1. Unit 9: Chemical Reactions

2. Changes of Matter Physical Changes: Chemical Changes: A B AB
do not change the composition of a substance.
melting, vaporizing, dissolving, size, etc.
Chemical Changes:
result in new substances.
burning, rusting, decomposition, etc. A B AB

3. 4 Clues of Chemical Change
Demo:
Zn + H+  Zn H2
Pb I–  PbI2(s)
Mg + O2  MgO
gas production
color change
energy transfer (temp change, light)
precipitate formation
Do three reactions and summarize BEFORE revealing list.
(solid forms from solution)
Chemical Change: new & different substances formed.

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

5. Law of Conservation of Mass
In a chemical reaction, matter is neither created nor destroyed.
Atoms are rearranged. 4H 2O 4H 2O

6. Law of Conservation of Mass
mass of reactants = mass of products ?

7. mass is conserved (same before & after)
Chemical Equations
Balancing Equations
WHY?
mass is conserved (same before & after)
N2H4 + O2  N2 + H2O
N2H4 + O2  N H2O
Coefficient - # of units of each substance

8. Chemical Equations 2 H2 + O2  2 H2O 3 Balancing Steps
1. Count atoms on each side. 2. Add Coefficients to make # of atoms equal. coefficient  subscript = # atoms How many N’s in: 2 Fe(NO3)2 3. Check atom balance!!!

9. Chemical Equations 3 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 2 3 3 2 2 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

11. Which of the following is a chemical reaction?
Quick Quiz!
Which of the following is a chemical reaction?
melting of lead
dissolving sugar in water
rusting of iron
crushing of stone
(new substance)

12. 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

13. Why do we have to balance equations?
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

14. 4. During any chemical change, the mass of the final products is…
Quick Quiz.
4. During any chemical change, the mass of the final products is…
always equal to the mass of the starting reactants.
always greater than the mass of the starting reactants.
always less than the mass of the starting reactants.
sometimes different than the mass of the starting reactants.

15. __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