Transcript CHEMICAL REACTIONS

CHEMICAL REACTIONS
Balancing, States of Matter, and
Writing
Evidence for A Chemical Reaction
 Five Signs of a Chemical Change
1.
2.
3.
4.
5.
Gas Given Off (Bubbles)
Color Change (BIGGIE!!!!!!)
Heat is Produced or Absorbed
Odor Change
Precipitate Forms (Solid is Made)
Chemical Equations
 Chemical Equations are used to represent
chemical reactions.
Reactants  Products
 127. Law of Conservation of Matter (Mass)
States: “Matter cannot be created or
destroyed during a chemical reaction.”
 We must balance chemical reactions (add
coefficients) to comply with the law.
Chemical Equations (Cont)
 Example:
CH4 +
O2  CO2 +
H2O
Methane + Oxygen  Carbon + Water
Dioxide
Reactants
Products
1–C
1–C
4–H
2–H
2–O
3–O
Balancing
 Chemical reactions change the
groupings of the atoms, but DO NOT
create or destroy atoms, so there
must be the same number of each
type of atom on each side of the
arrow. We must use coefficients to
Balance a Chemical Equation
Balancing Chemical Equations
1. Determine the correct formula for each
reactant and product by supplying the
subscripts to the formulas.
1. Once you write the correct formula, DO NOT
change the subscripts.
2. Add coefficients in front of balanced
formulas to give the same number and
kind of atoms on both sides.
1. Use the lowest possible ratio of coefficients.
Example
 CH4
+
2 O2  CO2 +
2 H2O
 If I place a 2 in front of the O2 and
the H2O, then I end up with:
Reactants
Products
1–C
1–C
4–H
4–H
4–O
4–O
You Try this One
 C2H4
+
O2
2
4
2
 CO2 +
–C-1
–H-2
–O-3
H2O
C2H4 + 3O2
 2CO2
2–C-2
4–H-4
6–O-6
+2H2O
Another Example
 C3H6
+
O2  CO2 +
3–C-1
6–H-2
4–O–3
H2O
2 C3H6 + 9 O2  6 CO2 + 6 H2O
6–C-6
12 – H - 12
18 – O – 18
You Try This One
 What would the coefficients be for
this reaction:
PCl5 +
H2O  H3PO4
+
HCl
Physical States
 Symbols in parentheses after each
formula:
1.
2.
3.
4.
Solid
Liquid
Gas
Aqueous
(s)
(l)
(g)
(aq)
1. Aqueous means dissolved in water.
– Example:
2 H2O2 (aq)

2 H2O (l) +
O2 (g)
Words to watch for
 States of matter – This is a symbol that is
used to identify the state of matter for each
substance in a reaction.
 Gas, Liquid, Solid, Aqueous
 Gas (g)
 Liquid (l)
 Solid (s) may also be called a Precipitant (ppt)
or a crystal (c)
 Aqueous (aq) means dissolved in water.