1. Chemical Reactions

2. What You’ll Learn…
How to write chemical equations to describe reactions
How to classify and identify chemical reactions
How to write ionic equations for reactions that occur in aqueous solutions

3. Reactions and Equations
How to recognize evidence of chemical change
How to represent chemical reactions with equations

4. Chemical Reaction:
A process by which atoms of one or more substances are rearranged to form different substances
A.K.A. chemical change

5. Evidence of Chemical Reactions
Change of:
Temperature:
Exothermic – energy released (heat…hot and light)
Endothermic- absorb energy (heat…cold)
Color
Production of:
Odor
Gas
Solid (precipitate)

6. What are the clues that a chemical reaction has occurred?

7. Production of a gas

8. Production of a solid (a precipitate.)
Production of a solid (a precipitate.)

9. http://www. youtube. com/watch
Change of color

10. Heat and/or light are emitted.

11. The key to chemical reactions!
The Law of Conservation of Mass
In a chemical reaction, no atoms are created or destroyed. The atoms in the reacting molecules simply re-arrange to form new molecules.
Since atoms aren’t created or destroyed, no mass is created or destroyed either.
Mass is “conserved.”

12. Count the atoms, before & after
2H2 + O2 2H2O

13. Representing Chemical Reactions
Reactants: Starting substances, usually on the left Products Substances formed, usually on the right

14. Terminology
In a chemical reaction, the materials to the left of the arrow are the reactants. Reactants are consumed in the reaction. The arrow is like an equal sign in math. It can be read “yields” or “produces” or “forms”. The materials to the right of the arrow are the products. They form as the reaction proceeds. H2O + SO3 H2SO4

15. Representing Chemical Reactions
Symbols Used in Equations
Symbol
Meaning +
Separates two or more reactants or products 
“yields” or
“react to produce”
Separates reactants and products
(s)
Solid state
(l)
Liquid state
(g)
Gas state
(aq)
“dissolved in water”
Aqueous solution
Representing Chemical Reactions
reactant 1 + reactant 2 
product 1 + product 2
Physical state can provide clues about how the reaction occurs

16. Phase Notation
A chemical equation can also show the physical state of the materials in the reaction. This is done with phase notations:
Phase notations are written to the lower right of each formula. Clues to phase notations are often given in equations written in words.

17. Other Phase Notations (ppt) or ( ) means “a precipitate forms”
( ) means “a gas bubbles off”
means “is heated”

18. Representing Chemical Reactions
Word Equations
Ex. Iron(s) + chlorine(g)  iron(III) chloride(s)
Chemical Equations  BALANCED!!!
2Fe(s) + 3Cl2(g)  2FeCl3(s)

19. Chemical Equation:
A statement that uses chemical formulas to show identities and relative amounts of the substances in a chemical reaction
MUST show that the law of conservation
of mass is obeyed
# atoms of each reactant and each product is equal on BOTH sides of the 

20. Balancing Chemical Equations
Steps
Step 1
Write the skeleton equation for the reaction
Step 2
Count the atoms of the elements in the reactants
Step 3
Count the atoms of the elements in the products
Step 4
Change the coefficients to make the # atoms of each element equal on both sides
Step 5
Write the coefficients in their lowest possible ratio
Step 6
Check your work
Balancing Chemical Equations
Coefficient: # written in front of reactant or product
Whole numbers
Value of 1 not usually written

21. REMEMBER!!
ALL chemical reactions obey the law that matter is neither created nor destroyed!

22. What else is conserved?
In N H2 2NH3 what besides mass (grams) is being conserved?
Atoms?
Molecules?
Moles?

23. Coefficients
In a chemical reaction, if more than 1 molecule of a substance reacts or is produced, the number of molecules is shown with a coefficient.
6CO H2O C6H12O O2
Coefficients multiply subscripts through the whole molecule that follows, so 6CO2 shows that there are 6 Carbon atoms and 12 Oxygen atoms.

24. How many?
How many of each atom are shown by
4Al2(SO4)3
3(NH4)3PO4