Recap Precipitation Reactions: ions combine to form insoluble products Neutralization Reactions: H + ions and OH - ions combine to form H 2 O Next: Oxidation-Reduction (Redox) Reactions: Atoms or ions donate or accept electrons

Redox Reaction Corrosion of a metal battery terminal iron rusting metal Oxygen, acid

Metal Corrosion metal loses electron forms a cation = metal is oxidized AND another reactant gains electrons = is reduced

Oxidation-Reduction Reactions An oxidation occurs when an atom or ion loses electrons. A reduction occurs when an atom or ion gains electrons. One cannot occur without the other.

Redox Reactions Oxidation-Reduction Reactions (Redox Reactions) –reactions that involve the transfer of electrons between two reactants –an element in one reactant is oxidized while an element in another reactant is reduced Mg (s) + 2 H + (aq)  Mg 2+ (aq) + H 2 (g) oxidizedreduced

Redox Reactions Oxidation: –the loss of electrons chemical species becomes more positively charged –the gain of oxygen –the loss of hydrogen

Redox Reactions Reduction: –the gain of electrons the chemical species becomes more negatively charged –the gain of hydrogen –the loss of oxygen

Redox Reactions LEO: –Lose Electrons Oxidation GER: –Gain Electrons Reduction GER LEO LEO says GER

Redox Reactions Oil Rig Oil :  Oxidation Involves Loss of e - Rig :  Reduction Involves Gain of e -

Redox Reactions Electrons are not explicitly shown in chemical equations. Oxidation Numbers are used to keep track of electrons gained and lost during redox reactions. Oxidation number –a hypothetical number assigned to an individual atom present in a compound using a set of rules. May be positive, negative, or zero

Rules for Oxidation Numbers Oxidation numbers are always reported for individual atoms or ions not groups of atoms or ions!!!!!!!!!!! For an atom in its elemental form, the oxidation number is always zero. –H 2 : oxidation # = 0 for each H atom –Cu: oxidation number = 0 –Cl 2 : oxidation # = 0 for each Cl atom

Rules for Oxidation Numbers For any monoatomic ion: oxidation # = charge on ion K + oxidation # = +1 Cl - oxidation # = -1 S 2- oxidation # = -2

Rules for Oxidation Numbers Group 1A Metal Cations: Always +1 Group 2A Metal Cations: Always +2 Hydrogen (H) +1 when bonded to nonmetals -1 when bonded to metals

Rules for Oxidation Numbers Oxygen (O) -1 in peroxides (O 2 2- ) -2 in all other compounds Fluorine (F) always -1

Rules for Oxidation Numbers The sum of the oxidation numbers of all atoms in any chemical species (ion or neutral compound) is equal to the charge on that chemical species H 2 O: MgCl 2 : MnO 4 - : (-2) = (-1) + (-1) = (-2) + (-2) + (-2) + (-2) = -1

Oxidation Numbers For many compounds, you will be able to directly apply the rules to determine the oxidation number of all atoms except for one. –Use the last two rules to determine the oxidation number of that last element.

Practice determining oxidation numbers Example: Determine the oxidation state of all elements in SO 3. Is it elemental? Are any monoatomic ions present? Which elements have rules? Set up an equation to find the remaining oxidation number. No O = -2 S + 3(-2) = 0  S = +6

Oxidation Numbers Example: Determine the oxidation number of Mn and O in MnO 4 -. Mn + 4(-2) = -1 so Mn = +7 Is it elemental? Are any monoatomic ions present? Which elements have rules? Set up an equation to find the remaining oxidation number. No O = -2

Oxidation Numbers Example: Determine the oxidation state of all elements in NaNO 3 Is it elemental? Are any monoatomic ions present? Which elements have rules? Set up an equation to find the remaining oxidation number. No Na + Na = +1, O = N + 3(-2) = 0  N = +5

Practice 1)Determine the oxidation number of P in HPO )Determine the oxidation state of all elements in Cr 2 O )Determine the oxidation state of Sn in SnBr 4.

