1. Chemical Reactions (Rxns)
Chapter 21
Chemical Reactions (Rxns)

2. Sec. 1 Chemical Changes
Chemical Reaction—a change in which 1 or more substances are converted into new substances.
Reactants—substances that react or the starting substances.
Products—new substances produced.
Ways to tell if a chemical reaction has occurred:
Odor, color, bubbles

3. Conservation of Mass
Law of Conservation of Mass—mass cannot be created or destroyed.
The mass of reactants is always equal to the mass of products.
Ex. 0.7g O g Hg  10.0g HgO2

4. Chemical Equations
Chemical equation—a way to describe a chemical rxn using chemical formulas and other symbols.
Reactants are written on the left, products on the right.
Reactants  Products
The  is read as “yields” or “produces”

5. States of Matter & Coefficients
The physical state of the reactants and products is written after the formula in parentheses.
Solid (s)
Liquid (l)
Gas (g)
Aqueous (aq): a substance dissolved in water.
Coefficients (#s to the left of formulas) tell the # of units of each substance in the rxn.

6. Writing Equations A chemical reaction can be written in words
Ex. One unit of solid zinc plus 2 units of hydrochloric acid dissolved in water produce one unit of hydrogen gas plus one unit of zinc chloride dissolved in water.
You can use chemical formulas & symbols in the chemical equation.
Ex. Zn(s) + 2HCl(aq)  H2(g) + ZnCl2(aq)

7. Remember: some elements are diatomic (exist as 2 atoms) Hydrogen: H2, Nitrogen: N2, Oxygen: O2, Fluorine: F2, Chlorine: Cl2, Bromine: Br2, Iodine: I2
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9. Balancing Chemical Equations
Determine the formulas for all reactants and products. (this may be given)
Write reactants on the left and products on the right of the arrow.
Count the # of each element in reactants & products
Ex. CH4 + O2  CO2 + H2O
C = 1 C = 1
H = 4 H = 2
O = 2 O = 3

10. Balancing Chemical Equations
4. Balance each element by using coefficients—number placed in front of the symbol.
5. Make sure all elements are balanced.
6. Make sure all coefficients are in the lowest ratio.
Ex. CH4 + O2  CO2 + H2O
C = 1 C = 1
H = 4 H = 2
O = 2 O = 3
____ CH4 + ____ O2  ____ CO2 + ____ H2O 2 2

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

13. Types of Chemical Rxns

14. Combination/ Synthesis Rxn
2 or more reactants combine to form a single product.
Examples:
2K(s) + Cl2(g) 2KCl(s)
CaO(s) + H2O(l) Ca(OH)2(aq)
a+b ab

15. Decomposition Rxn
A single compound is broken down into 2 or more products.
Examples:
PbO2(s)  Pb(l ) + O2(g)
CaCO3(s)  CaO(s) + CO2(g)
ab a+b

16. Single-Replacement Rxn
One element replaces a second element in a compound.
Example:
Mg(s) + 2AgNO3(aq)Mg(NO3)2(aq) + 2Ag(s)
a+bc ac +b

17. Double-Replacement Rxn
Involve an exchange of positive ions between 2 ionic compounds.
Examples:
Na2S + Cd(NO3)2  CdS + 2NaNO3
2NaCN + H2SO4 2HCN + Na2SO4
ab+cd ad+cb

18. Combustion Rxn Heat & light are also often given off
An element or compound reacts with oxygen (O2) often producing CO2 & H2O
Heat & light are also often given off
Example:
2CH3OH(l) + 3O2(g)2CO2(g) + 4H2O(g)
X + O2  CO2 + H2O

19. Romance analogy for the four types of reaction
Combination Falling in love
Decomposition The Break-up
Single Replacement The player
Double Replacement The swingers

21. Completing Chem Rxns Some steps for doing reactions
Identify the type of reaction
Predict the product(s)
Balance the equation using COEFFICIENTS
Remember: some elements are diatomic (exist as 2 atoms)
Hydrogen: H2, Nitrogen: N2, Oxygen: O2, Fluorine: F2, Chlorine: Cl2, Bromine: Br2, Iodine: I2

22. When Predicting Products
When predicting products, remember to write the formulas correctly—DON’T FORGET TO BALANCE OUT THE CHARGES!
Ex. Li + CuSO4 
Type of Rxn: Single Replacement
Li replaces Cu & bonds with SO4
Look at charges of ions: Li+1 SO4-2
Products are…
Li2SO4 + Cu

23. 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. _____________ __FeCO3 
__BaSO4 + __HCl 2
Double-Replacement
__CO2 + __H2O
Combustion 9 6 6
__ZnSO4 + __Cu
Single-Replacement
Balanced!!!
Combination 2
__CsBr 2
Decomposition
__FeO + __CO2
Balanced!!!

25. Activity Series for Metals
In Single-Replacement, not all metals elements can replace other metals.
The single metal element must be more reactive (ha! More attractive) than the one in the compound.
Look on P. 643 (figure 13) in textbook for the activity series
WRITE THIS DOWN!!!

26. Google: Metals in Aqueous Solutions
Follow Directions as you go and complete all 4 Activities.

27. REMEMBER!!!
Aqueous (aq): a substance dissolved in water.

29. 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.

30. 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.

31. 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.