1. Chemical Reactions
Chapter 10

2. How Many Atoms in a Molecule?
Part I: Counting Atoms
How Many Atoms in a Molecule?

3. Counting Atoms
Most substances that we encounter are compounds, not elements.
• A chemical compound is a pure substance formed from the combination of two or more different elements.
The properties of the compound may be completely unlike those of the elements that form it.
• The formula for a compound lists the symbols of the individual elements followed by subscripts which indicate the number of atoms of that element.
(If no subscript is given, it is understood to be “1.”) E.g., NaCl, H2O, C12H22O11.

4. Counting Atoms
A molecular formula gives the actual number of atoms of each element in a molecule of a compound.
Hydrogen peroxide H2O2
Water H2O
Glucose C6H12O6
A structural formula uses lines to represent covalent bonds, and shows how the atoms in a molecule are joined together:
H—O—O—H
H—O—H
O=C=O

5. Counting Atoms Example: How Many Atoms? C6H12O6
6 C’s + 12 H’s + 6 O’s = 24 atoms
K3PO4
3 K’s+1 P’s + 4 O’s = 8 atoms
C2H5OH
2 C’s+ 5 H’s + 1 O’s + 1 H = 9 atoms
H2O2
2 H’s and 2 O’s = 4 atoms

6. Counting Atoms with Polyatomic Ions - Al2(SO4)3= S O

7. Counting Atoms with Polyatomic Ions
When counting atoms with polyatomic ions;
Count number of atoms in one polyatomic ion
Ions inside the parentheses
Multiply by number of polyatomic groups in the molecule (number outside the parenthesis)
Examples:
Al2(SO4)3 - 2 Al’s + 3(1 S + 4 O’s) = 2 + 3(5) = 2+15 = 17 atoms
Mg(NO3)2 – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 = 9 atoms

8. Hydrates
Hydrates are ionic compounds which also contain a specific number of water molecules associated with each formula unit. The water molecules are called waters of hydration.
The formula for the ionic compound is followed by a raised dot and #H2O
Example: MgSO4•7H2O.
They are named as ionic compounds, followed by a counting prefix and the word “hydrate”
CuSO4•5H2O copper(II) sulfate pentahydrate
BaCl2•6H2O barium chloride hexahydrate
MgSO4•7H2O magnesium sulfate heptahydrate (Epsom salts)

9. How Many Atoms in a Hydrate?
When counting atoms in the hydrate, count the water atoms also.
Example:
CuSO4•5H2O
1 Cu + 1 S + 4 O’s + 5(2 H’s + 1 O’s)
= (3)
= 6+15 = 21 atoms
BaCl2•6H2O
1 Ba + 2 Cl + 6(2 H’s + 1 O’s)
= 1+2+6(3)
= = 21 Atoms

10. Part II: Conservation of Mass

11. Conservation of Mass
In a normal chemical reaction, the mass of substances in a closed system will remain constant, no matter what processes are acting inside the system.
How ever many atoms a reaction starts with, ends with the same number.
Atoms don’t change their identity in a chemical reaction
Number of atoms for EACH ELEMENT STAYS THE SAME in a chemical reaction
The elements just rearrange their organization
The beginning MASS of the reaction EQUALS the ending MASS of the reaction

12. Conservation of Mass Total Mass stays the same in a chemical reaction
2g H2 + 16g O2 yields 18g H2O
Number and Identity of Atoms stays the same in a chemical reaction
2 H2 + 1 O2 yields 2 H2O O H H H O H O

13. Part III: Writing Reactions
How Do You Write a Chemical Reaction?

14. III. Chemical Reactions
Definition – process by which the atoms of one or more substances are rearranged
KEY: new substances are formed
KEY: No Atoms are Gained or Lost
A chemical reaction is the process by which atoms of one or more substances are rearranged into new substances
Chemical change occurs
How do you know?

15. III. Evidences of a Chemical Reaction
1) gas production
2) light production
3) temperature change (endo/exothermic)
4) precipitate formed (solid from 2 liquids)
5) permanent color change

16. III. Energy Changes
Energy is stored in compounds as chemical potential energy
due to specific arrangements of atoms.
A chemical reaction changes the potential energy present.