Redox Reactions There are many different kinds of redox reactions. –Combustion CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) –Oxidation of Metals by Acids or Metal Salts Cu (s) + H 2 SO 4 (aq)  CuSO 4 (aq) + H 2 (g)

Redox Reactions Oxidation of Metals by Acids or Metal Salts The reaction between a metal and an acid or between a metal and a metal salt is called a displacement reaction. –a reaction in which an element reacts with a compound and displaces an element from that compound A + BX  AX + B

Redox Reactions Oxidation of Metals by Acids or Metal Salts Examples: Mg (s) + 2HCl (aq)  MgCl 2 (aq) + H 2 (g) Zn (s) + 2 HBr (aq)  ZnBr 2 (aq) + H 2 (g) Mn (s) + Pb(NO 3 ) 2 (aq)  Mn(NO 3 ) 2 (aq) + Pb (s)

Redox Reactions Oxidation of Metals by Acids or Metal Salts How do you know if a redox reaction has occurred? You must examine the oxidation number of each of the elements present in the reactants and products. –If the oxidation number changes, then a redox reaction has occurred.

Redox Reactions Oxidation of Metals by Acids or Metal Salts When oxidation occurs: –Electrons are lost –Oxidation number increases When reduction occurs: –Electrons are gained –Oxidation number is reduced (decreases) e-e- e-e-

Redox Reactions Oxidation of Metals by Acids or Metal Salts Mg(s) + 2HCl (aq)  MgCl 2 (aq) + H 2 (g) Oxidation # of Mg increased oxidation Oxidation # of H + ion reduced reduction

Redox Reactions Oxidation of Metals by Acids or Metal Salts Example: Identify the element that has been oxidized and the one that has been reduced. Zn (s) + 2 HCl (aq)  ZnCl 2 (aq) + H 2 (g)

Redox Reactions Oxidation of Metals by Acids or Metal Salts Find the oxidation numbers: Zn (s) + 2 HCl (aq)  ZnCl 2 (aq) + H 2 (g) oxidized reduced Zn is the reducing agent. HCl is the oxidizing agent.

Redox Reactions Oxidation of Metals by Acids or Metal Salts Oxidizing agent: –The reactant that causes another reactant to be oxidized –The reactant that contains the element that was reduced Reducing agent: –The reactant that causes another reactant to be reduced –The reactant that contains the element that was oxidized.

Redox Reactions Oxidation of Metals by Acids or Metal Salts Example: Identify the oxidizing and reducing agents in the following reaction. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g)

Redox Reactions Oxidation of Metals by Acids or Metal Salts You can also write a net ionic equation to describe a redox reaction: –Write all soluble strong electrolytes as ions –Eliminate the spectator ions Zn (s) + 2 HCl (aq)  ZnCl 2 (aq) + H 2 (g) Zn (s) + 2 H + (aq) + 2 Cl - (aq)  Zn 2+ (aq) + 2 Cl - (aq) + H 2 (g) Zn (s) + 2H + (aq)  Zn 2+ (aq) + H 2 (g)

Redox Reactions Oxidation of Metals by Acids or Metal Salts Example: Write the complete ionic and net ionic equations for the reaction. Which element is oxidized? What is the oxidizing agent? 2 Al (s) + 3 Ni(NO 3 ) 2 (aq)  2 Al(NO 3 ) 3 (aq) + 3 Ni (s) 2 Al (s) + 3 Ni 2+ (aq) + 6 NO 3 - (aq)  2 Al 3+ (aq) + 6 NO 3 - (aq) + 3 Ni (s) 2 Al (s) + 3 Ni 2+ (aq)  2 Al 3+ (aq) + 3 Ni (s)

Redox Reactions Oxidation of Metals by Acids or Metal Salts 2 Al (s) + 3 Ni(NO 3 ) 2 (aq)  2 Al(NO 3 ) 3 (aq) + 3 Ni (s) oxidized

Redox Reactions Oxidation of Metals by Acids or Metal Salts Based on the previous equation, we wouldn’t want to store a solution of Ni(NO 3 ) 3 in an aluminum container. –The aluminum container would react and dissolve!!! Metals differ in the ease with which they are oxidized. –Al(s) is oxidized by Ni(NO 3 ) 3 (aq) –Ag(s) is NOT oxidized by Ni(NO 3 ) 3 (aq)

Redox Reactions Oxidation of Metals by Acids or Metal Salts Activity series: –A list of metals arranged in order of decreasing ease of oxidation –Used to predict whether a metal will react with an acid or with a metal salt –See Table 4.5 –Any metal on the list can be oxidized by, i.e. will lose electrons to, the ions of a metal below it.

Activity Series

Activity series: Any metal on the list can be oxidized by, i.e. will lose electrons to, the ions of a metal below it.

Cu can lose electrons to Ag +