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Ch. 8 – Chemical Reactions. Intro to Reactions. A.Signs of a Chemical Reaction. Evolution of heat and light Formation of a gas Formation of a precipitate Color change. B.Law of Conservation of Mass. mass is neither created nor destroyed in a chemical reaction. total mass stays the same.

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a signs of a chemical reaction
A.Signs of a Chemical Reaction
  • Evolution of heat and light
  • Formation of a gas
  • Formation of a precipitate
  • Color change
b law of conservation of mass
B.Law of Conservation of Mass
  • mass is neither created nor destroyed in a chemical reaction
  • total mass stays the same
  • atoms can only rearrange

4 H

2 O

4 H

2 O

36 g

4 g

32 g

c chemical equations
C. Chemical Equations

A+B  C+D

REACTANTS

PRODUCTS

d writing equations
D. Writing Equations

2H2(g) + O2(g)  2H2O(g)

  • Identify the substances involved.
  • Use symbols to show:
  • How many? - coefficient
  • Of what? - chemical formula
  • In what state? - physical state
  • Remember the diatomic elements.
d writing equations1
D. Writing Equations

Two atoms of aluminum react with three units of aqueous copper(II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride.

  • How many?
  • Of what?
  • In what state?

 3

2

Al

(s)

+ 3

CuCl2

(aq)

Cu

(s)

+ 2

AlCl3

(aq)

e describing equations
Describing Coefficients:

individual atom = “atom”

covalent substance = “molecule”

ionic substance = “unit”

E. Describing Equations

3CO2

2Mg 

4MgO 

3 molecules of carbon dioxide

2 atoms of magnesium

4 units of magnesium oxide

e describing equations1
E. Describing Equations

Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)

to produce

  • How many?
  • Of what?
  • In what state?

One atom of solid zinc

reacts with

two molecules of aqueous hydrochloric acid

one unit

of aqueous zinc chloride

and one

molecule of hydrogen gas.

a balancing steps
A. Balancing Steps

1. Write the unbalanced equation.

2. Count atoms on each side.

3. Add coefficients to make #s equal.

Coefficient  subscript = # of atoms

4. Reduce coefficients to lowest possible ratio, if necessary.

5. Double check atom balance!!!

b helpful tips
B. Helpful Tips
  • Balance one element at a time.
  • Update ALL atom counts after adding a coefficient.
  • If an element appears more than once per side, balance it last.
  • Balance polyatomic ions as single units.
    • “1 SO4” instead of “1 S” and “4 O”
c balancing example
C. Balancing Example

Al + CuCl2 Cu + AlCl3

Al

Cu

Cl

Aluminum and copper(II) chloride react to form copper and aluminum chloride.

2

3

3

2

 2

 6

1 1

1 1

2 3

2 

3 

6 

 3