1. Representing Chemical Reactions

2. Representing Chemical Reactions with Equations
Reaction of sodium with water ((Phenolphthalein added)
Acid, neutral in phenolphthalein = colorless
Base in phenolphthalein = pink

3. Interpreting experimental evidence
Metal dissolves = + ion forming
Magenta pink = basic solution high concentration of hydroxide ion
Bubbles = gas
Possible gases oxygen or hydrogen
Na +1
OH -1

4. Actual Product is highly Flammable
Rxn is very exothermic (releases heat);
Hydrogen can ignite
Is Hydrogen
(monoatomic or diatomic?) H2

5. Common symbols in chemical reactions:
rxn = reaction NR = no reaction
→ yields or produces ∆
→ heat added to initiate rxn
(s) = solid ; ↓ = precipitate (solid product) formed
(l) = liquid
(g) = gas ; ↑ = gas released
(aq) = aqueous (dissolved in water)

6. Overall equation for sodium and water
Is elemental sodium monoatomic or diatomic?
What is the formula for water?
Na(s) + H2O(l) → Na+1 OH H2
Na(s) + H2O(l) → NaOH(aq) + H2(g) ↑
This equation is breaking the law! Which law is being broken?

7. Balanced Equations
Law of Conservation of Mass -Atoms are rearranged but conserved in chemical reactions
2Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g) ↑
Defn of Balanced Equation:
# and type of atoms on reactant side =
# and type of atoms on product side. + +

8. Balance Equations with Coefficients
Key idea: Chemical equations can only be balanced by changing coefficients, Never balance by changing subscripts.
Final coefficients must be simplest, whole #’s.

9. Notes on Coefficients and Subscripts
Subscript: # below line; indicates # of atoms bonded together Br subscript
Coefficient: # in front of formula; indicates number of units (atoms or molecules) present
Coefficient Na
Br = Br = Br =
Br2 = Br2 = Br2 =

10. Example Balancing with coefficients
H2 + O2 → H2O video link
Can we simply balance this equation by changing the subscript of O in the water to 2?
H2 + O2 → H2O2

11. Example Balancing with coefficients
H2 + O2 → H2O video link
Can we simply balance this equation by changing the subscript of O in the water to 2?
H2 + O2 → H2O2
NO! H2O2 is a different chemical than water

12. Hydrogen peroxide reacts with MnO2 while water does not!! Video link
Water + MnO2 = NR
Hydrogen peroxide + MnO2 =
MnO2
water

14. Balance equation by using coefficients
H2 + O2 → H2O
2 H2 + O2 → 2 H2O 2

15. 2 Mg(s) + O2(g) → 2 MgO(s) ∆