17. Energy Changes
When energy is lost as heat, it is called an __________________.
exothermic reaction
These reactions get hotter.
When energy is gained; heat is added for a reaction to occur. These are called ______________________,
endothermic reactions
These reactions get colder.
Energy in a reaction is shown with:
ΔH (heat) kJ
Joules
Heat
energy

18. III. Chemical Reactions
Representing Chemical Reactions:
Reactants – the ‘stuff’ you start with
An ‘arrow’ which means ‘yields’, or ‘becomes’
Products – the ‘stuff’ you end up with
Principle of “Conservation of Mass” applies to chemical reactions.
Why?

19. III. Chemical Reactions
Word Equations:
Reactant-A + Reactant-B yields Product-AB
Example:
Sodium(s) + Chlorine(g) → Sodium Chloride(s)
The small letters in paretheses () indicate the state of the reactant or product (solid, liquid, gas, or aqueous solution)
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous = dissolved in water

20. Part IV: Balancing Equations
Applying Conservation of Mass to Equations

21. VI. Chemical Equations Step 1: Write a Skeleton Equation
Skeleton Equation uses chemical formulas and symbols instead of words:
Words: Sodium + Chlorine gas yields Sodium Chloride
Symbols: Na(s) + Cl2(g) → NaCl
Skeleton Equations are not complete equations, but are the first step in writing a complete equation

22. IV. Chemical Equations Chemical Equation is BALANCED
Balanced means that “conservation of mass” is upheld
All atoms in reactants are also in products
No more, no less
Just rearranged

23. IV. Chemical Equations Balancing Equations
Use a number before the compound/element symbol to indicate how many of them are needed
Called a COEFFICIENT
Written in front of the atom/compound
KEY: Coefficient is a MULTIPLIER
Number of atoms per molecule is SUBSCRIPT
Change ONLY the COEFFICIENTS to balance the equation

24. IV. Chemical Reactions Steps to Balance Equations
Write the skeleton equation
Count the atoms of EACH element in the reactants
Count the atoms of EACH element in the products
Change the coefficients to make the number of atoms of each element equal on both sides of the equation
Write the coefficients in the lowest possible ratio
Check your work
NEVER CHANGE A SUBSCRIPT

25. IV. Chemical Equations Write the skeleton equation: Al + O2 → Al2O3
2. Count Number of atoms for each element on both sides
This is not balanced because the numbers don’t match
3. Multiply coefficients until they match – multiply the entire units 2
Al + O2 → Al2O3
Go to 6 Oxygens

26. IV. Balancing Equations
Al + O2 2
Al2O3
Multiply each atom by 2 4
Al + 2
Al2O3 O2 3
Balanced

27. IV. Balancing Equations 2
The work of balancing a chemical equation is in many ways a series of trials and errors.
Consider the equation given below.
Does this represent a balanced chemical equation?
N H  NH3

28. IV. Balancing Equations 3
To balance this reaction, it is best to choose one kind of atom to balance initially. Let's choose nitrogen in this case.
2 Nitrogen Atoms in Reactants requires 2 Ammonia molecules in Product to balance the nitrogen
2NH3 H2 + N2

29. IV. Balancing Equations 2
Once we know what the molecules are (N2, H2, and NH3 in this case) we cannot change them (only how many of them there are).
The nitrogen atoms are now balanced, but there are 6 atoms of hydrogen on the product side
only 2 of them on the reactant side.
The next step requires multiplying the number of reactant hydrogen molecules by three to give:
Balanced
                        
2NH3 H2 + N2
3H2

30. IV. Don’t Forget: Diatomic Elements
Definition – 7 elements that NEVER occur as singular atoms (always paired with an the same or different element)
H2 O2 F2 Br2 I2 N2 Cl2
Ex: HCl K  2 KCl H2

31. IV. Balancing Equations 3
1. Start with an unbalanced equation
2. Draw boxes around the compounds so you don’t mess with the groups
Don’t be threatened by how complex it looks!

32. IV. Balancing Equations 2
3. Make an element inventory – count number of atoms for each element on each side of the equation

33. IV. Balancing Equations 3
4. Write coefficients in front of each of the boxes until the inventory for each element is the same both before and after the reaction
Save Oxygen and Hydrogen for last, Treat Polyatomic like an atom.
Let’s start with Sodium
We have 2 in products, so I need 2 in reactants
Multiply reactant with sodium by 2 and recount atoms 2 3 1 H
SO4 O Na
Product
Reactant
Element
Balanced?
1 2 Y N
1 2 N Y
3 4 N Y

34. IV. Balancing Equations 3
Inventory Shows:
Reactant side has FOUR hydrogen atoms
Product side has TWO hydrogen atoms
Using your amazing powers of mathematics
two hydrogen multiplied two becomes four hydrogen
Balanced 2 4 1 H
SO4 O Na
Product
Reactant
Element
Balanced? Y
1 2 Y N
2 4 Y N Y

35. Helpful Hints Balance hydrogen and oxygen last
Balance polyatomic ions as a group if present on both reactants and products
You can consider a polyatomic ion as a single element
If the balancing starts to get very complex:
Stop
Start over
Select a different atom to balance first.

