1. Chemical Equations and Reactions
Unit 8
Chemical Equations and Reactions

2. Unit 8 Goals
Describe evidence and characteristics of chemical reactions
Write balanced chemical equations
Identify the types of reactions
Determine if an ionic compound is soluble or insoluble in water
Describe a precipitation reaction and relate it to solubility
Predict if a precipitate will form or not using solubility rules
Predict the products and balance molecular equations of precipitation reactions

3. Thinker:
List observations that indicate a chemical reaction has taken place in the following situations:
A cut apple turns brown
An egg changes when it cooks
A log burns
A car rusts

4. Reminder: Chemical Reactions are indicated by: Hallmark: Bubbling
Color change
Energy released as heat and light
Formation of solids
Hallmark:
At least one chemical has changed its formula

5. Writing Reactions Several ways to write equations:
Always have reactants on left
Always have products on the right
Word Equations
Methane + Oxygen  Carbon Dioxide + Water
Formula Equation
CH4 + O2  CO2 + H2O
Balanced Formula Equations
We’ll learn how to do this tomorrow
CH4 + 2O2  CO2 + 2H2O

6. Energy in Reactions Some reactions require energy Endothermic
Bonds breaking overall
Ex:
Dinitrogen tetroxide + energy  nitrogen dioxide
N2O4 + energy  NO2

7. Energy in Reactions Other reactions make energy Exothermic
Bonds created overall
Ex:
Methane + Oxygen  Carbon dioxide + Water + Energy
CH4 + O2  CO2 + H2O + energy

8. Symbols in Reactions Equations are like recipes (s) - solid
(l) - liquid
(g) - gas
(aq) - aqueous (dissolved in water)
 - yields
- reversible reaction
∆ - reactants are heated
Pd, Mn, etc. - catalyst

9. Homework
Reread p
On p. 266 do SR 11-17

10. What is Wrong With These Reactions?

11. HW Answers (side 1) NH3 + HCl  NH4Cl CaCO3 ∆→ CaO + CO2
BaO + H2O  Ba(OH)2
CH3CHO  CH4 + CO
Zn + Cu(NO3)2  Zn(NO3)2 + Cu
CaSO3 ∆→ CaO + SO2
Fe + H2SO4  FeSO4 + H2
C2H6N2 ∆→ C2H6 + N2
CO + Cl2  COCl2
MnI2  Mn + I2

12. HW Answers (side 2) N2O5 + H2O  HNO3 Mg + TiCl4  MgCl2 + Ti
C + ZnO  Zn + CO2
Br2 + NaI  NaBr + I2
PCl3 + Cl2  PCl5
P + Br2  PBr3
CaH2 + H2O  Ca(OH)2 + H2
H2SO4 + KOH  K2SO4 + H2O
C3H8 + O2 ∆→ CO2 + H2O
C6H6 + O2 ∆→ CO2 + H2O

13. Chemical Reactions: Conserve Mass Therefore: What happens?
No mass is created
No mass is destroyed
Therefore:
No atoms are created
No atoms are destroyed
What happens?
Bonds (energy) are created, destroyed or rearranged

14. What does this mean? Formation of Water: H2 + O2  H2O
The above equation is not correct!
It does not conserve matter.
H – 2 | H - 2
O – 2 | O - 1
We need to balance the equation
# of atoms at start = # of atoms at end

15. Balancing Equations
Purpose: To show how much of each reactant is needed to make a certain quantity of product.
What can we do?
Add coefficients to each formula
Acts as a multiplier for that formula
CO2 2CO2 2Ba(OH)2
C – 1 C – 2 Ba – 2
O – 2 O – O – 4
H – 4

16. General Rules for Balancing
Identify reactants & products
Write equation, predict products as necessary
Count atoms
Look for polyatomic ions
Insert coefficients
Balance one atom/polyatomic at a time
Delay balancing elements appearing in more than 1 formula on each side.
Do not rewrite formulas!
Do not change subscripts!
Odd-even technique
Verify your results

17. Example:

18. Let’s try these Ca2Si + Cl2  CaCl2 + SiCl4 P4 + O2  P2O5
C3H8 + O2  CO2 + H2O
C3H8 + 5O2  3CO2 + 4H2O
Is this correct?
2 P O2  4 P2O5

19. Homework:
Balancing Chemical Equations Worksheet

20. Thinker: What do the following terms mean to you? Synthesis
Decomposition
Displacement
Talk with others around you.
Compare definitions & examples.

