1. List the common indicators of a chemical change and use them to identify when a chemical reaction has probably occurred. They are: color change, change of state (gas bubbles, precipitate, etc), temperature change, pressure change in a closed container, and change in odor. Note the emphasis on change

There are some easily observed changes that usually indicate a chemical reaction
1.
Color change
2.
Change of state (gas bubbles, precipitate, etc),
3.
Temperature change
4.
Pressure change in a closed container
5.
Change in odor

 Release of gas bubbles when two substances mixed is often evidence of chemical reaction
Antacid in water

 Many reactions happen in solution
 If solid appears after two solutions mixed, a reaction has probably happened
 Example: Pb(NO
3
)
2
+ KI
 A bright yellow solid PbI
2 appears as the proof that the reaction has happened

 The solid that appears is called precipitate
 Precipitate  a solid that is made as a result of a chemical reaction in solution and that separates from the solution
 PbI
2 precipitate

Reaction between vinegar and baking soda is accompanied by drop of temperature
Reaction between glycerin
C
3
H
5
(OH)
3 and potassium permanganate KMnO
4 produces extensive heat
(burning)
Note: Heat by itself NOT necessarily sign of chemical change

If reaction is going in the gas phase and new gases are formed, the pressure in the closed container will change

Odor Changes It only takes one experience with a rotten egg to learn that they smell different that fresh eggs. When eggs and food spoil, they undergo a chemical change. The change in odor is a clue to the chemical change

Is it a chemical or physical change?
1.
“Dry ice”, solid carbon dioxide, is sublimated into gaseous CO
2 at room temperature
2.
Salt is dissolved in water
3.
Iron rusts in a damp environment
4.
Gasoline burns in the presence of oxygen
5.
Liquid hydrogen peroxide (H
2
O
2
) decomposes to water and oxygen
6.
Rotting of eggs

2. Given a word equation, write a chemical equation (this requires good formula writing skills)

 Chemical reaction is a process by which one or more substances are changed into one or more different substances
 Chemical equations represent chemical
reaction with symbols and formulas. Chemical equations show the identities, physical phases and relative amounts of the reactants and
products in a chemical reaction
Reactants (state) → Products (state)
 Reactants are compounds being brought together to react
 Products are the new compounds that are made in the reaction

• Solid (s)
• Liquid (l)
• Gas (g)
• Aqueous solution (aq) – solution in water
• Examples:
Na (s); O
2
(g); HCl (l); Ca(NO
3
)
2
(aq)

 First step in writing chemical equation is to identify facts to be represented
 Word equation  “equation” in which reactants and products are represented by words
 Does not give quantities of reactants/products
Reactants ==> Products chemicals that react ==> chemicals that are produced hydrogen + oxygen hydrogen peroxide
==> water
==> water + oxygen magnesium + oxygen ==> magnesium oxide sodium chloride ==> sodium + chlorine

Word equation: hydrogen (gas) + oxygen (gas)  water (liquid)
Formula equation: H
2
(g) + O
2
(g)  H
2
O (l) magnesium (solid) + oxygen (gas)  magnesium oxide
(solid)
Mg (s) + O
2
(g)  MgO (s)

1.
The equation must contain correct formulas for the reactants and products.
2.
The Law of Conservation of Mass must be satisfied: mass is neither created nor destroyed in any ordinary chemical reaction.
• The quantity of substances produced (products) by a chemical reaction is always equal to the quantity of the reacting substances (reactants).
• The “quantity” here means mass or the number of atoms for each element participating in reaction

1.
Write a formula equation
H
2
O(l)  H
2
(g) + O
2
(g)
2.
Create a table showing the number of atoms for each element: This equation is not balanced!
Reactants Products The number of atoms
H
O
2
1
2
2

3.
Correct the number of O by placing coefficient 2 in front of H2O
2 H
2
O(l)  H
2
(g) + O
2
(g)
4.
Correct the table:
The number of atoms
Reactants Products
H 4 2
O 2 2
Oxygen is now balance with 2 on left and 2 on right. H is not!
5.
Now balance hydrogen – 4 on left, 2 on right. Add coefficient 2 to H
2
2 H
2
O(l)  2 H
2
(g) + O
2
(g) balanced!
The number of atoms
Reactants Products
H 4 4
O 2 2

 Create a table of elements for reactant and product side of the equation
 Balance the different types of atoms one at a time.
 First balance the atoms of elements that are combined and that appear only once on each side of the equation.
 Balance polyatomic ions that appear on both sides as single units.
 Balance lone atoms, especially H and O atoms (or any), last.

