1. Unit 6 Chemical Reactions

2. Objectives Write chemical equations to describe chemical reactions
Balance chemical equations
Classify and identify chemical reactions
Predict products from chemical reactions

3. I. Reactions and equations
Chemical reactions:
A chemical change has occurred
Evidence of a chemical reaction
Temperature change (energy
change)
Change in color

4. Evidence of chemical change
Gas production (bubbles, fizzing, odor)
Formation of a solid (precipitate)

5. YouTube - ammonium dichromate volcano
YouTube - Nitrogen Triiodide Detonation
YouTube - Sodium and Water
YouTube - Reaction of Sodium & Chlorine (with subtitles)

6. Representing chemical reactions:
Reactants: starting substances (written on the left side)
Products: substances that result from the reaction (written on the right side)
An arrow is written between reactants and products and is read as “yields”

7. Symbols used in chemical equations (p 203)
Meaning 
Yield (placed between reactants and products)
Separates reactants and products; indicates reversible reaction +
Separate two or more reactants or products
(s)
Substance is in a solid state
(l)
Substance is in a liquid state
(g)
Substance is in a gaseous state
(aq)
Substance is in an aqueous (water) solution 
Heat is applied
A substance written above an arrow indicates it is a catalyst needed for the reaction Pt

8. Word Equations Skeleton equations
Statements used to indicate reactants and products
Lack important information
Ex. Hydrogen (g) + oxygen (g)  water (l)
Skeleton equations
Chemical formulas are used to represent reactants and products
Ex. H2 (g) + O2 (g)  H2O (l)

9. Learning Check: Write the skeleton equations for the following:
Hydrogen (g) + Bromine (l)  hydrogen bromide (g)
Carbon monoxide (g) + oxygen (g)  carbon dioxide (g)

10. Practice: Write the skeleton equations for the following:
Hydrogen (g) + Bromine (l)  hydrogen bromide (g)
H2 (g) + Br2 (l)  HBr (g)
Carbon monoxide (g) + oxygen (g)  carbon dioxide (g)
CO (g) + O2 (g)  CO2 (g)
CW p 284 # 3, p #1,2

11. II. Balancing chemical equations
Must obey “law of conservation of matter”
Must show that the number of atoms of each substance is the same before and after the reaction.
A chemical equation has to be BALANCED
To balance chemical equations, COEFFICIENTS (whole number)are written in front of a reactant or product.
If coefficient is 1, it is not written

12. Steps for balancing equations
Write the skeleton equation for the reaction. Ex. Hydrogen gas reacts with chlorine gas and yields gaseous hydrogen chloride.
H2 (g) + Cl2 (g)  HCl(g)
Count the atoms of each substance for the reactants and products.
REACTANTS
PRODUCTS
H :
H :
Cl :
Cl :

13. Place coefficients (ONLY IN FRONT OF A REACTANT OR PRODUCT) to make number of atoms of each element equal on both sides of the equation. (#atoms= coefficient x subscript)
H2 (g) + Cl2 (g)  HCl(g)
REACTANTS
PRODUCTS
H :
H :
Cl :
Cl :

14. Write coefficients in lowest ratio possible
H2 (g) + Cl2 (g)  2 HCl(g)
Check your answer.
REACTANTS
PRODUCTS
H : 2
H : 2
Cl : 2
Cl : 2

15. Practice
1. Write a balanced chemical equation if sulfur trioxide when heated yields sulfur dioxide and oxygen.

16. Write a balanced chemical equation for:
Iron (III) oxide iron (II) oxide + oxygen

17. Balance the following reaction
CaO + H2O  Ca(OH)2

18. Reactions for gummy bear sacrifice
KClO3  KCl O2

19. CW p balancing equations handout