2 Oxidation-Reduction Rxns or Redox Oxidation-Reduction Rxns are rxns where electrons are transferred between the reactants. Since electrons are electrical energy, there is a transfer of electrical energy. Many common examples are combustion, oxidation, and single displacement rxns. –AgNO 3 (aq) + Na(s) Ag(s) + NaNO 3 (aq) –HNO 3 (aq) + Na(s) H 2 (g) + NaNO 3 (aq)
3 Redox Rxns What do all these rxns have in common? One reactant gains electrons while another reactant loses electrons. The first redox rxns extensively studied were oxidation rxns. In oxidation rxns, the element or compound reacting with oxygen gas transferred electrons to oxygen. So they said that they were oxidized.
4 Vocabulary of Redox Rxns Oxidation: the process of losing electrons. The compound or element which loses electrons is oxidized. The compound or element which is oxidized is the reducing agent or the reductant, as it gives electrons to the other reactant. Reduction: the process of gaining electrons. The compound or element which gains electrons is reduced. The compound or element which is reduced is the oxidizing agent or the oxidant, as it takes or accepts electrons from the other reactant.
5 Vocabulary of Redox Rxns Oxidation Number or Oxidation State: A number which is assigned to an atom which is a “ pretend ” ionic charge. It is the charge the atom would have if it were ionic. We assign oxidation numbers with a set of rules. Memorization Hint: Use OIL RIG to remember which is which. It stands for: Oxidation Is Losing; Reduction Is Gaining.
6 Goals 1.Learn how to assign oxidation states to all the atoms in a substance. 2.Using oxidation numbers, determine whether the rxn is a redox rxn. 3.If it is a redox rxn, determine which reactant is being oxidized and which is being reduced. 4.For single displacement redox rxns, learn the activity series to predict whether a rxn will occur and what the products are.
7 Assigning Oxidation States Here are the rules for assigning oxidation states in order of precedence: 1.Atoms in their elemental states have an oxidation number of 0. 2.Monatomic ions have an oxidation state equal to their charge. 3.In ionic compounds with a monatomic cation, the cation ’ s oxidation state equals its ionic charge in the compound. 4.Hydrogen is +1 when it is combined with nonmetals, and it is -1 when it is combined with metals. 5.Fluorine has an oxidation number of -1 in compounds.
8 Assigning Oxidation States Here are the rules for assigning oxidation states in order of precedence: 6.Oxygen usually has an oxidation number of -2. It has an oxidation number of -1 in peroxides, and is -1/2 in superoxides (these are obtained by the cation charge). 7.Halogens usually have an oxidation number of -1, except when combined with oxygen or fluorine. 8.In molecular compounds or in polyatomic ions, the more electronegative element (one closer to F or to the right of the Periodic Table) is assigned an oxidation number equal to its normal ionic charge. 9.The sum of all of the individual oxidation numbers in a substance must equal the overall charge of the substance.
9 Assigning Oxidation States Assign oxidation numbers to all the atoms in the following: S 8 C 2 H 4 H 2 SO 4 H 2 SO 3
10 Determining Whether A Rxn is Redox Now you get to use the oxidation numbers! Once you assign oxidation numbers to all the atoms in a rxn, you look for changes. If the oxidation number of an atom changes from a reactant to a product, then it is a redox rxn.
11 Determining Whether A Rxn is Redox Ex: Is the following rxn redox? Al(s) + HCl(aq) AlCl 3 (aq) + H 2 (g) 1)Assign oxidation numbers to all atoms. 2)Are the oxidation numbers of any atoms changing? If yes, it is redox.
12 What ’ s Being Reduced and Oxidized Look at the following rxn again. Al(s) + HCl(aq) AlCl 3 (aq) + H 2 (g) You know that this is a redox as Al is going from 0 in Al(s) to 3+ in AlCl 3 ; and because H is changing from +1 in HCl to 0 in H 2.
