1. Chapter 21
Chemical Reactions

2. Sec. 1 Chemical Changes
Chemical Reaction—a change in which 1 or more substances are converted into new substances.
Reaction can be abbreviated as RXN
Ways to tell if a chemical rxn has occurred:
Color change, bubbles produced, NEW odor produced or energy change.

3. Reactants & Products Reactants—substances that combine or change
The things you mix together
Products—new substances formed.
The things made in the end

4. Chemical Equations
Chemical equation—a way to describe a chemical rxn using formulas & other symbols.
Write the reactants on the left, & products on the right.
Reactants  Products

5. Other Symbols
 : Between reactants & products & is read as “yields” or “produces”
+ (plus): separates 2 formulas on either the reactant or product side
Coefficients: (#s in front of formulas) tell the # of units of each substance in the rxn.
Ex. 2Na + Cl2  2NaCl
Subscripts tell you how many atoms are in the compound

6. Aqueous (aq): a substance that is dissolved in water.
State of Matter
The symbol for the state of matter for each compound is written after formula in parentheses.
Solid (s)
Liquid (l)
Gas (g)
Aqueous (aq): a substance that is dissolved in water.
2Na(s) + Cl2(g)  2NaCl(s)

7. 3 ways to write a Chemical Equation
Word Equation
Skeleton Equation
Balanced Equation

8. Word Equations
Describe chemical reactions using words with limited symbols.
Hydrogen(g) + Oxygen(g)  Water(l) (word)

9. Skeleton Equation
Equations that show the formulas of the reactants and products
NOT the amounts of reactants or products
H2 (g) + O2 (g) → H2O(l) skeleton

10. Balanced Chemical Equations (BCE)
Uses COEFFICIENTS to show the amount of reactants and products in the reaction
Follows the Law of Conservation of Mass
2H2 (g) + 1O2 (g) → 2H2O(l) balanced

11. Conservation of Mass
Law of Conservation of Mass—mass cannot be created or destroyed; it can just change forms
The mass of reactants is always equal to the mass of products.
Ex. 0.7g O g Hg  10.0g HgO2

12. Closed vs Open System Closed system—the chemical rxn is contained.
No product is lost
Open system—the chemical rxn is not completely contained.
Some product (mostly gases) can escape.
BOTH systems follow the LOCO mass—closed systems are easier to measure

13. Steps for Balancing Equations
Determine the formulas for all reactants and products. (this may be given)
Reactants on the left & products on the right.

14. Count the # of each element in reactants & products
CH4 + O2  CO2 + H2O
C = C = 1
H = H = 2
O = O = 3

15. 3. Balance each element by using coefficients—number placed in front of the symbol.
4. Make sure all elements are balanced.
Ex. CH O2  CO H2O
C = 1 C = 1
H = 4 H = 2
O = 2 O = 3 2 2 4 4 4

16. 5. Make sure all coefficients are in the lowest ratio.
Ex. 2CH4 + 4O2  2CO2 + 4H2O
Can be reduced to 

18. Chem Balancer Online
Complete all 11!
Draw out 12 and 13 like shown online!

20. Types of Chemical Rxns

21. Combination/ Synthesis Rxn
2 or more reactants combine to form a single product.
a + b  ab
Examples:
2K + Cl2 2KCl (Can start with 2 elements)
CaO + H2O Ca(OH) (or 2 compounds)

22. Decomposition Rxn
A single compound is broken down into 2 or more products.
ab  a + b
Examples:
PbO2  Pb + O2 (Can make 2 elements)
CaCO3  CaO + CO2 (or 2 compounds)

23. Single-Replacement Rxn
One element replaces a second element in a compound.
a + bc ac +b
Example:
Mg + 2AgNO3Mg(NO3)2 + 2Ag
2NaBr + Cl2  2NaCl + Br2

24. Double-Replacement Rxn
Involve an exchange of cations between 2 ionic compounds.
ab + cd ad + cb
Examples:
Na2S + Cd(NO3)2  CdS + 2NaNO3

25. Combustion Rxn Heat & light are also often given off
A compound containing C & H reacts with oxygen (O2), producing CO2 & H2O
Heat & light are also often given off
CxHy + O2  CO2 + H2O
Example:
2CH3OH + 3O2 2CO2 + 4H2O

26. Analogies for 4 of the types of reactions
Synthesis/Combination
Falling in Love
Decomposition
The Break-up
Single Replacement
Couple dancing, someone cuts in
Double Replacement
2 couples dancing switch partners

27. Activity Series a + bc ac +b
In Single-Replacement rxns, not all metals can replace other metals.
a + bc ac +b
The single metal element (a) must be more reactive than the one in the compound (b).
The activity series is a list of metals in order of reactivity.
On the back of your P.T.!!

28. Tips for balancing equations
Identify the type of rxn 1st
Start by balancing oxygen or another diatomic element
Make odds even by using a coefficient of 2
Balance O2 last in combustion rxns
CO2 & H20 are products
Go back & forth until all elements balance. Balance in Pencil!

30. Quiz: Balance the 4 equations and classify the type of rxn.
___1. __Al + __Cl2 __AlCl3 ___2. __Mg + __CuCl  __MgCl2 + __Cu ___3. __FeCl3 + __NaOH Fe(OH)3 + NaCl ___4. __CH4 + __O2  __CO2 + __H2O

32. Predicting Products
Identify the products that should be formed based on type of rxn
Write the formulas correctly—DON’T FORGET to look up & balance charges
If it is ionic, make sure you have a cation with an anion
Remember: some elements are diatomic (exist as 2 atoms):
H2, N2, O2, F2, Cl2, Br2, & I2
You MUST have correct formulas BEFORE you balance.

33. Example __LiCl + __CuSO4 Type of Rxn: Double Replacement
Li and Cu are both metals (cations) so they can switch.
Look at charges of ions: Li+1 Cl-1 Cu+2 SO4-2
Products are…
__Li2SO4 + __CuCl2
Balance

34. State the type of reaction, Predict the products & Balance the equation.
Type of Rxn Reactants Products
1. _____________ __BaCl2 + __H2SO4 
2. _____________ __C6H12 + __O2 
3. _____________ __Zn + __CuSO4 
4. _____________ __Cs + __Br2 
5. _____________ __FeCl3 
__BaSO4 + __HCl 2
Double-Replacement
__CO2 + __H2O
Combustion 9 6 6
__ZnSO4 + __Cu
Single-Replacement
Balanced!!!
Synthesis 2
__CsBr 2 2
__Fe + __Cl2 2 3
Decomposition

36. Sec. 4 Chem Rxns & Energy
Some chemical rxns require heat in order to happen
Some rxns give off heat.
Thermochemistry—heat changes that occur during chemical reactions.

37. Exo & Endo Exothermic—a rxn where energy (heat) is given off.
Ex: burning wood, exploding dynamite.
Exothermic rxns feel warm.
Endothermic—a rxn where energy (heat) is absorbed.
Ex. Cold packs
Endothermic rxns feel cold.

38. Catalysts & Inhibitors
Catalyst—a substance that speeds up a chem rxn without being used in the rxn.
Written above the arrow in an equation.
Used in making plastics
H2O2
MnO2
H2O + O2
Heat as a catalyst
Inhibitors—prevent certain rxns from occurring.
Used in food preservatives to prevent spoiling.

39. Some elements are diatomic (exist as 2 atoms)
They have the same name as the element (ex. not dihydrogen)
Hydrogen: H2
Nitrogen: N2
Oxygen: O2
Fluorine: F2
Chlorine: Cl2
Bromine: Br2
Iodine: I2