1. Unit 4 – Chemical Equations & Reactions
Conceptual Chemistry
Unit 4 – Chemical Equations & Reactions

2. Objective 1
Read and express information given in chemical formulas.

3. Chemical Formula
Shows the type and number of atoms present in a substance.

4. Chemical Formula
In the chemical formula, subscripts show how many atoms are in the compound.
C6H10N2O2
C = 6
H= 10
N= 2
O= 2

5. Chemical Formula
The large numbers in front of the chemical formula, called coefficients, show the number of particles.
3C6H10N2O2
C6H10N2O2 = 3 total

6. Practice Problems: How atoms of each element are in the following?
AlCl3
4 H2O
10 Fe3P2
6 Zn(NO3)2
Al2(SO3)3

7. Objective 2
Balance chemical equations.

8. Chemical Equation Reactants  Products Ex: 2 KClO3  2 KCl + 3 O2
Reactant = KClO3
Products = KCl + O2

9. Symbols (s) = (l) = (g) = (aq) = + =  = ∆ = H2 (subscripts) =
(l) =
(g) =
(aq) =
+ =
 =
∆ =
H2 (subscripts) =
2 H2 = (Coefficients) =

10. Balancing Equations
The law of conservation of mass states that matter cannot be created or destroyed.
The number of atoms reacting must equal the number of atoms produced.
A chemical change rearranges these atoms into new substances.

11. To balance equations, follow these steps:
1. Count atoms on each side of the arrow. They should be equal. If not, you need to balance them. 2. Balance both sides by using coefficients. (They multiply through everything inside of that compound.) NEVER change subscripts! 3. Double check work.

12. Practice Problems
____ KClO3  ____ KCl + ____ O2 ____AlBr3 + ____ K2SO4  ____ KBr + ____ Al2(SO4)3 ____ C3H8 + ____ O2  ____ CO2 + ____ H2O

13. Objective 3
Describe chemical reactions and calculate percent yield.

14. Percent Yield Used to determine completeness of a reaction
Actual Yield: actual amount of products produced in an experiment
Theoretical Yield: maximum amount of products predicted from conservation of mass
Percent Yield = Actual Yield x 100%
Theoretical Yield

15. Practice Problem
When a measured amount of hydrogen gas reacted with a measured amount of oxygen gas, 2.5 g of water was produced. The expected amount was 3.0 g. What was the percent yield?