Chemical Reactions (Ch. 7)

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
• 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
O
O
H
S
O
O
H
O
O
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
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 O2 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 O2 CO2 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. Continue balancing if
  needed. Rule - 4
• CH4 O2 CO2 H2O
• 1 C 1
• 4 H 2
• 2 O 3

2
2
4
4
4
17
3 Very Helpful Hints

• 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 1
1 1 1 2
18
Hint 2 If a molecule has only 1 type of atom,
balance that atom last

• Example
• CH4 O2 CO2 H2O
• Notice the O2, dont 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
• S2 N2 S3N2
• MgCl2 HF HCl MgF2
• BaCl2 H2SO4 BaSO4 HCl