1. Chemical Equations & Reactions

2. Count the atoms
Cl Cl Cl2
Compounds can also have brackets to indicate the number of atoms.
E.g. Ca(OH)2 O H Ca
Remember that “OH” is a polyatomic ion
The two outside of the brackets represents 2 atoms of hydrogen and two atoms of oxygen.

3. Try these! a) NaOH b) Be(NO3)2 Na = 1, O = 1, H = 1 c) 3Ca(OH)2
Be = 1, O = 6, N = 2
Ca = 3, O = 6, H = 6

4. Counting Atoms
The symbol of an element represents 1 atom of this element. 
  Example:   Ca = ______ Ca?
Subscript numbers only apply to the number it touches. 
N2 = ______N?
H2O means 2 H and 1 O
CaCl2 means ______ Ca and _____ Cl?

5. When brackets are used, a number outside of the brackets will apply to all atoms inside:
Ca(OH)2 means 1 Ca, 2 O and 2 H
Ba3(PO4)2 = ______Ba, _____P, ______O?
A coefficient before the symbol represents the number of atoms of that particular element.
Example: 3C = _____ C?

6. Multiplication is used when two numbers are present
2H2O means 4 H and 2 O
3SrCl2 means… _____Sr and _____Cl?
2Mg(NO3)2 means…___Mg, ____N, ____O?

7. Writing Chemical Equations
Products on right side of the arrow
Reactants on left side of the arrow
hydrogen + oxygen → water
Plus sign means “reacts with”
Arrow means “produce”

8. So, let’s consider this chemical equation:
H2 + O2 → H2O
How many hydrogen and oxygen atoms in the
a) reactants? _____ H, _____O?
b) products? _____H, _____O? 

9. What about now?
2H2 + O2 → 2H2O
How many hydrogen and oxygen atoms in the
a) reactants? _____H, _______O?
b) products?  _____H, _____O?
What do you notice about the second equation?

10. Notes and practice – Counting atoms

11. Writing Chemical Equations
Products on right side of the arrow
Reactants on left side of the arrow
hydrogen + oxygen → water
Plus sign means “reacts with”
Arrow means “produce”

12. Balancing Chemical Equations
The law of conservation of mass states that the mass of the reactants in a chemical equation must equal the mass of the products.
Remember: “atoms cannot be created or destroyed” so we must have the same number of atoms of each type in the reactants and products

13. Balancing Chemical Equations

14. So… Here is a balanced equation
Mg + O2 → MgO
2 Mg + O2 → 2 MgO - Balanced!

15. Ex. Hydrogen reacts with oxygen to produce water
Types of equations
Word equation:
All elements are written out in words to describe the reaction
Ex. Hydrogen reacts with oxygen to produce water

16. Types of Equations Skeleton equation:
Summarizes the reaction in symbol form.
Does not follow the law of conservation of mass.
Ex: H2 + O2 → H2O
HOFBrINCl!

17. Types of Equations Balanced Chemical equation:
Includes coefficients to represent equal number of atoms on both sides of the equation
Ex: 2 H2 + O2 → 2 H2O
Coefficient – a number placed in front of an atom or chemical formula.

18. Tricks for Balancing
The ONLY number you can change is the number in front of the formula (coefficient)
Balance all atoms other than O and H (leave them for the end!)
Balance O, then H
Count polyatomic ions together if they are together on both sides. Ex: CaSO NaCl → CaCl2 + Na2SO4
CHECK to make sure ALL atoms are balanced at the end

19. Notes and Practice
Word Equations

20. Video – Chemical Equations
Video – Mr. Anderson
Video - Introduction to Balancing Chemical Equations

21. Web Site
Web Site - Phet

22. Try Some! __Mg + __O2 → __ MgO __SF4 + __H2O → __ SO2 + __ HF
__ Al + __ H2SO4 → __ Al2(SO4)3 + __H2
__ Ca(OH)2 + __ H3PO4 → __ Ca3(PO4)2 + __H2O

23. Balance the following skeleton equation:
a) Mg + HCl → MgCl2 + H2
b) Ca + N2 → Ca3N2
c) NH4NO3 → N2O + H2O
d) AlCl3 + H2S → Al2S3 + HCl
e) C4H10 + O2 → CO2 + H2O
f) O2 + C6H12O6 → CO2 + H2O
g) NO2 + H2O → HNO3 + NO
h) Cr2(SO4)3+ NaOH → Cr(OH)3+ Na2SO4
i) Al4C3 + H2O → CH4 + Al(OH)3

24. Answers: a) Mg + 2HCl → MgCl2 + H2 b) 3Ca + N2 → Ca3N2
c) NH4NO3 → N2O + 2H2O
d) 2AlCl3 + 3H2S → Al2S3 + 6HCl
e) 2C4H O2 → 8CO2 + 10H2O
f) 6O2 + C6H12O6 → 6CO2 + 6H2O
g) 3NO2 + H2O → 2HNO3 + NO
h) Cr2(SO4)3+ 6NaOH → 2Cr(OH)3+ 3Na2SO4
i) Al4C3 + 12H2O → 3CH4 + 4Al(OH)3