities: X attracts electrons more than Y 2. Y attracts electrons more than X X and Y attract electrons exactly the same way (important: this happens only if X = Y) Case 1. Even if the attraction is only slightly stronger by X than by Y, we pretend that the entire electron pair belongs to X. Case 2. Similarly to case 1, we pretend that the entire electron pair belongs to Y. Case 3. We assign one electron to X, and the other electron to Y. If we carry out this formal assignment of electrons for each bond of the molecule or ion, then the “charge” obtained on each atom is the oxidation number of the given atom. Redox Reactions, Acid, Base , Paul G. Mezey

5. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] A numerical measure of how strongly atoms attract electrons is electronegativity We may phrase the concept of oxidation number in terms of electronegativity: In calculating oxidation numbersthe two electrons in a bond are completely assigned to the more electronegative element, UNLESS the bond is between two atoms of the same element, in this case the electrons are shared equally so that one electron is assigned to each of the atoms. Redox Reactions, Acid, Base , Paul G. Mezey

6. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 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 number rules (Rule 1) Atoms of pure elemental compounds (e.g. metals, solid carbon, O2 gas, Br2 liquid, I2 solid, etc.) have an oxidation number of ZERO Oxidation number rules (Rule 2) Monatomic ions (like Mg2+, Li+, F-, S2-, etc.) have an oxidation number equal to the charge Oxidation number rules (Rule 3) Fluorine, as the most electronegative element, will ALWAYS have an oxidation number of -1 EXCEPT in F2 where it has anoxidation number ofZERO (Rule 1) Redox Reactions, Acid, Base , Paul G. Mezey

7. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Oxidation number rules (Rule 4) 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 5) Hydrogen, will have an oxidation number of +1 unless it is bonded toa metal atom, where it will have an oxidation number of -1, or if it is bonded toanother H atom, where it will have an oxidation number of 0 (Rule 1). Oxidation number rules (Rule 6) Halogens (Cl, Br, I, and At), generally have an oxidation number of -1 EXCEPTwhen bonded toF, O, or halogens of the same type or above it on the periodic table. Oxidation number rules (Rule 7) The sum of the oxidation 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). Redox Reactions, Acid, Base , Paul G. Mezey

8. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 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. If in any doubt, please remember that the main idea is to artificially assign the entire bonding electron pair to the more electronegative atom in each bond. If all bonding electron pairs are distributed this way, then the charge obtained on each atom becomes the oxidation number. (Of course, the charge on each atom includes the protons in the nucleus as well as all the electrons of the atom). Redox Reactions, Acid, Base , Paul G. Mezey

9. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Problems: Assign oxidation numbers to every atom in the following compounds and ions: S8 LiH TiO2 H2O H2O2 HSO4- Cr2O72- CaCO3 Redox Reactions, Acid, Base , Paul G. Mezey

10. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Using oxidation numbers, the balancing of redox chemical equations becomes simpler. The main steps in balancing redox equations: Using oxidation numbers, identify what is oxidized (what loses electrons) and what is reduced (what gains electrons). What are the products after the oxidation and reduction take place? Is the redox reaction done under acidic or basic conditions? The information obtained from these 3 steps results only in an unbalanced skeleton equation where we know generally what reactants and products are specifically involved in the electron transfer (redox) process. Redox Reactions, Acid, Base , Paul G. Mezey

11. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Examples of skeleton equations with oxidation numbers shown: Examples Redox Reactions, Acid, Base , Paul G. Mezey

12. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] How to proceed further? Brake up the problem to two formal half-reactions : We 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 reactionhas an atom where the oxidation number becomes more negative. Redox Reactions, Acid, Base , Paul G. Mezey

13. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Oxidation half-reaction: Reduction half-reaction: Example: Half-reactions from skeleton reaction Redox Reactions, Acid, Base , Paul G. Mezey

14. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 1. Balance all atomsEXCEPT H and O in each half reaction: 2. Balance O atomsby adding water to the side missing O atoms: Steps for balancing half-reactions in ACIDIC solution: Redox Reactions, Acid, Base , Paul G. Mezey

15. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 3. Balance H atomsby adding H+ to the side missing H atoms: Oxidation half-reaction Reduction half-reaction Steps for ACIDIC solution Redox Reactions, Acid, Base , Paul G. Mezey

16. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 4. Balance chargeby adding electrons to the side with more total positive charge: Oxidation half-reaction Reduction half-reaction Steps for ACIDIC solution Redox Reactions, Acid, Base , Paul G. Mezey

17. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 5. Make the number of electrons the samein both half-reactionsby multiplication, while avoiding a fractional number of electrons: Oxidation half-reaction Reduction half-reaction Steps for ACIDIC solution Redox Reactions, Acid, Base , Paul G. Mezey

18. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 6. Add the half reactions together and then simplify by cancelling out species that show up on both sides: Added together Simplified (should have NO extra electrons!) Steps for ACIDIC solution Redox Reactions, Acid, Base , Paul G. Mezey

19. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 7. Confirm that the reaction is balancedin number of atoms and total charge on both sides of the arrow. If the reaction stoichiometrycan be simplified by divisionwithout giving fractional coefficients, you can simplify further: Steps for ACIDIC solution Redox Reactions, Acid, Base , Paul G. Mezey

20. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] First follow steps one to sevenas seen in the case of acidic solution. 8. Add the same number of OH- groups as there are H+ present to BOTH sides of the equation: Steps for balancing half-reactions in BASIC solution: Redox Reactions, Acid, Base , Paul G. Mezey

21. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 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: Steps for BASIC solution Redox Reactions, Acid, Base , Paul G. Mezey

22. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] 10. Simplify by cancelling out an equal number of water from each side until one side has no waterand confirm that the reaction is balancedin number of atoms and total charge on both sides of the arrow: Steps for BASIC solution Redox Reactions, Acid, Base , Paul G. Mezey

23. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] Balance the following unbalanced redox skeleton equation in BASIC solution Problem Redox Reactions, Acid, Base , Paul G. Mezey

24. Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1] End of section Redox Reactions, Acid, Base , Paul G. Mezey