1. Unit 9: Chemical Reactions and Redox

2. 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

3. 4 Clues of Chemical Change
Demo:
Zn + H+  Zn H2
Pb 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.

4. 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

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

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

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

8. 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!!!

9. 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.
Al CuCl2  Cu AlCl3 Al Cu Cl
2 
 2
3 
6   6
 3

11. 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)

12. 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

13. 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

14. 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.

15. __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 B. 2 C. 3 D. 6
2 Al HCl → 2 AlCl H2 1 1 1 2 1 3

16. 5 Types of Reactions 1. Synthesis 2. Decomposition
3. Single Replacement
4. Double Replacement
5. Combustion

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

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

19. 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

20. _____ > _____ > _____
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
_____ > _____ > _____

21. 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

22. 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

23. 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

24. 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

25. 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 CO H2O
A. double-replacement
B. decomposition
C. single-replacement
D. combustion

26. 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

27. 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 + SO decomposition
C. H2SO4  2H2O2 + SO2 combination
D. H2SO4  2H2O2 + SO2 decomposition

28. 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.

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

30. 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

31. 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–

32. 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–

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

34. 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 H2 2
Classify the reaction in two ways.
Single-Replacement and Redox

35. 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)

36. 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)

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

38. 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 ↓)

39. 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)

40. 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

41. 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

42. 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

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

44. Quick Quiz.
2. Which element is reduced in the following chemical reaction?
Fe2O CO  2 Fe CO2 O Fe C
none (not redox) +2 –2 +3 –2 +4 –2