Chemical Reactions

1
Chemical Reactions

• Warm-Up
• On a sheet of paper, draw a picture of a chemical
  reaction that happens in your daily life, or
  write down three things you think of when you
  think about chemical reactions.

2
Chemical Reactions

• When you hear the phrase Chemical Reaction,
  what do you think of?
• Explosions?
• Acids dissolving doors?
• Glowing liquids?
• Purple clouds?

3
Changes in Matter

• We can classify changes in matter as either
  physical or chemical changes.
• Physical changes only change the shape or state
  of an object.
• Chemical changes rearrange atoms among molecules.
• Can you think of some examples of each?

4
Physical
Chemical

• Crushing ice
• Melting ice
• Boiling water
• Bending a piece of metal
• Separating iron filings and sulfur powder
• Melting iron and tin, mixing, and letting the
  mixture cool

• Using a battery to break water into hydrogen and
  oxygen gas
• Burning a piece of wood
• Heating iron filings and sulfur until they bond,
  making pig iron

5
Bonding and Energy

• Chemical reactions are the rearrangement of atoms
• What do you have to do in order to rearrange
  atoms?
• You have to break their bonds!
• It takes energy to break bonds.
• How can we give a molecule energy it can use to
  break a bond?

6
Giving Molecules Energy

• Slam other molecules into it (heating)
• Like when you bake a cake
• Pull them apart with an electric field or hit
  them with electrons (electricity)
• Like when you put wires from a battery in water
  to make hydrogen and oxygen
• Hit them with light
• Like when your dentist uses a UV light to set a
  polymer filling

7
Getting Energy from Molecules

• Of course, you can get energy from molecules,
  too.
• Thats why we burn gasoline in our cars - to get
  the energy to turn the wheels
• Heat - the new molecules are moving more quickly
• Electricity - the molecules get rid of electrons
  (like in a battery!)
• Light - like in a glowstick

8
Exothermic Reaction

• If molecules release more energy than they
  absorb, the reaction is exothermic.

9
Sodium and Water
10
Endothermic Reaction

• If molecules take in more energy than they
  release, the reaction is endothermic.

11
Endo or Exo?

• You mix two substances together, and
• they heat up.
• Exothermic
• they cool down.
• Endothermic
• they glow.
• Exothermic
• nothing happens until you shine light on them.
• Endothermic

12
Chemical Equations

• Methane gas combines with oxygen gas to produce
  carbon dioxide and water
• Thats quite a mouthful, eh?
• Theres a shorter way to write that

combines with
? to produce
CH4 methane
O2 oxygen
CO2 H2O carbon dioxide and water
13
Types of Reactions

• synthesis - two different elements combine
  together to form one compound
• Fe (II) O ? FeO (rust!)
• Decomposition - one compound is broken into its
  two individual elements
• H2O ? 2H O

14
Types of Reactions (cont.)

• single displacement - a lone element replaces
  some part of a compound.
• 2Li 2H2O ? 2LiOH H2
• double replacement - atoms in compounds switch
  partners
• BaBr2 K2SO4 ? 2KBr BaSO4

15
Conservation of Atoms

• Conservation means, in science, keep the same
  or the same everywhere
• It really means, theres no such thing as a free
  lunch
• When you have a chemical reaction, the atoms IN
  must be the same as the atoms OUT. The atoms
  must come from somewhere!
• Are atoms conserved in this reaction?
• CH4 O2 ? CO2 H2O

16
CH4 O2 ? CO2 H2O

• C - Carbon
• 1 goes in 1 comes out
• H - Hydrogen
• 4 go in 2 come out (??)
• O - Oxygen
• 2 go in 3 come out (??)
• Nope, the atoms arent conserved.

17
Balancing Equations

• Balancing equations is adjusting the numbers of
  each molecule until theres the same number of
  each atom on each side.
• CH4 O2 ? CO2 H2O
• Hrm what if I had TWO oxygen molecules on the
  left?
• CH4 2 O2 ? CO2 H2O
• C - 1, H - 4, O - 4 on the left
• C - 1, H - 2, O - 3 on the right

18
Balancing Equations (cont.)

• Closer I need 2 more H and 1 more O on the
  right.
• How about another water molecule?
• CH4 2 O2 ? CO2 2 H2O
• C - 1, H - 4, O - 4 on the left
• C - 1, H - 4, O - 4 on the right
• It is balanced!

19
Balancing Equations (practice)

• If I mix aluminum (Al) and sulfur (S) together,
  what compound will they make?
• Hrm Al has an oxidation of 3, S has an
  oxidation of 2-

Al2S3
2
3
Al S ?