1. St. 11: Introduction to Chemical Reactions
Making new substances

2. Main Ideas Chemical Reactions are represented by Chemical Equations.
Chemical Equations are balanced to show the same number of atoms of each element on each side.
The Law of Conservation of Mass says that atoms won’t be created or destroyed in a chemical reaction. That is why you have to balance chemical equations!

3. Objectives Level 3 Is able to balance chemical equations.
Is able to interpret the 5 types of chemical reactions.
Level 2
Is able to identify the number of atoms in a formula
Is able to understand the concept of conservation of mass as related to the balancing of chemical equations.

4. Chemical Reactions are Everywhere
Cooking
Respiration

5. Chemical Reactions are Everywhere
Hair Dye
Auto Fuel

6. What do you think is going to happen?
Demo
Elephant Toothpaste
What do you think is going to happen?

7. What Happened during Demo?
What did you observe during demonstration?
Share your observations with someone sitting next to you

8. How do you know when a chemical reaction takes place?
Color Change
Precipitate Formation

9. How do you know when a chemical reaction takes place?
Gas Formation
Odor

10. How do you know when a chemical reaction takes place?
Temperature Change
Change in Acidity

11. Recognizing formulas and counting the # of atoms
1. Each element has its own chemical symbol. Ex. Sodium- Na or Chlorine- Cl
2. Each compound has its own chemical formula. Ex. NaCl or Sodium Chloride
A formula tells us two things:
The type of elements that the compound is made of
The number of atoms of each element for that compound
NaCl Fe2O3 Be(OH)2
1 atom Na atoms of Fe 1 atom of Be
1 atom of Cl atoms of O 2 atoms of O
2 types of elements types of elements 2 atoms of H types of elements

12. What if there are parentheses?
In the case of parentheses, multiply the subscript throughout the number of atoms that are in the parentheses to get the total number of those atoms.
Ex. Be(OH)2- Take 2 x 1 H and 2 x 1 O, so there are 2 H and 2 O
Ex. (NH4)2SO4- Take 2 x 4 H and 2 times 1 N, so there are 8 H and 2 N

13. Representing Chemical Reactions
Chemists observe chemical reactions and have come up with a way to represent or model what is happening.
Making NaCl
Solid Sodium combines with Chlorine gas to make solid Sodium Chloride:
2Na (s) + Cl2 (g)  2NaCl

14. Chemical Equations are different from Numerical Equations
Numerical Equation: 3x + 2y = 47
Chemical Equation 2Na + Cl2  2NaCl
ReactantA + Reactant B  Product
The reactants are used up in forming the product
The arrow  shows the direction of the reaction

15. Symbols used in Chemical Equations
Purpose +
Separates more than one reactant or product 
Separates reactants from products. Indicates direction of reaction
(s)
Identifies a solid state
(aq)
Identifies that something is dissolved in water
(l)
Identifies liquid state
(g)
Identifies gaseous state

16. Law of Conservation of Mass
In a chemical reaction, matter is neither created nor destroyed.
Atoms won’t change their identity (e.g. a Carbon atom can’t become an Iron atom)
This means that you have to have the same number of each type of atom on each side of the chemical equation.
Conservation of Mass Video

17. How would you balance this equation?
Law of Conservation of Mass Video
Balancing Equations
After you write a chemical equation you have to balance it to make sure that the same number of atoms of each element are on each side.
How would you balance this equation?
Li + H2O  H2 + LiOH

18. Steps to Balancing a Chemical Equation
6. Check your work
5. Write the Coefficients in their lowest possible ratio
4. Use coefficients to balance atoms on each side of the arrow.
2Li(s) + 2 H2O  H2(g) + 2LiOH(aq)
3. Count the atoms of the elements in the products
1 atom Li, 3 atoms H, 1 atom O
2. Count the atoms of the elements in the reactants
1 atom Li, 2 atoms H, 1 atom O
1. Write the Skeleton Equation
Li(s) + H2O(l)  H2 (g) + LiOH (aq)

19. Steps to balancing equations (from previous slide)
1. Write/Read the Skeleton Equation
2. Count the atoms of the elements in the reactants
3. Count the atoms of the elements in the products
4. Use coefficients to balance atoms on each side of the arrow. (the coefficients multiply the # of atoms of the elements in that compound or molecule)
5. Write the Coefficients in their lowest possible ratio
6. Check your work

20. Balancing Chemical Equations
Use coefficients to balance equations.
Coefficients: Small whole numbers placed in front of the formulas in an equation.
A chemical equation is balanced when:
Atoms of reactants = atoms of products