• DO NOT WRITE INCORRECT FORMULAS, THIS WILL
MESS UP YOUR BALANCING
• DO NOT CHANGE SUBSCRIPTS IN FORMULAS TO
BALANCE THE EQUATION!
H
2
O(l)  H
2
(g) + O
2
(g) H
2
O(l)  H
2
(g) + O(g)

• When you think you have balanced the equation,
COUNT THE NUMBERS OF EACH TYPE OF ATOM
ON EITHER SIDE OF THE EQUATION
2H
2
O(l)  2H
2
(g) + O
2
(g)
H
O
Reactant side Product side
4 4
2 2

Write balanced chemical equations for the following reaction: Solid sodium combines with chlorine gas to produce solid sodium chloride.
• Word reaction:
Sodium (solid) + chlorine (gas)  sodium chloride (s)
• Formula reaction:
Na(s) + Cl
2
(g) → NaCl(s)
• Balanced reaction
2Na(s) + Cl
2
(g) → 2NaCl(s)

Na(s) + Cl
2
(g) → NaCl(s)
1.
Count atoms:
1.
Na (in compound): 1 on the left, 1 on the right - balanced
2.
Cl (lone element): 2 on the left, 1 on the right – not balanced!
2.
Adjust Cl: Na(s) + Cl
2
(g) → 2 NaCl(s)
Count atoms:
1.
Na: 1 on the left, 2 on the right – not balanced!
2.
Cl: 2 on the left, 2 on the right – balanced
3.
Adjust Na: 2 Na(s) + Cl
2
(g) → 2 NaCl(s)
Count atoms – all balanced!
1.
Na: 2 on the left, 2 on the right
2.
Cl: 2 on the left, 2 on the right

Write word, formula, and balanced chemical equations for magnesium and hydrochloric acid
(HCl) react to produce magnesium chloride and hydrogen.
 Word equation: Magnesium (solid) + hydrochloric acid (liquid)  magnesium chloride
(solid) + hydrogen (gas)
 Formula equation: Mg (s) + HCl (l)  MgCl
2
H
2
(g)
 Count atoms:
(s) +
1.
Mg (in compound): 1 on the left, 1 on the right

 Word equation: Methane (gas) + oxygen (gas)  carbon dioxide (gas) + water (gas)
 Formula equation:
CH
4
(g) + O
2
(g)  CO
2
(g) + H
2
O(g)
Reactants Products Balanced?
1. C (in compounds in both sides) 1 1 Yes
2. H (in compound)
3. O (in compound and a lone element)
4
2
2
3
No
No

• Adjust H: CH
4
(g) + O
2
(g)  CO
2
(g) + 2 H
2
O(g)
Reactants Products Balanced?
1. C (in compounds in both sides) 1 1 Yes
2. H (in compound)
3. O (in compound and a lone element)
4
2
4
4
Yes
No
• Adjust O: CH
4
(g) + 2 O
2
(g)  CO
2
(g) + 2 H
2
O(g)
Reactants Products Balanced?
1. C (in compounds in both sides) 1 1 Yes
2. H (in compound)
3. O (in compound and a lone element)
4
4
4
4
Yes
Yes

Fe + Cl
2
= FeCl
3
2 Fe + 3 Cl
2
= 2 FeCl
3
Zn + HCl  ZnCl
2
+ H
2
Zn + 2 HCl  ZnCl
2
Al + O
2
 Al
2
O
3
4 Al + 3 O
2
 2 Al
2
O
3
+ H
2