Electrochemistry

both atoms and the electrons must be balanced in redox reactions gains 1e- x 2 atoms = 2e- oxidation numbers: +1 -1 Cu + Cl2 → Cu+ + Cl- 2 2 2 loses 1e- x 2 = 2e- done by assigning oxidation numbers to all elements

Mg (s) + Cl2 (g) → MgCl2 (s) Mg → Mg2+ + 2e- Cl + 1e- → Cl1- **There is a written difference between ion charge and oxidation number** Mg (s) + Cl2 (g) → MgCl2 (s) Mg → Mg2+ + 2e- Cl + 1e- → Cl1- Magnesium ion ion charge: 2+
 oxidation #: +2 Chlorine ion ion charge: 1-
 oxidation #: -1 Sometimes these numbers are the same (like above) sometimes they are very different – which is why we write them differently

O#s are based on the electronegativity of the atoms Oxidation number represents the charge the atom would have if every bond were ionic. O#s are based on the electronegativity of the atoms 1. Assign known numbers first, calculate others. All uncombined elements (& diatomics) – zero Cu O2 Monatomic ion - equals ion charge Fe3+ +3 Cl- -1

Alkali metals (IA) – always +1 Earth metals (IIA) – always +2 Halogens (VIA) – usually -1 Hydrogen – usually +1 Oxygen – usually -2 Except metal hydrides (NaH) Except peroxides (H2O2) 2. Neutral compound: Sum of O#s must equal zero 3. Charged compound: Sum of O#s must equal charge of compound

Multiple the O#s by the subscripts for each atom to get the sum 2. Neutral compound: Sum of O# must equal zero Assign O#s for each atom in K2Cr2O7 Step 1: Start with atoms which are known. O: –2
 K: +1 Step 2: Solve for other atoms. +1 +6 -2 Multiple the O#s by the subscripts for each atom to get the sum K2Cr O7 2 +2 +12 ?? -14 = 0 K = +1 Cr = +6 O = –2.

NO3- Assign O#s for each atom in NO3- O: –2 N = +5 O = –2. +5 -2 +5 ?? 3. Charged compound: Sum of O# must equal charge of compound Assign O#s for each atom in NO3- Step 1: Start with atoms which are known. O: –2
 Step 2: Solve for other atoms. +5 -2 NO3- +5 ?? -6 = -1 N = +5 O = –2.

Use ox.numbers to determine if reaction is a redox reaction. +4 -2 +1 -2 +1 +4 -2 SO2 + H2O → H2SO3 +4 -4 +2 -2 +2 +4 -6 Remember: Redox involves the transfer of electrons – no change in O#s during the reaction means NO redox NOT a redox reaction

Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) Is the following reaction a redox reaction? +1 +5 -2 +1 +5 -2 Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) +1 +5 -6 +1 +5 -6 Oxidation cannot occur without reduction Redox reaction • Cu – oxidized (loss of electrons)
 • Ag – reduced (gain of electrons)

Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) Oxidizing agent - causes the oxidation of another substance. AgNO3 is the oxidizing agent Reducing agent - causes the reduction of another substance. Cu is the reducing agent Think opposites: if the compound is the “oxidizing agent” of the reaction, then it is reduced in the process and vice versa +1 Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s) +1

2 HNO3(aq) + 3 H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l) Identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent. +1 +5 -2 +1 -2 +2 -2 +1 -2 2 HNO3(aq) + 3 H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l) +1 +5 -6 +2 -2 +2 -2 +2 -2 S – oxidized N – reduced H2S – reducing agent HNO3 – oxidizing agent Substance - means element Agent – means compound

How many electrons are transferred in the reaction below: gains 3e- +1 +5 -2 +1 -2 +2 -2 +1 -2 HNO3(aq) + H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l) 2 3 loses 2e- Stoichiometry (coefficients and subscripts) used to determined total electrons transferred S: (3 atoms) x (2e- lost) = 6 electrons lost N: (2 atoms) x (3e- gained) = 6 electrons gained

CAN YOU / HAVE YOU? Determine the oxidation numbers for atoms in compounds and ions Identify reactions as redox or non-redox Include: oxidizing agent, reducing agent Additional KEY Terms