36. Example Using PolyAtomics
Before
MgCl2 + NaOH  Mg(OH)2 + NaCl
1 Mg
1 Na
1 Mg
1 Na
2 Cl
1 OH
1 Cl
2 OH
After
MgCl NaOH  Mg(OH) NaCl
1 Mg
2 Na
1 Mg
2 Na
2 Cl
2 (OH)
2 Cl
2 (OH)

37. Types of Chemical Reactions
Part V

38. Classifying Chemical Reactions
Synthesis
Decomposition
Single replacement
Double Replacement
Combustion

39. Synthesis
Definition – two or more substances react to form ONE product
Product is usually bigger or more complex than either reactant
A + B  AB

40. Hey baby let’s get jiggy.

41. Al3+ Cl1- Synthesis 2 3 2 ___Al + ___Cl2  ___AlCl3
reaction of two elements 2 3 2
___Al + ___Cl2 
___AlCl3
Al3+
Cl1-

42. CaO + H2O  Ca(OH)2 SO2 + O2  SO3 Synthesis 2 2
reaction of two compounds
reaction of element and compound
CaO + H2O 
Ca(OH)2 2
SO2 + O2  2
SO3

43. Decomposition
definition – one substance breaks down into two or more simpler products
AB  A + B

44. Break yoself fool!

45. Decomposition __ NaN3 (s)  ___ Na (s) + ___ N2 (g) __ CaO (s) 
Example reaction: 2 2 3
__ NaN3 (s) 
___ Na (s)
+ ___ N2 (g) 2 2 1
__ CaO (s) 
___ Ca (s)
+ ___ O2 (g)

46. Single Replacement Reactions
Definition – one element replaces another element in a compound to form new compound
A + BX 
AX + B

47. I’m gon’ dance with yo’ lady

48. A B + C D  AD + CB Double Replacement
Defn – exchange of cations between two ionic compounds
A B + C D 
AD + CB
switch

50. 3 possible products of double replacement reactions
Precipitate
Gas
Water

51. Reactivity Series (or Activity Series)
More active will replace less active
Less active will NOT replace more active
metals
Li K Ca Na Mg Al Mn Zn Fe Ni Sn Pb Cu Ag Au
most active
least active
halogens
F Cl Br I
most active
least active

52. + Al3+ O2- examples + Al Fe2O3 Fe Al2O3 Fe3+ O2- 2 1 2 1
aluminum + iron (III) oxide
Fe3+
O2-
Stronger? 2 1 2 + 1 Al +
Fe2O3 Fe
Al2O3
Al3+
O2-

53. + examples Ag CuNO3 Cu1+ NO31- silver + copper (I) nitrate NO RXN
Stronger? Ag +
CuNO3
NO RXN

54. + examples F2 NaBr NaF + Br2 1 2 2 1 Na1+ F1-
fluorine gas + sodium bromide
Stronger? 1 2 2 1 F2 +
NaBr
NaF +
Br2
Na1+
F1-

55. examples
chlorine gas + hydrogen fluoride
Stronger?
Cl2 + HF
NO RXN

56. Example Problem Li I Li1+ I1- Ag1+ NO31- Ag I (s) + AgNO3 LiNO3 +
lithium iodide and aqueous silver nitrate react
Li1+
I1-
Ag1+
NO31-
Li I
Ag I (s) +
AgNO3
LiNO3 +

57. Combustion definition – compound reacts with O2
Hydrocarbon – compound w/ only carbon and hydrogen

58. Combustion CxHy + O2 CO2 + H2O Combustion of hydrocarbons
ALWAYS produces CO2 and H2O
CxHy + O2
CO2 + H2O

59. I sell propane and propane accessories!
Ex problem
show combustion of propane (C3H8) gas 1
C3H8 + 5 O2 3
CO2 + 4
H2O
I sell propane and propane accessories!