13 What ’ s Being Reduced and Oxidized So if the oxidation state of Al is changing from (from reactant to product) 0 to 3+, is it gaining or losing electrons? So is it being oxidized or reduced? If the oxidation state of H is changing from 1 to 0, is it gaining or losing electrons? So is it being oxidized or reduced?
14 What ’ s Being Reduced and Oxidized How do we state this? We say that Al is being oxidized, while HCl or H in HCl is being reduced. Or we say that Al is the reductant, while HCl is the oxidant. Notice that I need say nothing about the products. The above highlights a very important fact about redox rxns: if something is being reduced, something else MUST be oxidized! You can ’ t have one without the other.
15 What ’ s Being Reduced and Oxidized Determine what substance is being oxidized and what ’ s being reduced in the following: CH 4 + 2O 2 2H 2 O + CO 2
16 Activity Series & Single Displacement Rxns Single displacement rxns occur when a metallic element reacts with a cation (or the cation of an ionic compound) or it reacts with acid. M + NX N + MX M + HX H 2 + MX Cr + CuCl 2 Cu + CrCl 3 Sn + HCl SnCl 2 + H 2
17 Activity Series & Single Displacement Rxns The product is another element and another ionic compound. So the cation swaps places with the element. What are the predicted products: silver metal is mixed with cupric nitrate sln?
18 Activity Series & Single Displacement Rxns But how do you know whether a single displacement rxn occurs or not? After all, not all metals are equally reactive. You know for example, that Group 1 is the most reactive metal group. So is it likely that silver metal would react with sodium nitrate to form silver nitrate and sodium metal? To predict whether a single displacement rxn actually occurs, we use a Metal Activity Series.
19 Metal Activity Series Li, Rb, K, Ba, Ca, Na (in order of decreasing reactivity) These metals react with oxygen to produce oxides; acids to produce hydrogen gas and a salt; and cold water to produce hydrogen gas and an oxide or a hydroxide salt.
20 Metal Activity Series Mg, Al, Mn, Zn, Cr, Fe (in order of decreasing reactivity and less than those before) These metals react with oxygen to produce oxides; acids to produce hydrogen gas and a salt; and steam to produce hydrogen gas and an oxide or a hydroxide salt.
21 Metal Activity Series Co, Ni, Sn, Pb (in order of decreasing reactivity and less than those before) These metals react with oxygen to produce oxides; and acids to produce hydrogen gas and a salt.
22 Metal Activity Series H 2, Cu, Ag (in order of decreasing reactivity and less than those before) These “ metals ” react with oxygen to produce oxides. Remember hydrogen isn ’ t a metal, but it is a dividing line between metals that react with acids and metals that don ’ t!
23 Metal Activity Series Hg, Pt, Au (in order of decreasing reactivity and less than those before) Mostly unreactive (please note that the position of Ag and Hg is sometimes reversed).
24 Metal Activity Series Classically, the activity series tells you what metal will react in a single displacement rxn: A + BC B + AC If A is the more reactive metal than B, or A is less stable than B, or A is higher on the Table than B, then the rxn will occur. If A is a less reactive metal than B, or A is lower on the Table than B, then the rxn will not occur.
25 Metal Activity Series What this really says is that the more stable metal (the less reactive metal) is produced. So a more reactive metal will react with an aqueous ionic salt sln of a less reactive metal, forming the less reactive metal and an ionic salt sln of the more active metal. Metals which are above hydrogen gas on the Table also react with strong acids to produce hydrogen gas and an ionic salt sln.
26 Metal Activity Series For the following, predict whether the rxn will occur and if it does, write the balanced equation: Nickel shot is added to 6M hydrochloric acid.
27 Halogen Activity Series There is a halogen series just as there is a metal activity series. Another type of single displacement rxn is when a halogen element reacts with another halogen ion (or an ionic compound with a halogen anion). F 2 + NaCl Cl 2 + NaF
28 Halogen Activity Series Fluorine is the most active halogen, while iodine is the least active halogen (size factor which affects bond strength). Problem: Will the following rxn occur? If so, write the balanced equation. Fluorine gas is mixed with sodium bromide.