21. Classifying Reactions
Classified to help predict products
Five Basic Types:
Synthesis
Decomposition
Single Displacement
Double Displacement
Combustion
. . . more, but beyond scope of this unit.

22. Combustion Often used to generate energy
Gas, coal, starches, alcohol, sugars, cellulose, oil, etc. (has C compound)
Ex:
CHX + O2  H2O + CO2 + energy
C3H8 + 5O2  3CO2 + 4H2O + energy
CH3CH2OH + 3O2  2CO2 + 3H2O + energy

23. Synthesis Greek - “to put together”
2 or more formulas combine to form 1 formula
Ex:
A + B  AB
C + O2  CO2
2C + O2  CO
2Na + Cl2  2NaCl2
CaO(s) + H2O(l)  Ca(OH)2(s)

24. Decomposition Opposite of synthesis Binaries usually  elements
Compounds 3+ usually  elements
Ex:
AB ∆ A + B
CaCO3 ∆ CaO + CO2
2KClO3 ∆ 2KCl + 3O2
Mg(OH)2 ∆ MgO + H2O
Air Bags: NaN3(s) ∆ Na(s) + N2(g)

25. Single Displacement A single atom replaces another in a compound.
Like charges exchange places:
Metals replace metals or H
Nonmetals replace nonmetals
Ex:
A + BC  AC + B
2Al(s) + 3CuCl2(aq)  2AlCl3(aq) + Cu(s)
Cu(s) + AgNO3(aq)  CuNO3(aq) + Ag(s)

26. Reactivity (Activity Series)
Just because we put chemicals together will there be a reaction?
Not necessarilly
See the Activity Series (p.281 or Appendix A-9 p.832)
More Rx elements will replace less Rx elements in compounds
Ex:
K will replace H in H2O (forms KOH + H2)
Cu will replace Ag in AgNO3

27. Will the following React?
Ag in Cu(NO3)2
Cu in AgNO3
K in Mg(ClO)2
Al in ZnCrO4
Zn in Al2(CrO4)3
CuCl2 containing Pb
CuCl2 containing Ag No
Yes

28. Double Displacement Similar to single displacement
2 reactants; 2 products
Like charges exchange places
Must result in: a solid, a gas, or a molecule
Ex:
AB + YZ  AZ + YB
2KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2KNO3(aq)
BaO + H2O  Ba2OH + H2

29. Questions?
Rx Type Flowchart on p 284
HW:
Read section 8-3
S.R. 1-10

30. Thinker:
What is a spectator?
What role does a spectator play?

31. Writing Net Ionic Equations
What is the purpose to outlining?
Summarizing important information
How does this relate to spectators and reactions?
Hint:
Are there any types of reactions where some things do not change (phase)?
Yes: Ionic Equations (1 & 2 displacement)

32. Ionic Compounds Dissolve in H2O Ions separate from each other Thus:
2KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2KNO3(aq)
Looks more like:
2K+(aq) + 2I-(aq) + Pb2+(aq) + 2(NO3)-(aq)  PbI2(s) + 2K+(aq) + 2NO3-(aq)

33. But Wait!
2K+(aq) + 2I-(aq) + Pb2+(aq) + 2(NO3)-(aq)  PbI2(s) + 2K+(aq) + 2NO3-(aq)
This is too long to write.
Let’s outline the net equation.
What does net mean?
Hint: What does net profit mean?
We can remove the spectator ions.
Leaving us with:
2I-(aq) + Pb2+ PbI2(s)

34. Another Example: Single Displacement This Time Formula Equation
Word Equation
Zinc + Copper (II) Sulfate  Copper + Zinc Sulfate
Formula Equation
Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq)
All Ions:
Zn(s) + Cu2+(aq) + SO42-(aq)  Cu(s) + Zn2+(aq) + SO42-(aq)
Net Ionic Equation:
Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq)