1. Chapter 8: Chemical Reactions
College Prep Chemistry
Mr. Pompilii
Chapter 8: Chemical Reactions

2. Section 8.1: Describing Chemical Change
Word Equations: describes chemical reactions using words instead of symbols

3. Chemical Equations
Definition: chemical formulas written to express chemical and physical changes
Skeletal Equation: chemical equation that does not indicate the relative amounts of the reactants and products involved in the reaction

4. Section 8.1: Describing Chemical Change
Reactants -= those substances that are reacting or changing
written on the left
Products - those substances that are being formed from the reactants
written on the right

5. Section 8.1: Describing Chemical Change
Coefficient: number used to represent the amount of a substance in a chemical reaction
Located before the chemical

6. Symbols Used in Chemical Rxns
+ = used to separate two reactants or two products
 = “yields” separates reactants from products
↔ or  = used in place of the  for reversible rxns
(s) = solid state
(g) = gaseous state

7. Symbols Used in Chemical Rxns
(l) = liquid state; only used for water, mercury, bromine usually
(aq) = aqueous soln (made w/ H2O)
 or heat = written over the  to show heat was added
catalyst = the actual substance is written over the  to show the presence of a catalyst

8. Catalyst
Definition: substance that speeds up the rate of a reaction but that is not used up in the reaction Pt
Not a reactant 
Not a product
Formula written over the “yields” sign
Examples:
MnO2, Pt, heat, …

9. Practice Problems Write a skeletal equation:
1. Sulfur burns in oxygen to form sulfur dioxide
2. heating potassium chlorate in the presence of the catalyst manganese (IV) oxide produces oxygen gas and solid potassium chloride

10. Practice Problem Answers
1. S (s) + O2 (g)  SO2 (g)
MnO2, Δ
2. KClO3 (s) --- O2 (g) + KCl (s)

11. Practice Problem 2 Write a sentence: 3. KOH (aq) + H2SO4 (aq) 
H2O (l) + K2SO4 (aq)
4. Na (s) + H2O (l)  NaOH (aq) + H2 (g)

12. Practice Problem Answers 2
3. aqueous potassium hydroxide and aqueous sulfuric acid produces water and aqueous potassium sulfate
4. solid sodium is added to water hydrogen gas and aqueous sodium hydroxide are produced

13. Balancing Equations
Definition: each side of the equation has the same number of atoms of each element
Must obey the Law of Conservation of Mass
lance.html

14. Balancing Equations
Sometimes we write equations that are already balanced
C (s) + O2 (g)  CO2 (g)
Other times the skeleton equation is not balanced
C (s) + O2 (g)  CO (g)

15. Rules for Balancing Eqns
Write an atomic inventory
Balance all non-H’s and O’s first
Add a coefficient to the front of the substance with the lesser side. NEVER change a subscript!
Re-tally your atomic inventory after putting a coefficient in

16. Rules for Balancing Eqns
Balance all other non-H’s and O’s next.
Then balance H’s next
Then balance O’s last
Recheck inventory!

17. Example #1
H2 (g) + O2 (g)  H2O (l)

18. Example #1 Answer
2 H2 (g) + O2 (g)  2 H2O (l)

19. Example #2
NaCl (s) + H2SO4 (aq) 
Na2SO4 (s) + HCl (aq)

20. Example #2 Answer
2 NaCl (s) + H2SO4 (aq) 
Na2SO4 (s) + 2 HCl (aq)

21. Example #3
Fe (s) + O2 (g)  Fe2O3 (s)

22. Example #3 Answer
4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s)

23. Balancing Helpful Hints
If a polyatomic ion exists as the same on each side of the , you do not have to split the atoms
Only diatomic elements can have ½ or decimal coefficents

24. Diatomic Elements 7…
N2, O2, F2, Cl2, Br2, I2, H2

25. Sect Types of Rxns
1. Combination (Synthesis) rxns
2. Decomposition rxns
3. Single-Replacement rxns
4. Double-Replacement rxns
5. Combustion rxns
(Later) 6. Oxidation / Reduction rxns
(Later) 7. Acid / Base rxns

26. Sect Types of Rxns
In terms of the relationships…
1. Combination = marriage
2. Decomposition = divorce
3. Single-Replacement = “the other man or the other woman”
4. Double-Replacement = “wife swap”
5. Combustion = “the argument”

27. 1.Combination Rxns (synthesis)
Definition: two or more substances react to form a single substance
The reactants are normally two elements
General equation: (marriage) A + B  AB

28. 1. Combination Rxns Examples
1. Synthesis of an element and oxygen to form an oxide
4 Al (s) + 3 O2 (g) 2 Al2O3
Al O2-
2. Synthesis of a metal and a nonmetal forms an ionic compound Mg (s) + N2 (g)  Mg3N2
Mg N3-

29. How to Predict Equations
A + B  (2 elements added together)
Combination Rxn
1. Determine charges of each atom (review ion sheet)
2. Form a neutrally charged product
3. Balance equation
*Don’t let diatomic elements throw you off!*

30. 2. Decomposition Rxns
Definition: a single compound is broken down into two or more elements
Don’t forget the 7 diatomics!
Requires energy in the form of heat, light, or electricity (written above the  sign)
General formula
(divorce) AB  A + B

31. Decomposition Examples
electricity
2 H2O (l) --- 2 H2 (g) + O2 (g) 
2 HgO (s)  2 Hg (l) + O2 (g)

32. How to Predict Equations
AB  (1 compound)
Decomposition Rxn
1. Break compound into elements.
2. Be mindful of diatomic elements.
3. Balance equation.
*Usually under the presence of heat () or electrolysis!*

33. 3. Single-Replacement Rxns
Definition: atoms of an element replace the atoms of a second element in a compound
Two Types (the other man/woman)
1. Metal replacing a metal
General Rxn: A + BC  AC + B
2. Nonmetal replacing a nonmetal
General Rxn: A + BC  BC + A

34. 3. Single-Replacement Rxns
1. Metal replacing a metal
General Rxn: A + BC  AC + B
Metals can only replace metals and will be shown as the first substance (cation replaces cation)
Must consult the Activity Series of Metals to see if the reaction will take place
Activity Series of Metals: lists metals in order of decreasing reactivity

35. Activity Series of Metals
The reactive metals will replace any metals below it
A non-reactive metal will not replace a metal above it = NO RXN

36. 3. Single-Replacement Rxns
Ex) iron will replace copper
Fe + CuCl2  FeCl2 + Cu
Fe2+ Cu2+ Cl-
Ex) but copper will not replace iron… Cu + FeCl2  NO RXN

37. 3. Single-Replacement Rxns
2. Nonmetal replacing a nonmetal
General Rxn: A + BC  BC + A
Nonmetals can only replace nonmetals and will be shown as the second substance in the compound (anion replaces anion)
Nonmetal replacement limited to the halogens
Activity decreases as you go down group 17 on the periodic table

38. 3. Single-Replacement Rxns
Cl2 + 2 NaBr  2 NaCl + Br2
Cl Na+ Br (Cl can replace Br)
I2 + NaBr  NO RXN
I Na+ Br (I cannot replace Br)

39. How to Predict Equations
A + BC  (1 atom & 1 compound)
Single Replacement Rxn
1. Identify ions of each atom.
2. Determine what can replace what (cation-to-cation OR anion-to-anion).
*Don’t let diatomic elements throw you off!*

40. How to Predict Equations
Single Replacement Rxn (cont’d)
3. Consult Activity Series Chart! (only elements higher on the chart can replace an atom in a compound!)
4. Make new neutral compound and atom.
5. Balance equation!

41. 4. Double Replacement Rxns
Definition: involve an exchange of positive ions between compounds
Between two ionic compounds in aqueous solutions
Generic formula (wife swap)
AB + CD  AD + CB
A+ B- C+ D-

42. 4. Double Replacement Rxns
For the rxn to occur, at least one of the following must be true for at least one of the products of the rxn
1.) precipitate (ppt or ↓), a solid is formed
2.) gas is formed (↑), ie) H2S ↑
3.) molecular compound, ie) H2O (l), is formed

43. 4. Double Replacement Rxns
Solubility Table: table that shows the solubility of two ions
Cations on left & anions on top
Where they intersect is either:
S = Soluble  meaning an aqueous substance is formed and a (aq)
I = Insoluble – meaning ppt is formed and a ↓

44. 4. Double Replacement Rxns
2 NaOH (aq) + H2SO4 (aq)  Na2SO4 (aq) + 2 H2O (l) Driving force is the formation of water
BaCl2 (aq) + K2CO3 (aq)  BaCO3  + 2 KCl (aq) Driving force is the formation of a precipitate
FeS (s) HCl (aq)  H2S  + FeCl2 (aq) Driving force is the formation of a gas

45. 4. Double Replacement Rxns
Common Solubility Rules:
All nitrates are soluble
All potassium compounds are soluble
Others on Salmon Sheet
If there is no ppt, gas, or water made and only aqueous substances made – NO RXN!
AgNO3 (aq) + KCl (aq)  AgCl (aq) + KNO3 (aq)
NO RXN!

46. How to Predict Equations
AB + CD  (2 aqueous compounds)
Double Replacement Rxn
1. Identify ions of each atom.
2. Switch the cations by bonding the outside ions together and the inside ions together.

