1. Chemical Reactions
Chapter 8

2. Chemical reactions…
involve a rearrangement of the ways atoms are grouped together.

3. 2H2 + O2 → 2H2O Chemical Reaction – Ex:
Remember, in a chemical change, a new substance is formed!
2H2 + O2 → 2H2O

4. Signs a Chemical Reaction has occured
Color Change
Solid forms
Bubbles form
Heat and/or flame produced or heat is absorbed

6. 2H2 + O2 → 2H2O Parts of a chemical reaction:
Reactants
Products
Yield
Sign
Reactants – substances that are reacting; the starting substances
Products – new substances that are made; substances formed during reaction

7. Physical states of compounds are often given in a chemical equation.

8. Conservation of Mass
- During a chemical reaction, mass is not created or destroyed.
– Mass is conserved!
-All atoms present in the reactants must be present in the products.
-Same number of each type atom on both sides of arrow
reactants
products

9. The equation is balanced!
Balancing Chemical Equations
- The number of atoms of each element must be the same on both sides of the yield sign.
2H2 + O2 → 2H2O
4 Hydrogen
2 Oxygen
4 Hydrogen
2 Oxygen
The equation is balanced!

10. When balancing an equation, you can only change the coefficient before the compound. Never change the subscript!
2Ca(NO3)2
coefficient
subscript

11. Multiply the coefficient by any subscripts to determine the number of atoms of each element.
2 calcium atoms
4 nitrogen atoms
12 oxygen atoms
For Oxygen:
For Calcium:
For Nitrogen:
2Ca(NO3)2 2 2
x___ 6
distribute - just like math class!
coefficient
subscript
2 x 1 = 2
2 x 2 = 4
2 x 6 = 12

12. Balancing equations: Al + O2  Al2O3
Step 1: Determine the formulas for reactants and products
Step 2: Write reactants on the left and products on the right
Example:
Aluminum + oxygen  aluminum oxide
Al + O2  Al2O3

13. Step 3: Count the number of each element in the reactants and products.
Al + O2  Al2O3
1 Al, 2 O 2 Al, 3 O
Step 4: Balance the elements one at a time by using coefficients (you should not write 1s). Do not change the formulas as you balance!
4Al + 3O2  2Al2O3
4 Al, 6 O Al, 6 O

14. Finally,
Step 5: Check to make sure each atom is balanced, and make sure all the balanced coefficients are in the lowest possible ratio.

15. CH4 + 2O2 → CO2 + 2H2O Reactants 1 carbon atom 4 hydrogen atoms
2 oxygen atoms
Products
1 carbon atom
2 hydrogen atoms
3 oxygen atoms
4 hydrogen atoms
4 oxygen atoms
4 oxygen atoms
What went in came out…
the reaction is balanced!

16. CH4 + 2O2 → CO2 + 2H2O What went in came out…
the reaction is balanced!

17. 2Al + 6HCl → 2AlCl3 + 3H2 Reactants 1 aluminum atom 1 hydrogen atom
1 chlorine atom
Products
1 aluminum atom
2 hydrogen atoms
3 chlorine atoms
2 aluminum atoms
2 aluminum atoms
6 hydrogen atoms
6 hydrogen atoms
6 chlorine atom
6 chlorine atoms
Congratulations…
it’s balanced!

18. Practice 1. The reaction below shows the process of oxidizing iron
Practice 1. The reaction below shows the process of oxidizing iron. Balance the equation by calculating the coefficients.
2, 3, 1
1, 1, 1
4, 3, 2
4, 2, 2 4 3 2

19. Practice 2. Which chemical equation
Practice 2. Which chemical equation supports the law of conservation of mass?
2H2O → H2 + O2
Zn + HCl → ZnCl2 + H2
Al4C3 + H2O → CH4 + Al(OH)3
CH4 + 2O2 → CO2 + 2H2O

20. 2 2
Practice 3. What are the coefficients that will balance the chemical reaction above?
2, 1, 1
3, 4, 2
2, 2, 1
4, 3, 2

21. 2K + 2H2O → 2KOH + H2
Practice 4. What is the coefficient for H2O when the equation above is balanced? 1 2 3 4

22. Ways to Classify Reactions
There are 5 general types of chemical reactions:
Combination
Decomposition
Single-replacement
Double replacement
Combustion

23. Types of Reactions
Combination or synthesis reactions- occur when two elements combine to form a single compound as a product.
X + Y  XY
A synthesis reaction may also involve the combination of an element plus a compound or the combination of two compounds, always forming a single compound as a product.

24. Examples:
Ca + Cl2  CaCl2
PCl3 + Cl2  PCl5
Na2O + H2O  2NaOH
Rules to remember:
Any metallic oxide plus water yields a metallic hydroxide, as in
K2O + H2O  2KOH
Any nonmetallic oxide plus water yields an acid
N2O H2O  2HNO3

25. Decomposition reaction- occurs where one compound breaks down into two or more elements or compounds. Heat, electricity, or a catalyst is usually necessary.
XY  X + Y
Examples:
2NaCl  2Na + Cl2
H2SO3  H2O + SO2

26. Rules to remember:
When heated, metal carbonates break down to yield metal oxides and carbon dioxide
Example: MgCO3  MgO + CO2
Hydrogen peroxide breaks down into water and oxygen gas
H2O2  H2O + O2
Carbonic acid decomposes into water and carbon dioxide
H2CO3  H2O + CO2

27. Single replacement reaction (or single displacement)…
occurs when one element displaces another element in a compound.
A new element and a new compound are formed.
A + BC  B + AC

28. Details about single replacement reactions…
Active metals displace less active metals or hydrogen from their compounds in aqueous solution. Consult an activity series of metals.
A + BY  AY + B

30. In general, Group I and Group II metals and aluminum are most active, transition metals are more active than hydrogen, and the jewelry metals (Cu, Ag, Au) and Hg are the least active. You do not need to memorize the activity series.
Examples:
Mg + NiCl2 
Zn + CuSO4 
2HNO Pb 

31. More single replacement…
Some nonmetals can also displace other nonmetals: active halogens replace less active halogens from their compounds in aqueous solution.
Fluorine at the top of group 7A is the most active, with activity decreasing as you move down the group.
MX + Y  MY + X

32. b) Some nonmetals can also react in single replacement reactions: active halogens replace less active halogens from their compounds in aqueous solutions. Fluorine at the top of the group is the most active, with the activity decreasing as you move down the group.
Examples:
2NaCl + F2  NaF + Cl2
Cl KI  2KCl + I2

33. Double replacement reactions
Two compounds (in aqueous solution) react to form two new compounds. These are also called exchange reactions.
AB + CD  AD + CB

34. Double replacement reactions require one of three kinds of chemical change:
Formation of a precipitate (solid)
Formation of a gas
Formation of water

35. Types of double replacement…
Formation of a precipitate
(a solid product)
BaNO3(aq) + K2CrO4(aq)  2KNO3(aq) + BaCrO4(s)

36. Types of double replacement…
Formation of a gas
Since H2CO3 and H2SO3 are unstable, if produced in a reaction, they will decompose.
H2CO3 decomposes into H2O and CO2.
H2SO3 decomposes into H2O and SO2.

37. Types of double replacement…
Formation of water
Includes acid-base reactions…
acid + base  water and salt
HCl + NaOH  NaCl + H2O

38. Combustion Reactions Involve combining with oxygen
These reactions produce energy so rapidly that a flame results
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)