1. 4.3 Conservation of Mass and Chemical Equations

2. Learning Goals Learn what is the law of conservation of mass
Learn how to represent chemical reactions by word equations and skeleton equations

3. Reactants and Products
In chemical reactions we have
Reactants: pure substance undergoing a chemical change
products that we combine (or start the reaction with)
Products: pure substance formed by a chemical change
the result of the reaction
Contain different properties than the reactants

4. Chemical Reactions Are represented as either Word equations
Skeleton equations (periodic symbol)
What are the states that an element can exist in?
- Solid (s)
- Liquid (l)
- Gas (g)
In reactions, compounds can also be AQUEOUS (aq)
-This means that the compound dissolves in water

5. Law of Conservation of Mass
In a chemical reaction, the total mass of the reactants is equal to the total mass of the products
This means that we will have the same number of atoms of each element of reactants, as products

6. Balancing Chemical Equations
In order to create a balanced chemical equation, you must know how to determine the total number of atoms in a compound using coefficients.
What is the total number of atoms of each element?
2 NaCl
6 H2O
2NaNO3
(NH4)2SO4

7. Writing Chemical Equations
1. Word Equations
hydrogen + oxygen  water
“Reacts with”
“yields OR reacts to produce”
Product
Reactants

8. Writing Chemical Equations
2. Skeleton Equation
H2(g) O2(g)  H2O(l)
What is wrong with this chemical equation?
It must be balanced!!!

9. Writing Chemical Equations
H2(g) O2(g)  H2O(l)
Determine the number of atoms for each element in the reactant.
Determine the number of atoms for each element in the product.
H =
O = 2
H =
O = 2 2 1
Balance by adding coefficients to the unequal element

10. Writing Chemical Equations
H O2  H2O
Determine the number of atoms for each element in the reactant.
Determine the number of atoms for each element in the product.
H =
O = 2
H =
O = 4 2 2
Continue balancing by adding coefficients to the unequal element

11. Writing Chemical Equations
2 H2(g) O2(g)  H2O(l)
Determine the number of atoms for each element in the reactant.
Determine the number of atoms for each element in the product.
H =
O = 4
H =
O = 4 2 2

12. Writing Chemical Equations
2. Balanced Skeleton Equation
2 H2(g) O2(g)  H2O(l)

14. Practice Balance each chemical equation: A) Mg (s) + O2 (g)  MgO (s)
B) Li (s) Br2 (g)  LiBr(s)
Al (s) CuO (s)  Al2O3 (s) Cu (s)

15. Homework Textbook: Read Section 4.3 Balancing Equations Worksheets
Chapter 4 Quiz Wednesday!