47. How to Predict Equations
Double Replacement Rxn (cont’d)
3. Make new neutral compounds.
4. Consult Solubility Table and match up new ion pairs.
S = soluble, place an (aq) after the cmpd
I = insoluble, place a  after the cmpd
5. Balance equation!

48. 5. Combustion Rxns CxHy + O2  CO2 + H2O
Definition: hydrocarbons react with oxygen, often producing energy in the form of heat and light
Products are always carbon dioxide and water
General equation: (the argument)
CxHy + O2  CO2 + H2O

49. benzene: methyl alcohol: 5. Combustion Rxns
C6H6 (l) O2  6 CO H2O
methyl alcohol:
CH3OH (l) O2  CO2 + 2 H2O

50. How to Predict Equations
CxHy + O2  (hydrocarbon + oxygen)
Combustion Rxn
1. Products are always CO2 + H2O
2. Balance equation.
X + O2  (metal + oxygen)
Combustion & Combination Rxn
1. Follow steps for combination rxns.

51. Your Turn! Predict the Following!
1. PbO2 
2. Ca + S 
3. C5H12 + O2 
4. NaCl + K2SO4 
5. Ca + Fe2O3 

52. Your Turn! Predict the Following!
1. PbO2  1 compound
2. Ca + S  2 elements
3. C5H12 + O2  hyrdrocarbon & O2
4. NaCl + K2SO4  2 compounds
5. Ca + Fe2O3  1 element & 1
compound

53. Your Turn! Predict the Following!
1. PbO2  divorce
2. Ca + S  marriage
3. C5H12 + O2  argument
4. NaCl + K2SO4  wife swap
5. Ca + Fe2O3  the other man

54. Your Turn! Predict the Following!
1. PbO2  Pb (s) + O2 (g)
2. Ca + S  CaS
3. C5H12 + O2  CO2 + H2O (balance!)
4. NaCl + K2SO4  Na2SO4 (aq) + KCl (aq)
 NO RXN! = BOTH (aq)
5. Ca + Fe2O3  CaO + Fe (s)

55. Section 8.3: Complete Ionic Eqns
Definition: an eqn that shows dissolved ionic compounds as their free ions
All aqueous compounds actually exist as ions free floating in the solution
Therefore, they are written as aqueous ions
AgNO3 (aq) = Ag+ (aq) + Cl- (aq)

56. Complete Ionic Eqns Reactants really exist as and are written as:
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
Reactants really exist as and are written as:
Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) …
Only aqueous products are written as ions:
… AgCl (s) + Na+ (aq) + NO3- (aq)

57. Complete Ionic Eqns Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) …
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
Ions that appear on both sides did NOTHING in the reaction and are referred to as spectator ions:
Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) …
AgCl (s) + Na+ (aq) + NO3- (aq)
Spectator Ions: Na+ (aq) + NO3- (aq)

58. Complete Ionic Eqns Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) …
AgCl (s) + Na+ (aq) + NO3- (aq)
Spectator Ions: Na+ (aq) + NO3- (aq)
Can be omitted from both sides of the original eqn
Net Ionic Eqn: shows the particles that actually take part in the rxn (making ↓, ↑, or H2O)

59. Complete Ionic Eqns Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) …
AgCl (s) + Na+ (aq) + NO3- (aq)
Spectator Ions: Na+ (aq) + NO3- (aq)
Net Ionic Eqn:
Ag+ (aq) + Cl- (aq)  AgCl (s)

60. Complete Ionic Eqns
Your turn: write the complete ionic equation and net ionic equations for the following reaction. Identify the spectator ions as well.
Ex) Aqueous calcium hydroxide reacts with aqueous phosphoric acid

61. Complete Ionic Eqn Ex Calcium phosphoric Hydroxide (aq) + acid (aq) 
Ca2+ OH H+ PO43-
Ca(OH)2 (aq) + H3PO4 (aq)  ? + ?

62. Complete Ionic Eqn Ex Complete Ionic:
Ca(OH)2 (aq)+H3PO4 (aq)  Ca3(PO 4)2 ↓ + H2O (l)
Complete Ionic:
Ca2+ (aq) + OH- (aq) + H+ (aq) + PO43- (aq) 
Ca3(PO 4)2 (s) + H+ (aq) + OH- (aq)
Spectator Ions: H+ (aq) + OH- (aq)
Net Ionic Eqn:
Ca2+ (aq) + PO43- (aq)  Ca3(PO 4)2 (s)
Balance Ca2+ (aq) + 2 PO43- (aq)  Ca3(PO 4)2 (s)

63. Chapter 8 Review Questions
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#32, 34-39, 40, 43-48