Ms Stephens Saturday, 12 December 2015 Redox Introduction Oxidation Numbers Review of Keywords Identifying Redox Reactions Common Oxidants Common Reductants

Oxidation Numbers 1. Any element, oxidation # of zero. 2. An Ion = its charge 3.The sum O # = charge for the formula. 4.Group 1 metals is +1, group 2 is +2, and aluminum (in group III) is +3 5.nonmetal or polyatomic ion = charge 6.F =–1, O = –2, H = +1 (almost always) H N O 3H N O 3 C2H6OC2H6O K 2 Cr 2 O 7 AgI H 2 PO 4 – rule total Ox.#

Why bother with Oxidation #s? Sign of oxidation number Positive has less control over it’s electrons in a bond (less electronegative) Negative has more control over it’s electrons in a bond (more electronegative) Value of oxidation Number Indicates the number of electrons involved in bonding Oxidation is regarded as an increase in oxidation number of an atom in an element ion or compound

R Reduct ion I is G Gain L oss E lectrons O xidation G ain E lectrons R eduction Says…

Some Keywords Oxidation – a chemical reaction which results in the loss of electrons from a chemical species Reduction – a chemical reaction which results in a chemical species gaining electrons Oxidant – a chemical species that has the ability to take electrons from other chemicals – i.e. it causes oxidation Reductant – a chemical species that has the ability to give electrons to another species – i.e. it causes reduction Electrolysis – the process by which ionic compounds are split into their atoms using electric currents

Identifying Redox Reactions A redox reaction must result in the movement of electrons from one element to another. To identify whether this is happening we must prove that one element is changing it’s oxidation number For example: SO 2 → SO 3 Oxidation Number = +4Oxidation Number = +6 Therefore, an oxidation number change has occurred Increase means it is Oxidation

Common Oxidants Oxygen:O 2 + 4e - → 2O 2- Permanganate: MnO H + + 5e - → Mn H 2 O Dichromate:Cr 2 O H + +6e - → 2Cr H 2 O Fe 3+ Fe 2+ Iron(III) ion:Fe 3+ + e - → Fe 2+ Halogens: I 2 + 2e -  2I - Cl 2 + 2e -  2Cl - Hydrogen peroxide:H 2 O 2 + 2H + + 2e - → 2H 2 O

Common Reductants Hydrogen:H 2 → 2H + + 2e - Thiosulfate: 2S 2 O 3 2- → S 4 O e - Fe 2+ Fe 3+ Iron(II) ion:Fe 2+ → Fe 3+ + e - Halides (Halogen ions): 2X - → X 2 + 2e - Zinc:Zn → Zn e - Sulfur dioxide: SO 2 + 2H 2 O→ SO H + + 2e -

Practice Questions 1.Find the oxidation numbers of the N atoms in each of the molecules. NO 2, HNO 3, NO, N2, N 2 O, 2.(a) Calculate the oxidation number of S in each of these ions: I) SO 4 2 , II)SO 3 2 , III) S 2 O 3 2 , IV) S 4 O 6 2  2.(b) By assigning oxidation numbers show which of the following reactions is not a redox reaction. (i) CuCO 3  CuO + CO 2 (ii) Cu + 2AgNO 3  Cu(NO 3 ) 2 + 2Ag (iii) Cr 2 O 7 2  + 6Fe 2+  6Fe Cr H 2 O (iv) Cr 2 O 7 2  + 2OH   2 CrO 4 2  + H 2 O

Answers a) I. +6II. +4III. +2IV b) I noII. yesIII. yesIV. no