1. Notes – Chemical Equations
Chapter 8, Lesson 2

2. Conservation of Mass
When the end of the last period bell rang, everyone moved to a new room, and ended up in different groups in different rooms. The number of students IN the whole school did not change, only their arrangement.
The amount of matter in a chemical reaction does not change, so the total mass at the start MUST equal the total mass at the end.

3. Conservation of Mass Matter is ALWAYS conserved in chemical reactions.
Chemical equations show that in chemical reactions, atoms rearrange, but no atoms are gained or lost.

4. Conservation of Mass
Antoine Lavoisier invented a balance to make more precise measurements.
He also determined that the mass before and after a chemical reaction was the same.
Mass of elements
BEFORE
Mass of elements
AFTER

5. Conservation of Mass
The law of conservation of mass states that the total mass before a chemical reaction is equal to the total mass after the reaction.

6. Reactant + Reactant → Product
Chemical Equations
Reactants are the starting materials in a chemical reaction and are placed on the LEFT side.
Products are the ending materials in a chemical reaction and are placed on the RIGHT side.
Reactant + Reactant → Product
H O → H2O2

7. Chemical Equations
“Tin + oxygen → tin oxide” reads as “tin plus oxygen produces tin oxide.”
Word equations can be long and do not show that mass is conserved.

8. Chemical Equations
Instead of writing long word equations, scientists use symbols and formulas.
Symbols represent atoms.
Formulas represent molecules.
Molecules may be elements or compounds.

9. Chemical Equations A molecule is the combination of 2 or more atoms.
Some molecules have atoms that are alike, called diatomic (O2).
Most molecules are made of more than one type of atom (H2O) – a compound.

10. Chemical Equations

11. Chemical Equations
Chemical equations use the symbols and formulas instead of words.

12. Balancing a Chemical Equation
Atoms are neither gained nor lost in a reaction, so both sides of the equation must have the same number of atoms.

13. Balancing a Chemical Equation
A subscript tells how many atoms of an element are in one molecule.
A coefficient tells how many atoms, molecules, or formula units are in a reaction.
subscript
2H2O
coefficient

14. Balancing a Chemical Equation
(i.e.) “Oxygen reacts with hydrogen to form water.”
H2 + O2 → H2O
Reactants
Products H O 2 1
This is unbalanced, and we CANNOT change the subscripts!
We add coefficients, a number in FRONT of a chemical formula that tells you how many molecules or atoms of each reactant & product act in the reaction. We do not write “1.”

15. We add a coefficient of “2” in front of the product: H2 + O2 → 2 H2O
Reactants
Products H O 2 4

16. The oxygen atoms are now balanced, but the hydrogens are not anymore
The oxygen atoms are now balanced, but the hydrogens are not anymore. Add a coefficient of “2” in front of the H2:
2 H2 + O2 → 2 H2O
Reactants
Products H O 4 2

17. Balancing a Chemical Equation

18. Writing Balanced Equations
Step 1 Determine the correct symbols and formulas for reactants and products.
Step 2 Write reactant symbols and formulas to the left of an arrow and product symbols and formulas to the right.
Step 3 Count the number of each kind of atom on both sides.
Step 4 Use coefficients to make the number of each kind of atom the same on both sides of the arrow.
Step 5 Check to see that each kind of atom balances.

19. Writing Balanced Equations
Reaction of methane:
CH4 + O2 → CO2 + H2O balance hydrogen
CH4 + O2 → CO2 + 2H2O balance oxygen
CH4 + 2O2 → CO2 + 2H2O balanced equation

20. Writing Balanced Equations
Baking soda and vinegar:
NaHCO3 + HC2H3O2 → CO2 + H2O + NaC2H3O2

21. Writing Balanced Equations
When counting atoms inside parentheses, the subscript multiplies all atoms inside.
Ca + H2O → Ca(OH)2 + H2
Ca + 2H2O → Ca(OH)2 + H2

22. Which is a diatomic molecule? A O2 B H2O C CH4 D N
8.2 Chemical Equations
Which is a diatomic molecule?
A O2
B H2O
C CH4
D N

23. What is potassium bromide (KBr)? A an element B a mixture C a compound
8.2 Chemical Equations
What is potassium bromide (KBr)?
A an element
B a mixture
C a compound
D a diatomic molecule

24. In the following equation, which is a reactant?
8.2 Chemical Equations
In the following equation, which is a reactant?
Ca +2H2O → Ca(OH)2 + H2
A H2
B (OH)2
C Ca
D none of the above

25. What is the coefficient of H2O?
Ca + 2H2O → Ca(OH)2 + H2
A 1
B 0
C 4
D 2

26. How many (OH) molecules are in the products?
Ca + 2H2O → Ca(OH)2 + H2
A 1
B 2
C 3
D 4

27. In a chemical reaction, which law states the products have the same mass as the reactants?
A Law of Conservation of Water
B Law of Conservation of Energy
C Law of Conservation of Mass
D Law of Conservation of Particles

28. If an equation is balanced, the number of atoms in the products is ____ the number of atoms in the reactants.
A greater than
B less than
C equal to
D none of the above

29. Symbols represent ____ and ____ represent molecules.
A elements; formulas
B formulas; equations
C molecules; formulas
D elements; equations