Oxidation-Reduction
An oxidation-reduction (redox) reaction involves the transfer of electrons (e - ). Sodium transfers its electrons to chlorine 2
Chemists need a way to keep track of what happens in a redox reaction - ie who gains electrons and who loses electrons They use a book-keeping system called oxidation numbers
Oxidation numbers look like the charge on the ion but are written with the - or + in front of the number!
Oxidation numbers are sometimes the ionic charge on an element but must be determined from a set of rules.
Find the oxidation numbers of each element in a reaction and see which ones have changed. 6
Rules for assigning oxidation number Any uncombined element is 0. A monatomic ion equals the charge of the ion Fluorine is -1 Oxygen is -2, unless it is combined with F, when it is +2, or it is in a peroxide such as H 2 O 2 when it its -1 Hydrogen is usually +1, unless combined with a metal, then it is -1 Group 1 is +1, Group 2 is +2, Aluminum is +3 The sum of the oxidation numbers of all atoms in a neutral compound is 0. The sum of the oxidation number of all atoms in a polyatomic ion equals the charge of the ion. 7
The sum of the oxidation numbers of all the atoms in a compound is zero. CuO Oxygen is -2 The oxidation number of copper must be calculated X + -2 = 0 X = +2 Na 2 SO 4 Na is +1 because it is a group 1 metal O is -2 The oxidation number of Sulfur must be calculated 2(+1) + X + 4(-2) = 0 (2 ) + X + (-8) =0 X = +6 8
The sum of the oxidation numbers in an ion is equal to the charge of the ion. Monatomic Cl - -1 Fe 3+ +3 Polyatomic NO 3 - O is -2 The oxidation number of nitrogen must be calculated X + (3)(-2) = -1 X + (-6) = -1 X = +5 9
The same element can have a different oxidation number in different substances
Practice Question What is the oxidation number of potassium in K 2 Cr 2 O 7? (a) +1 (b) +2 (c) -2 (c) -7 11
Practice Question What is the oxidation number of oxygen in K 2 Cr 2 O 7? (a) +1 (b) +2 (c) -2 (d) -7 12
Practice Question What is the oxidation number of chromium in K 2 Cr 2 O 7? (1) +12 (2) +2 (3) +3 (4) +6 13
Practice Question In which substance does chlorine have an oxidation number of +1? (1) Cl 2 (2) HCl (3) HClO (4) HClO 2 14
Cl 2 = 0 Rule #1 15
HCL H = +1 Rule #6 +1 + X = 0 Rule #8 Cl = -1 16
HClO H = +1 Rule #6 O = -2 Rule #5 +1 + X + -2 = 0 Rule #8 X: Cl = +1 17
HClO 2 H = +1 Rule #6 O = -2 Rule #5 +1 + X + (2x-2) = 0 Rule #8 X: Cl = +3 Rule #8 18
Continue with where we left off yesterday (page 81 or 78 or beyond) Identifying Redox Reactions 19
During redox, one atom is oxidized while another atom is reduced. Reduction and oxidation happen together. The oxidation numbers of the atoms will change. one goes up (0 to +1) and one goes down (0 to -2) 0 0 2H 2 + O 2 Oxidation + 2H 2 O 2- Reduction 20
Oxidation numbers (states) can be assigned to atoms and ions. Changes in oxidation numbers indicate that oxidation and reduction have occurred. 0 +2 +5-2 +2 +5-2 0 Zn + Cu(NO 3 ) 2 Zn(NO 3 ) 2 + Cu Find the oxidation numbers and see which ones change. Nitrate NO 3 is -1 Zn 0 Zn 2+ + 2e - Oxidation Cu 2+ + 2e - Cu 0 Reduction 21
Oxidation is the loss of electrons. Metal atoms lose electrons to become + ions Oxidation Number INCREASES Cr 2+ Cr 4+ + 2e - 2N 3- N 0 + 2 6e - 22
A half-reaction can be written to represent oxidation. Zn 0 Zn 2+ + 2e - In oxidation half reactions, electrons are written on the right because electrons are lost 23
Reduction is the gain of electrons. Metals gain electrons to form new ions or elements Oxidation Number DECREASES Fe 3+ + 3e - Fe 0 24
A half-reaction can be written to represent reduction. Cu 2+ + 2e - Cu 0 In reduction half reactions, electrons are written on the left because electrons are gained 25
Practice Question Given the equation: 2 Al + 3 Cu 2+ 2 Al 3+ + 3Cu The reduction half-reaction is (1) Al Al 3+ + 3e (2) Cu 2+ + 2e Cu (3) Al + 3e Al 3+ (4) Cu 2+ Cu + 2e 26
LEO (the lion) says GER Loss of Electrons Oxidation Gain of Electrons Reduction 27
Practice Question Given the reaction: Mg(s) + 2H + (aq) + 2Cl (aq) Mg 2+ (aq) + 2Cl (aq) + H 2 (g) Which species undergoes oxidation? (1) Mg(s) (2) H + (aq) LEO says GER (3) Cl (aq) (4) H 2 (g) 28
Practice Question Given the equation: C(s) + H 2 O(g) CO(g) + H 2 (g) Which species undergoes reduction? (1) C(s) (2) H + LEO says GER (3) C 2+ (4) H 2 (g) 29
Agents Reducing Agent: element or a compound in a redox reaction that reduces another species. It becomes oxidized, is the electron donor in the redox. A reducing agent is oxidized. 30
Oxidizing Agent: element or a compound in a redox reaction that oxidizes another species. It becomes reduced, is the electron acceptor in the redox. An oxidizing agent is reduced.
Practice Question In any redox reaction, the substance that undergoes reduction will (1) lose electrons and have a decrease in oxidation number (2) lose electrons and have an increase in oxidation number (3) gain electrons and have a decrease in oxidation number (4) gain electrons and have an increase in oxidation number 32
An electrochemical cell can be either voltaic or electrolytic. In an electrochemical cell, oxidation occurs at the anode and reduction at the cathode. Red Cat An Ox Reduction at the Cathode Oxidation at the Anode 33