21. Another Example 7 ≠ 6! CH4 (methane gas) + O2  CO2 + H2O
Where did our atoms go?
Reactants
Products
# of Carbons = 1
# of Hydrogens = 4
# of Hydrogens = 2
# of Oxygens = 2
# of Oxygens = 3
Total atoms = 7
Total atoms = 6

22. Example Continued
Change the Coefficients to make the number of atoms of each element equal
Balance the Hydrogens:
CH4 + O2  CO H2O
Balance the Oxygens:
CH O2  CO H2O

23. Example Continued CH4 + 2 O2  CO2 + 2 H2O
Are your coefficients in their simplest ratio?
Count your atoms again to check your work:
Reactants
Products
# of Carbons = 1
# of Hydrogens = 4
# of Oxygens = 4
Total atoms = 9

24. Think – Pair - Share Try These! C2H6 + O2  CO2 + H2O
Fe2O3 + H2SO4  Fe2(SO4)3 + H2O
Hint : balance the polyatomic ion first!
CaCl2 + AgNO3  AgCl + Ca(NO3)2
Think – Pair - Share

25. Again, be sure you know:
How many atoms? 2Ca3(PO4)2

26. Try these!
Zn HCl  H ZnCl2
Al HCl  AlCl H2
SnS O2  SnO SO2

27. Try a few more!
C2H O  CO H2O
Ba(NO3) H2SO4  BaSO HNO3
FeCl (NH4)3PO4  FePO NH4Cl

28. Review Matter is not destroyed or created
Atoms are rearranged in chemical reactions
Chemical equations represent chemical reactions
You have to have the same number of each type of atom on the left and right hand side of a chemical equation

29. WARNING!
Don’t mess with the insides of polyatomic ions – put a square around them, or label them as X – treat the WHOLE polyatomic ion as though it were an element!
Don’t ever play around with subscripts (those little numbers that tell you how many atoms are in a molecule) e.g. C6H22O11

30. Five Types of Chemical Reactions
1. Combination/synthesis = two simpler substances (element or compounds) combine to form a single new substance.
A + X  AX
Mg + O2  MgO

31. 2. Decomposition = single compound breaks down into two or more simpler substance.
AX  A X
H2O2  H2O O2

32. 3. Single Replacement = one element replaces a second element in a compound.
A BC  B AC
Al CuCl2  AlCl Cu

33. Activity Series of Metals: Lists metals in order of decreasing reactivity. A reactive metal will replace any metal listed below it in the activity series.

34. 4. Double Replacement = A chemical change where the elements in 2 compounds switch places to form 2 new compounds
AB XY  AY XB
Ba(NO3) H2SO4  BaSO HNO3

35. 5. Combustion = an exothermic reaction (when heat is released out) where an element or a compound reacts with oxygen
C2H O  CO H2O

36. 1) Combination/synthesis Reactions
•Is also referred to as a synthesis reaction
•It is a chemical change in which two or more substances react to form a new singular substance
•The product is a compound in this form of reaction
•You can tell this reaction has occurred because on the reactant side there are multiple substances, while on the product side there is only one product

37. 2) Decomposition Reactions
•This is a chemical change where a single compound (the reactant) breaks down into two or more simpler products.
•This type of reaction is easily identifiable, because:
•It is the exact opposite of synthesis; if you know one you know them both
•There is always one reactant. If you see one reactant that’s a compound and several products, a decomposition reaction occurred.

38. 3) Single-Replacement Reactions
•A chemical change where one element replaces a second element in a compound.
•It is identifiable by its symmetry. On the reactant side there is a single element and a compound, as there is on the product side.
•No reaction will occur if the most reactive element is already in the compound

39. Notice that the couple in white are a compound on the reactant side
Notice that the couple in white are a compound on the reactant side. However, on the compound side, the man in orange steals the man in white’s date. So, a part of one of the reactants trades places and is in a different place among the products.

40. 4) Double-Replacement Reactions
•A chemical change where the elements in 2 compounds switch places to form 2 new compounds
•May also be called double-displacement reactions
•Generally take place in aqueous solution
•Often produce a precipitate, a gas, or a molecular compound (i.e. water)

41. Notice here that there are all of the same aspects on both side of the equation, however the two men traded hats. This should help you visualize what a double-replacement reaction will look like.

42. 5) Combustion Reaction
•A chemical change where an element or a compound reacts with oxygen
•This will often yield energy in the form of heat and light
•Oxygen is ALWAYS going to be on the reactant side
•This is the main indicator that a combustion reaction has occurred