Unit 9: Chemical Reactions and Redox

Changes of Matter Physical Changes: Chemical Changes: A B AB do not change the composition of a substance. melting, vaporizing, dissolving, size, etc. Chemical Changes: result in new substances. burning, rusting, decomposition, etc. A B AB

4 Clues of Chemical Change Demo: Zn + H+  Zn2+ + H2 Pb2+ + I–  PbI2(s) Mg + O2  MgO gas production color change energy transfer (temp change, light) precipitate formation Do three reactions and summarize BEFORE revealing list. (solid forms from solution) Chemical Change: new & different substances formed.

A+B AB Chemical Reaction REACTANTS PRODUCTS Chemical Equation: yields REACTANTS PRODUCTS present at the start of the rxn produced in the rxn at the end

Law of Conservation of Mass In a chemical reaction, matter is neither created nor destroyed. Atoms are rearranged. 4H 2O 4H 2O

Law of Conservation of Mass mass of reactants = mass of products ?

mass is conserved (same before & after) Chemical Equations Balancing Equations WHY? mass is conserved (same before & after) N2H4 + O2  N2 + H2O N2H4 + O2  N2 + 2 H2O Coefficient - # of units of each substance

Chemical Equations H2 + O2  H2O 3 Balancing Steps 1. Count atoms on each side. 2. Add Coefficients to make # of atoms equal. coefficient  subscript = # atoms How many N’s in: 2 Fe(NO3)2 3. Check atom balance!!!

Chemical Equations 3 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 2 3 3 2 2 3 2 Aluminum and copper(II) chloride form copper and aluminum chloride. 3 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 2 3 3 2 2 3 2 2   2 3  6   6 1 1 2 3  3

Which of the following is a chemical reaction? Quick Quiz! Which of the following is a chemical reaction? melting of lead dissolving sugar in water rusting of iron crushing of stone (new substance)

Quick Quiz. Which of the following is NOT a possible clue that a chemical change is taking place? a change of state a change in color production of a gas formation of a precipitate

Why do we have to balance equations? Quick Quiz. Why do we have to balance equations? it’s so much fun! it’s the law of the west it’s the law of consternation of moose it’s the law of conservation of mass

4. During any chemical change, the mass of the final products is… Quick Quiz. 4. During any chemical change, the mass of the final products is… always equal to the mass of the starting reactants. always greater than the mass of the starting reactants. always less than the mass of the starting reactants. sometimes different than the mass of the starting reactants.

__Al + __HCl → __AlCl3 + __H2 Quick Quiz. What is the coefficient of HCl when the following reaction is balanced? __Al + __HCl → __AlCl3 + __H2 ___Al___ ___H___ ___Cl___ A. 1 B. 2 C. 3 D. 6 2 Al + 6 HCl → 2 AlCl3 + 3 H2 1 1 1 2 1 3

5 Types of Reactions 1. Synthesis 2. Decomposition 3. Single Replacement 4. Double Replacement 5. Combustion

A + B  AB “The Marriage” 2 H2 + O2  2 H2O 1. Synthesis only 1 product A + B  AB 2 H2 + O2  2 H2O

AB  A + B “The Break-Up” 2H2O2  2H2O + O2 2. Decomposition only 1 reactant 2H2O2  2H2O + O2

1 element replaces another element 3. Single Replacement 1 element replaces another element “The Kickout” A + BX  AX + B Demo Demo – Mg + HCl in test tube Mg + 2 HCl  MgCl2 + H2

_____ > _____ > _____ 3. Single Replacement Activity Series of Metals more active metals will replace less active metals Demo Cu(s) + 2 Ag+(aq)  Cu2+(aq) + 2 Ag(s) K + CuCl  KCl + Cu Demo – AgNO3 and Cu wire in test tube Li + KCl  LiCl + K Cu + LiCl  no rxn Li K Cu _____ > _____ > _____

two compounds “switch ions” NOT REDOX 4. Double Replacement “Switchin’ Partners” two compounds “switch ions” AX + BY  AY + BX Demo Demo – Pb(NO3)2 + KI  PbI2 in test tube 2 KI + Pb(NO3)2  2 KNO3 + PbI2

CxHy+O2  CO2+H2O “Burnin” C3H8 + 5 O2  3 CO2 + 4 H2O 5. Combustion burning a hydrocarbon (CxHy) in O2 always forms CO2 and H2O CxHy+O2  CO2+H2O C3H8 + 5 O2  3 CO2 + 4 H2O

Quick Quiz! 1. Which type of reaction is described by the following equation? 6 Li + N2  2 Li3N A. synthesis B. decomposition C. single-replacement D. combustion

Quick Quiz. Mg + 2 HNO3  Mg(NO3)2 + H2 2. Which type of reaction is described by the following equation? Mg + 2 HNO3  Mg(NO3)2 + H2 A. synthesis B. decomposition C. single-replacement D. double-replacement

Quick Quiz. 2 C2H5OH + 7 O2  4 CO2 + 6 H2O 3. Which type of reaction is described by the following equation? 2 C2H5OH + 7 O2  4 CO2 + 6 H2O A. double-replacement B. decomposition C. single-replacement D. combustion

Quick Quiz. Pb(NO3)2 + 2 NaCl  PbCl2 + 2 NaNO3 4. Which type of reaction is described by the following equation? Pb(NO3)2 + 2 NaCl  PbCl2 + 2 NaNO3 A. synthesis B. decomposition C. double-replacement D. single-replacement

Quick Quiz. 5. Balance the equation and describe it as a combination or decomposition reaction. H2SO4  H2O2 + SO2 A. H2SO4  H2O2 + SO2 combination B. H2SO4  H2O2 + SO2 decomposition C. H2SO4  2H2O2 + SO2 combination D. H2SO4  2H2O2 + SO2 decomposition

via Wikipedia: Oxidation-Reduction Reactions: An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.

Oxidation-Reduction Reactions (REDOX) (One cannot occur without the other) LEO says GER lose e-, charge up….gain e-, charge down (Video – OxRed I)

Oxidation Numbers Is it a redox reaction? To find out… assign oxidation numbers* (or oxidation states) to each element in a reaction. check if any oxidation states changed (↓ reduced , ↑ oxidized) *oxidation numbers of elements describe electrons that would be lost or gained IF the compound was 100% ionic. of ions show electrons transferred IN an ionic compound *charges

Assigning Oxidation Numbers Any pure element—even if it forms diatomic molecules like chlorine (Cl2) —has an oxidation state of 0. Examples of this are Cu or O2 – via Wikipedia All compounds are 0. Monatomic ion is its charge. (Ex. Na+ ion in water (aqueous)) Most nonmetals tend to be negative, but some are positive in certain compounds or ions. (Ex. SO3) O is −2 always, but in peroxide ion is −1 (O22–). H is +1 with nonmetals, −1 with a metals. F is always −1. other halogens are −1, but can be positive, like in oxyanions. Ex. ClO3–

Oxidation Numbers The sum of the ox. #’s in a neutral compound is 0. The sum of the ox. #’s in a polyatomic ion is the charge on the ion. Calculate the oxidation number of each: Sulfur in… SO3 Chromium in… K2Cr2O7 Nitrogen in… NH4+ Cobalt in… [CoCl6]3–

Classifying REDOX Reactions All rxns (but…NOT double replacement) Synthesis A + B → AB 2 → 1 (0 0 → +/–) Decomposition AB → A + B 1 → 2 (+/– → 0 0) Single Replacement AB + C → A + CB (+/– 0 → 0 +/–) Combustion CxHy + O2 → CO2 + H2O (–/+ 0 → +/– +/–) Zn in acid releases H2 gas 33

Writing REDOX Reactions Write the net ionic equation for the reaction of solid zinc in a solution of hydrochloric acid. comp – diss – cross – net – bal +1 –1 +2 –1 Mg(s) + HCl(aq)  MgCl2(aq) + H2(g) Mg + H+  Mg+2 + H2 2 Classify the reaction in two ways. Single-Replacement and Redox

4 Fe(s) + 3 O2(g) → 2 Fe2O3(s) Oxidation: lose e– charge ↑ Reduction: gain e– charge ↓ REDOX: transfer e– –2 +3 –2 –2 used to be gaining oxygen +3 +3 –2 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)

Mg(s) + S(s) → MgS(s) Metals: tend to be oxidized (lose e–) Nonmetals: tend to be reduced (gain e–) magnesium magnesium sulfide sulfur +2 –2 +2 –2 +2 –2 Mg(s) + S(s) → MgS(s)

LEO says GER Metals: tend to be oxidized (lose e–) Nonmetals: tend to be reduced (gain e–) LEO says GER +2 –2

CH4(g) + O2(g) → CO2(g) + H2O(g) -4 +1 +4 -2 +1 -2 CH4(g) + O2(g) → CO2(g) + H2O(g) Oxidized? (charge ↑) Reduced? (charge ↓)

NOT REDOX because… (NO e– loss or gain) 2NaF(aq) + CaCl2(aq) -1 +2 -1 +1 +1 -1 +2 -1 2NaF(aq) + CaCl2(aq) → 2NaCl(aq) + CaF2(s) Oxidized? (charge ↑) Reduced? (charge ↓) NOT REDOX because… NO charges changed (NO e– loss or gain)

Redox in Living Systems Photosynthesis O–2 is oxidized to O20 +4 –2 +1 –2 +1 –2 6CO2 + 6H2O → C6H12O6 + 6O2 C+4 is reduced to C0

Redox in Living Systems Cellular Respiration C0 is oxidized to C+4 +1 –2 +4 –2 +1 –2 C6H12O6 → 6CO2 + 6O2 + 6H2O O0 is reduced to O–2

1–4 Redox Synthesis A0 + B0 → A+B– Single Replacement A0 + B+X– → A+X – + B0 2H2 + O2  2H2O Zn + 2HCl  ZnCl2 + H2 Decomposition A+B – → A0 + B0 Combustion CxHy + O2 → CO2 + H2O – + +4 –2 + –2 H2O2  H2 + O2 CH4 + 2O2  CO2 + 2H2O

Quick Quiz! 1. During oxidation, an element _______ electrons. During reduction, an element ________ electrons. gains, loses loses, gains gains, donates gains, accepts

Quick Quiz. 2. Which element is reduced in the following chemical reaction? Fe2O3 + 3 CO  2 Fe + 3 CO2 O Fe C none (not redox) +2 –2 +3 –2 +4 –2