1. Chemical Reactions (Ch. 7)

2. The reactants are like ingredients in a recipe. Example:
2 Pieces of bread
1 tablespoon of peanut butter
peanut butter and jelly sandwich
1 tablespoon of jelly + + + +

3. What is a chemical equation?
Ca(s) + H2O(l)  Ca(OH)2(s) + H2(g)
“Calcium reacts with water to form calcium hydroxide and hydrogen gas.
Short hand way to show a reaction
“+” could be translated as “reacts with” or “added to”
an arrow () could be translated to “reacts to form”

4. Phases K(s) + H2O(l)  H2(g) + KOH(aq)
s, l, g, and aq are abbreviations for phases.
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (dissolved in water)
Remember: At room temperature, which elements are gases? Liquids? Solids?

5. More Terms 2H2 + O2  2H2O Reactants Products – On the left
chemicals mixed before the chemical reaction happens (like ingredients)
Products
– On the right
The chemicals present after the reaction.

6. Balancing chemical Equations
Remember these terms:
Subscript –tells you how many atoms are part of the molecule
Coefficient – tells you how many molecules are involved

7. Subscripts tell us that each molecule has 2 hydrogen, 1 sulfur, and 4 oxygen

8. Coefficients tell us how many molecules we have

9. A chemical equation
CH4 + 2O2  CO2 + 2H2O

10. Observing Conservation of Mass
VERY IMPORTANT –
Define Law of Conservation of Mass
Mass is neither created nor destroyed in a chemical reaction
Translation – the number and type of atoms that you have must be the same before and after the reaction

11. Conserving Mass (continued)
In other words, if I have 85 carbon atoms and 172 hydrogen atoms before a reaction, how many carbon atoms will I have after the reaction?
85 carbon atoms
How many Hydrogen atoms will I have?
172 hydrogen atoms
This means you must “balance” the chemical equations so that the number and type of atoms are equal before and after the chemical reaction

12. Balancing Chemical Equations
7.3
Balancing Chemical Equations

13. Rules for Balancing Equations
Rule # 1 Only change a coefficient!!!
*** Changing a subscript MAGICALLY creates new molecules. BIG NO NO!!!
Example of water and hydrogen peroxide
Rule #2 – keep track of your atoms
Rule #3 – Change coefficients to Balance the Equation
Rule #4 – Recount and continue

14. Example:
Count the number of each type of element on each side – Rule #2
CH4 + O CO2 + H2O
I recommend making a table
1 C 1
4 H 2
2 O 3

15. Balance 1 of the elements on both sides of the equation by changing a coefficient – Rule 3
Example: figure out the smallest # both sides will multiply evenly into
CH4 + O CO H2O
1 C 1
4 H 2
2 O 3
We can “Balance” the hydrogen with a “2” 2

16. After changing the coefficient, recount all the atoms on both sides
After changing the coefficient, recount all the atoms on both sides. Continue balancing if needed. Rule - 4
CH4+ O CO H2O
1 C 1
4 H 2
2 O 3 2 2 4 4 4

17. 3 Very Helpful Hints Li Cl 1 2 1 2
Hint #1: If you see a group of atoms that occurs in the same order both before and after the reaction, treat them like one atom in your table
Ex: Ca(OH)2 + LiCl CaCl2 + LiOH Ca OH Li Cl
Notice that “OH” is found both before the reaction and after? Keep it together in your table 1 2 1 2

18. Hint # 2: If a molecule has only 1 type of atom, balance that atom last
Example:
CH4 + O CO2 + H2O
Notice the “O2”, don’t balance the oxygen atoms until the very end.
Hint # 3: Simplify all coefficients to smallest #’s

19. Notebook Assignment Balance the following: P + O2 P4O10 Na + Cl2 NaCl
I don’t wanna!!
Balance the following:
P + O2 P4O10
Na + Cl NaCl
S2 + N2 S3N2
MgCl2 + HF HCl MgF2
BaCl2 + H2SO BaSO4 + HCl