1. Chemical Equations

2. Coefficient  subscript = # atoms
Chemical Equations (Shorthand communication for a great deal of information)
1. Write the unbalanced equation making sure each chemical formula is correctly written.
2. Count atoms on each side.
3. Add coefficients to make #’s equal.
Coefficient  subscript = # atoms
4. Reduce coefficients to lowest possible ratio, if necessary.
5. Double check atom balance!!!

3. Law of Conservation of Mass
In a chemical reaction, mass (or matter) is not created or destroyed.
Atoms can only rearrange to form new substances
“We may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal amount of matter exists both before and after the experiment. Upon this principle, the whole art of performing chemical experiments depends.”
--Antoine Lavoisier, 1789

4. Chemical Equations
Are concise representations of chemical reactions

5. Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

6. Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)
Reactants appear on the left side of the equation.

7. Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)
Products appear on the right side of the equation.

8. Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)
The states of the reactants and products are written in parentheses to the right of each compound.

9. Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)
Coefficients are inserted to balance the equation.

10. Subscripts and Coefficients Give Different Information
Subscripts tell the number of atoms of each element in a molecule

11. Subscripts and Coefficients Give Different Information
Subscripts tell the number of atoms of each element in a molecule
Coefficients tell the number of molecules, formula units or atoms

12. Chemical Equations 2 H2 + O2 2 H2O N2O5(g) + H2O 2 HNO3
Must have equal numbers of atoms of each element on each side of the equation = BALANCED EQUATION
NOTE
The coefficients in front of the formula for a compound refers to the number of particles (intact) involved while a subscript refers to the ratio of atoms within the particle
2 H2 + O2 2 H2O
4 hydrogen 4 hydrogen
2 oxygen 2 oxygen
N2O5(g) + H2O 2 HNO3
2 nitrogen 2 nitrogen
6 oxygen 6 oxygen
2 hydrogen 2 hydrogen

13. Never change subscripts in formulas when balancing chemical reactions!
Chemical Equations
Chemical Equations
balancing equations often requires some trial and error of coefficients
NOTE
NOTE
Never change subscripts in formulas when balancing chemical reactions!
Changing subscripts change the identity of compounds; coefficients change amounts
PCl3(l) + 3 H2O(l) H3PO3(aq) + 3 HCl
6 hydrogen 6 hydrogen
3 oxygen 3 oxygen
1 phosphorus 1 phosphorus
3 chloride 3 chlorine
C6H12(l) + 9 O2(g) CO2(g) + 6 H2O(l)
6 carbon 6 carbon
18 oxygen 18 oxygen
12 hydrogen 12 hydrogen

14. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
C2H4 + O CO2 + H2O

15. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
C2H4 + O CO2 + H2O
C C
H H
O O

16. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
C2H4 + O CO2 + H2O
C 2 C 1
H H 2
O 2 O 3

17. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
C2H4 + O CO2 + H2O
C 2 C (1)2= 2
H H 2
O 2 O 3 5
Chapt. 3.1

18. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
C2H4 + O CO H2O
C 2 C (1)2= 2
H H (2)2 = 4
O 2 O
Chapt. 3.1

19. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
C2H4 + 3O CO H2O
C 2 C (1)2= 2
H H (2)2 = 4
O(2)3 = 6 O

20. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
Al + HCl AlCl3 + H2

21. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
Al + HCl AlCl3 + H2
Al Al
H H
Cl Cl

22. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
Al + HCl AlCl3 + H2
Al 1 Al 1
H H 2
Cl Cl 3

23. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
Al + 3HCl AlCl3 + H2
Al 1 Al 1
H(1)3 = 3 H 2
Cl(1)3 = 3 Cl 3

24. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
Al + 6HCl AlCl H2
Al 1 Al 1
H(1)6 = 6 H (2)3 = 6
Cl(1)6 = 6 Cl 3

25. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
Al + 6HCl AlCl H2
Al 1 Al (1)2 = 2
H(1)6 = 6 H (2)3 = 6
Cl(1)6 = 6 Cl (3)2 = 6

26. Chemical Equations
Sample exercise: Balance the following equations by providing the missing coefficients:
2Al + 6HCl AlCl H2
Al(1)2 = 2 Al (1)2 = 2
H(1)6 = 6 H (2)3 = 6
Cl(1)6 = 6 Cl (3)2 = 6

27. Chemical Reactivity
Chemical Reactions
The course of a chemical reaction can often be predicted by recognizing general patterns of reactivity through similar reactions previously observed. Elements in same family (column of table) have similar reactions.
The periodic table is helpful in predicting products of reactions. Atoms like to assume electron configurations of the Noble Gases.

