1. Balancing Equations
Chapter 6

2. How do we know a chemical reaction has occurred?
The color changes.
A solid forms.
Bubbles form.
Heat and/or a flame is produced, or heat is absorbed.

3. Vocab.
A chemical change involves a rearrangement of the atoms; this is called a chemical reaction.
A chemical reaction is represented by a chemical equation.
Reactants-to the left of the arrow
Products-to the right of the arrow

4. Label the reactants and products in the below equations.
2Na+Cl22NaCl
2NaCl2Na + Cl2

5. Chemical Equations Are sentences.
Describe what happens in a chemical reaction.
Reactants ® Products
Equations should be balanced.
Have the same number of each kind of atoms on both sides because ...

6. Law of Conservation of Mass
In a chemical reaction, atoms are neither created or destroyed.
The atoms on the left (reactants) must equal (balance) the atoms on the right (products).

7. Balancing equations CH4 + O2 ® CO2 + H2O 1 C 1 4 H 2 2 O 3 Reactants
Products 1 C 1 4 H 2 2 O 3

8. Balancing equations CH4 + O2 ® CO2 + 2 H2O 1 C 1 4 H 2 4 2 O 3
Reactants
Products 1 C 1 4 H 2 4 2 O 3

9. Balancing equations CH4 + O2 ® CO2 + 2 H2O 1 C 1 4 H 2 4 4 2 O 3
Reactants
Products 1 C 1 4 H 2 4 4 2 O 3

10. Balancing equations CH4 + 2O2 ® CO2 + 2 H2O 1 C 1 4 H 2 4 4 4 2 O 3
Reactants
Products 1 C 1 4 H 2 4 4 4 2 O 3

11. Physical States
Physical states are represented by abbreviations beside the symbols of a chemical equation.
s solid
l liquid
g gas
aq dissolved in water (aqueous)

12. Abbreviations
(s) ¯
(g) ­
(aq)
heat D
catalyst

13. Meaning
A balanced equation can be used to describe a reaction in molecules and atoms.
Not grams.
Chemical reactions happen molecules at a time
or dozens of molecules at a time
or moles of molecules.

14. What is wrong with this equation?
Al2S3 + Cl2 AlCl3 + S

15. Balancing Equations __Al2S3 + __Cl2 __AlCl3 + __S
In order to balance the equation, we have to write numbers in front. Try to balance the previous example by writing numbers in front (coefficients) of each element or compound.
__Al2S3 + __Cl2 __AlCl3 + __S

16. Answer
1_Al2S3 +_3_Cl2_2_AlCl3 + _3_S

17. Practice balancing these
Ca(OH)2 + H3PO4 ® H2O + Ca3(PO4)2
Cr + S8 ® Cr2S3
KClO3(s) ® KCl(g) + O2(g)
Solid iron(III) sulfide reacts with gaseous hydrogen chloride to form solid iron(III) chloride and hydrogen sulfide gas.
Fe2O3(s) + Al(s) ® Fe(s) + Al2O3(s)

18. Practice balancing these
3Ca(OH)2+2H3PO4 ®6H2O + 1Ca3(PO4)2
16Cr + 3S8 ® 8Cr2S3
2KClO3(s) ® 2KCl(g) + 3O2(g)
Solid iron(III) sulfide reacts with gaseous hydrogen chloride to form solid iron(III) chloride and hydrogen sulfide gas.
1Fe2S3 + 6HCl ® 2FeCl3 + 3H2S
1Fe2O3(s) + 2Al(s) ® 2Fe(s) + 1Al2O3(s)

19. See if you can translate this chemical equation into a word equation.
K(s) + H2O(l)H2(g) + KOH (aq)

20. Answer
Solid potassium reacts (+) with liquid water to produce () hydrogen gas and (+) aqueous potassium hydroxide