1. The Law of Conservation of Mass

2. We’ve talked about changes in matter…
The evaporation of a puddle of water
Rust forming on a metal fence
PHYSICAL
CHEMICAL

3. The total amount of matter remains constant.
HOWEVER…
No matter is created or destroyed by such changes!
The total amount of matter remains constant.

4. The Law of Conservation of Mass
During a chemical reaction, matter cannot be created or destroyed.
Even though the matter may change from one form to another, the same number of atoms exists before and after the change takes place!

5. Since the overall mass of the matter does not change…
The mass of the reactants – (the substances there before the reaction occurs) – must equal the mass of the product – (the substances there after the reaction takes place.)
Mass of
Reactants
Products

6. Open System There is exchange of matter and energy with surroundings
Mass is lost to the environment; once gone, that mass cannot be measured.
Example: Burning Wood

7. Closed System
No matter can enter/leave the system only energy

8. An Example of the Closed System.
Test tube containing lead nitrate is placed upright in a flask that contains potassium iodide.
What are these?
REACTANTS!
They don’t mix!

9. A stopper is put in place to create a closed system – an environment where matter cannot enter or escape.
The mass of the system is found.

10. After the mass is found, the system is turned upside down so the lead nitrate can mix with the potassium iodide.
They create lead iodide and potassium nitrate.

11. Although the matter changed in form, the total mass of the system remains unchanged.

12. When iron reacts with oxygen in the air, they form rust or IRON OXIDE.
This is written as:
Iron + Oxygen Iron Oxide

13. TAKE A LOOK AT THE NUMBERS
REACTANTS
PRODUCT O2
Fe2O3 Fe Fe O2 O2
What about the PRODUCT?
How many Oxygen molecules?
How many Iron molecules?

14. All Chemical Reactions follow the Law of Conservation of Matter
With Iron Oxide, the MASS STAYS THE SAME!
The total number of iron atoms and oxygen atoms in the reactants is the same as that in the product.
The atoms were just rearranged!
No new atoms were created and none were destroyed!

15. Determining the Mass of Reactants and Products
REMEMBER: The mass of the reactants is always equal to the mass of the products.
Mass of
Reactants
Mass of
Products

16. What are Chemical Equations
A chemical change is represented by a chemical reaction.
A chemical equation identifies the starting and ending chemicals as reactants and products:
reactants products

17. Balancing Equations Reactants Products
Chemical reactions result in chemical changes.
Chemical changes occur when new substances are created.
The original substance(s), called reactants, change into new substance(s) called products.
Reactants
Products
(c) McGraw Hill Ryerson 2007
See pages

18. Balancing Equations Reactants Products
(c) McGraw Hill Ryerson 2007
See pages

19. Balancing Equations
Chemical reactions can be written in different ways.
A word equation:
Nitrogen monoxide + oxygen  nitrogen dioxide
A symbolic equation:
2NO(g) + O2(g)  2NO2(g)
STATE OF MATTER
Letters indicate the state of each compound.
(aq) = aqueous/dissolved in water
(s) = solid
( ) = liquid
(g) = gas
COEFFICIENTS
Indicates how many of each molecule there is.
Ie: there are 2 molecules of NO.
(c) McGraw Hill Ryerson 2007

20. Law of Conservation of Mass
When a chemical reaction occurs, new compounds are created, BUT…
No new matter is created or destroyed; atoms are just rearranged as the atoms change partners to form new compounds.
If there are 3 atoms of oxygen in the reactants, there MUST be 3 atoms of oxygen in the products.
Number of each atom in reactants = number of each atom in products.
The law of conservation of mass:
Mass of reactants = mass of products
If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gas + O2) = mass of products (exhaust).
(c) McGraw Hill Ryerson 2007

21. Writing & Balancing Equations
The simplest form of chemical equation is a word equation.
Potassium metal + oxygen gas  potassium oxide
A skeleton equation shows the formulas of the elements/compounds.
A skeleton equation shows which atoms are involved, but not how many molecules are involved.
K + O2  K2O

22. Writing & Balancing Equations
A balanced chemical equation shows all atoms and the coefficients tells us how many molecules (and atoms) there are.
Balancing ensures that the number of each atom is the same on both sides of the reaction arrow.
4K O  2K2O K K O O K O K K K K O K

23. Counting Atoms to Balance Equations
Using the law of conservation of mass, we can count atoms to balance the number of atoms in chemical equations.
Word equation: methane + oxygen  water + carbon dioxide
Skeleton equation: CH4 + O2  H2O + CO2
To balance the compounds, take note of how many
atoms of each element occur on each side of the
reaction arrow.
See Page 207
(c) McGraw Hill Ryerson 2007

24. Balancing Equations
CH4 + O H2O + CO2
C C
H H
O O

25. Balancing Equations
CH4 + O H2O + CO2
C- 1 C -1
H H- 2
O O -3

26. Balancing Equations CH4 + O2 2 H2O + CO2 C- 1 C -1 H - 4 H- 2 4
O O -3 4

27. Balancing Equations CH4 + 2 O2 2 H2O + CO2 C- 1 C -1 H - 4 H- 2 4
O – O -3 4

28. Counting Atoms to Balance Equations
Skeleton equation: CH4 + O2  H2O + CO2
Carbon = 1 Carbon = 1
Hydrogen = 4 Hydrogen = 2
Oxygen = 2 Oxygen = 3
Balanced equation: CH4 + 2O2  2H2O + CO2 Carbon = 1 Carbon = 1
Hydrogen = 4 Hydrogen = 4
Oxygen = 4 Oxygen = 4
The same number of atoms must be on each side.
(c) McGraw Hill Ryerson 2007
See Page 207

29. Balancing Equations H2 + O2  H2O hydrogen + oxygen  water Reactants
Products H 2 O 2 1

30. YOU CANNOT CHANGE THE SUBSCRIPTS
Balancing Equations X
hydrogen oxygen  hydrogen peroxide
H O  H2O2
YOU CANNOT CHANGE THE SUBSCRIPTS
Reactants
Products H 2 O

31. Balancing Equations 2 H2 + O2  H2O hydrogen + oxygen  water
Reactants
Products H 2 O 1

32. Balancing Equations H2 + O2  H2O 2 hydrogen + oxygen  water
Reactants
Products H 2 4 O

33. Balancing Equations 2 H2 + O2  H2O 2 hydrogen + oxygen  water
Reactants
Products H 4 O 2

34. ON BALANCING EQUATIONS
WATCH THIS LESSON
ON BALANCING EQUATIONS
Turn sound “on”

35. Balance the following:
Fe + Br2  FeBr3
C2H6 + O2  CO2 + H2O

36. ____Ba + ____H2O  ____Ba(OH)2 + ____H2
 ____CO2 + ____H2O  ____H2CO3   
____Fe2O3 + ____C  ____Fe + ____CO
____Fe + ____H2O  ____H2 + ____Fe2O3