28. Example, if you know that
Chemical Reactivity
Chemical Reactions
Example, if you know that
2Li + 2H LiOH + H2
then you should be able to predict the products from the reaction of Na, K and the other members of group 1 (alkali metals) with water. Thus a general reaction would be;
2 M H2O 2 MOH + H2

29. Types of Chemical Reactions:
Direct Combination/Synthesis Reactions
Decomposition/Analysis Reactions
Single Replacement Reactions
Double Replacement Reactions
Combustion Reactions

30. Direct Combination or Synthesis
the combination of 2 or more substances to form a compound
only one product forms
A + B  AB
ex: 2P + 3Br2  2PBr3

31. Decomposition or Analysis
a compound breaks down into 2 or more simpler substances
only one reactant
AB  A + B
ex: 2H2O2  2H2O + O2

32. A + BC  AC + B ex: Zn + 2HCl  ZnCl2 + H2 Single Replacement
one element replaces another in a compound
metal replaces metal (+) OR
nonmetal replaces nonmetal (-)
A + BC  AC + B
ex: Zn + 2HCl  ZnCl2 + H2

33. ex: 2KOH + CuSO4  K2SO4 + Cu(OH)2
Double Replacement
ions in two compounds “change partners”
cation(+) of one compound combines with anion(-) of the other
AB + CD  AD + CB
ex: 2KOH + CuSO4  K2SO4 + Cu(OH)2

34. Combustion - means add oxygen
the burning of a substance in O2 to produce heat
Often releases energy in the form of heat and light
If the combustion is complete, the products will be CO2 and H2O.
If the combustion is incomplete, the products will be CO and H2O.
CxHy+O2  CO2+H2O
ex: CH4 + 2O2  CO2 + 2H2O

35. Direct Combination/Synthesis Reactions
- when two or more elements or compound(s) react to form a product (opposite of decomposition reactions)
A + B C
often energy (heat) is released
2 Na(s) + 3 N2(g) 2 NaN3(s)
(ex. Air Bag Inflator)

36. Decomposition/Analysis Reactions
when one compound reacts to form two or more elements or compounds (opposite of synthesis reactions)
C A + B
often energy (heat) is required
2 NaN3(s) Na(s) N2(g)
(ex. Air Bag Inflator)

37. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
Solid mercury (II) sulfide decomposes into its component elements when heated.

38. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
Solid mercury (II) sulfide decomposes into its component elements when heated.
Hg+2 S-2
HgS  Hg + S

39. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
The surface of aluminum metal undergoes a combination reaction with oxygen in air.

40. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
The surface of aluminum metal undergoes a combination reaction with oxygen in air.
Al+3 O-2
Al + O2  Al2O3

41. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
The surface of aluminum metal undergoes a combination reaction with oxygen in air.
Al+3 O-2
4Al + 3O2  2Al2O3

42. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
Si2H6 burns when exposed to air.

43. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
Si2H6 burns when exposed to air. Hint, Si is in the same group as C, and therefore reacts similarly.
Si2H6 + O2  SiO2 + H2O

44. Chemical Reactivity
Sample Exercise: Write balanced chemical equations for the following reactions:
Si2H6 burns when exposed to air. Hint, Si is in the same group as C, and therefore reacts similarly.
2Si2H6 + 7O2  4SiO2 + 6H2O

45. Single Replacement Reactions
- a free element becomes an ion and an ion in solution becomes a neutral atom
A + BC  B + AC
Cl2(g) + 2NaBr(aq)  Br2(l) + 2NaCl(aq)

46. Double Replacement Reactions
when ionic “partners” switch
AB + CD AD + BC
(often in aqueous solutions)
Ag(NO3) + KCl AgCl(s) + KNO3
BaCl2 + Na2SO4 BaSO4(s) + 2NaCl