1. Unit 5: Chemical Reactions
Section 1: Describing Reactions
Chemical Reaction
Law of Conservation of Mass
Chemical Equations and Balancing

2. A. Physical and Chemical Changes
During a physical change, some properties of a material change, but the composition of the material does not change.
Examples of physical changes:
Grinding
Cutting
Folding
Cracking
Filing
Bending
Phase changes (melting, boiling, condensing, etc)

3. A. Physical and Chemical changes
A change that involves one or more substances turning into new substances is called a chemical change.
Examples of chemical changes:
decomposing
rusting
exploding
burning
oxidizing
tarnishing
fermenting

4. PA. Physical and Chemical Changesand Chemical Changes
A magnet separates iron from sulfur. This is an example of a physical change.
A mixture of iron filings and sulfur can be changed. a) A magnet separates the iron from the sulfur. b) Heat combines iron and sulfur in a compound. Classifying Which change is a chemical change? Explain.

5. PhA. Physical and Chemical Changesand al and Chemical Changes
A mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. This is an example of a chemical change.
A mixture of iron filings and sulfur can be changed. a) A magnet separates the iron from the sulfur. b) Heat combines iron and sulfur in a compound. Classifying Which change is a chemical change? Explain.

6. B. Chemical Reaction
Chemical Change: new & different substances formed.
4 Evidences of Chemical Reaction:
energy transfer
(hot or cool)
color change
gas formed
precipitate formed
(solid)

7. A+B AB B. Chemical Reaction REACTANTS PRODUCTS
Chemical Equation:
A+B AB
yields
REACTANTS
PRODUCTS
present at the start of the rxn
produced in the rxn at the end

9. C. Law of Conservation of Mass
In a chemical reaction, matter is neither created nor destroyed.
Atoms can only rearrange. 4H 2O 4H 2O

10. D. Chemical Equations N2H4 + O2  N2 + H2O N2H4 + O2  N2 + 2 H2O
Balancing Equations
N2H4 + O2  N2 + H2O
N2H4 + O2  N H2O
Coefficient - # of units of each substance

11. coefficient  subscript = # atoms
D. Chemical Equations
3 Balancing Steps
1. Count atoms on each side.
2. Use coefficients to make atoms equal.
coefficient  subscript = # atoms
How many N’s in: 2 Fe(NO3)2
3. Double check atom balance!!!

12. D. Chemical Equations 3 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 2 3 3 2
Aluminum and copper(II) chloride form copper and aluminum chloride.
Al CuCl2  Cu AlCl3 Al Cu Cl
2 
 2
3 
6   6
 3

13. Quick Quiz! Which of the following is a chemical reaction?
melting of lead
dissolving sugar in water
rusting of iron
crushing of stone

14. Quick Quiz.
Which of the following is NOT a possible clue that a chemical change is taking place?
a change of state
a change in color
production of a gas
formation of a precipitate

15. Quick Quiz. Why do we have to balance equations? it’s so much fun!
it’s the law of the west
it’s the law of consternation of moose
it’s the law of conservation of mass

16. __Al + __HCl → __AlCl3 + __H2
Quick Quiz.
What is the coefficient of HCl when the following reaction is balanced?
__Al + __HCl → __AlCl3 + __H2
___Al___
___H___
___Cl___
A B. 2 C. 3 D. 6
2 Al HCl → 2 AlCl H2 1 1 1 2 1 3