1. Chemical Equations and Conservation of Mass
What are they?

2. chemical reaction- one or more substances are changed into new substances.
Na + Cl NaCl
products
reactants
Reactant – substances that react
Product – new substances produced

3. Evidence of Reactions
Bubbling
Turns cloudy
Temperature change
Color change

4. Aqueous-solid dissolved in water
Chemical Equation
Expression that describes a chemical reaction using chemical formulas and other symbols
Symbol
Meaning
(cr)
Crystalline solid
(l)
Liquid
(g)
Gas
(aq)
Aqueous-solid dissolved in water

5. Symbols for Chemical Equations
Pb(NO3)4 + 4KI  PbI4 + 4KNO3
“Pb(NO3)4” is a formula.
 Means produces (or yields)
+ Means added together
Subscripts - Number of a particular atom in a molecule
Coefficients – Number of molecules

6. Father of Modern Chemistry
Antoine Lavoisier
First Described the “Law of Conservation of Mass”

7. Conservation of Mass HgO Hg + O2
Antoine Lavoisier found that the mass of the reactants and the products are equal, even when the states of matter change.
0.7g Oxygen became a gas
HgO
Hg + O2
He started with: 10g of Mercury Oxide (HgO)
He ended up with: and 9.3g Mercury…
...But what happened to the O2?
10 g. = g.
Matter is neither created nor destroyed.

8. Conservation of atoms-the number of each type of atom on the reactants side of the chemical equation MUST be equal to the number of each type of atom on the products side of the equation.
Coefficient-represent the number of units of each substance taking part in the reaction
Balanced chemical equation-the same number of atoms of each element on both sides of the equation

9. Hg (mercury) can exist by itself... This balances the equation!
Lavoisier’s Chemical Equation: 2
HgO  2
Hg + O2
Hg (mercury) can exist by itself...
but, oxygen will need to bond
with another oxygen to make O2 (diatomic)
To balance the atoms we need to:
Put the coefficient of 2 in front of reactant HgO.
Put the coefficient of 2 in front the product Hg.
This balances the equation!

10. N2 H2 NH3 Why or Why Not? Is this balanced? 10 Let’s Count the Atoms:
There are 2 nitrogen atoms
There are 2 hydrogen atoms
1 nitrogen and 3 hydrogen…

11. Atoms can only bond in certain ways..H2 N2 N H2 H N2 H N H H2 H2 H N
That’s why we can’t change the subscripts. N2 H2 H N N2 N N2 N H

12. Here is what it means...
Subscripts - Small #’s below an element.
Coefficients - Large #’s in front of the formulas. H2
I can’t live without you!
2H2 H H H
When balancing equations, we can only change the coefficients!

13. 1 N2 1 H2 1 NH3 We can only change coefficients before the symbols.13
Now, back to the balancing... 1 N2 1 H2 1
NH3
We can only change coefficients before the symbols. N H N H
1 molecule of nitrogen
1 molecule of
hydrogen
1 molecule of ammonia

14. Do both sides have the same amount of atoms?
N H NH3 N H 2 1 2 3

15. Then it is a balanced equation.
Do both sides have the same amount of atoms?
N H NH3 3 2 N H
(2) 2 1
(6)
(6) 2 3
N H NH3
Then it is a balanced equation.

16. Four Steps to Balance Equations:
1. Set up your equation.
List the metals, nonmetals, oxygen, and hydrogen below equation.
2. Count the number of atoms you have on both sides.
3. Balance by changing the coefficients and recounting.
4. Start the process again if it still does not balance.

17. H2 + O2  H2O O H 1. Set up your equation.
List the elements in this order below the equation: Metals, Nonmetals, Oxygen, and Hydrogen
H2 + O2  H2O O
Metals
Nonmetals
Oxygen
Hydrogen H

18. 2. Count the number of atoms you have of each on both sides.
H2 + O2  H2O 2 O 1 2 H 2

19. 3. Balance by changing the coefficients and recounting.2 2
H2 + O2  H2O O H 2 1
(2)
Need to have at least 2 “O”
(4)
(4)
But it changes the number of “H”
How are you going to make “H” add up to 4?
Is this balanced?
Yes!

20. Need to have at least 2 “Cl” Changing the Cl changes the “H”?
Let’s try another: 2
Mg + HCl  H MgCl2 1 Mg 1
(2) 1 Cl 2
Need to have at least 2 “Cl”
Is this balanced?
Yes! 2
Changing the Cl changes the “H”?
(2) 1 H

21. Na + HCl  NaCl + H2 Na Cl H 1 2 2 (2) Let’s try another:
In this case, we will start with hydrogen since it is the only one unbalanced.
(2)

22. Na + HCl  NaCl + H2 Na Cl H 1 2 2 2 (2) (2) (2)
But, changing the hydrogen in HCl affects the number of chlorine atoms.
Na + HCl  NaCl + H2 Na Cl H 1 2 2 2
(2)
(2)
(2)

23. Na + 2HCl  NaCl + H2 (2) 1 1 (2) (2) 1 (2) 1 (2) 1 2
Changing the chlorine on the product side affects the sodium (Na) on the reactants side. So we must now change sodium as well.
Na + 2HCl  NaCl + H2 2 2
(2) 1 Na 1
(2)
(2) Cl 1
(2) 1
(2) 1 H 2

24. Independent Practice