REPRESENTING CHEMICAL REACTIONS

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

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

Actual Product is highly Flammable Rxn is very exothermic (releases heat); Hydrogen can ignite Is Hydrogen (monoatomic or diatomic?) H 2

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)

Overall equation for sodium and water Is elemental sodium monoatomic or diatomic? What is the formula for water? Na (s) + H 2 O (l) → Na +1 OH -1 + H 2 Na (s) + H 2 O (l) → NaOH (aq) + H 2(g) ↑ This equation is breaking the law! Which law is being broken?

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

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.

Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br 2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br 2 = 2 Br 2 = 3 Br 2 =

Example Balancing with coefficients H 2 + O 2 → H 2 O video link video link Can we simply balance this equation by changing the subscript of O in the water to 2? H 2 + O 2 → H 2 O 2

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

H YDROGEN PEROXIDE REACTS WITH MnO 2 WHILE WATER DOES NOT !! V IDEO LINKV IDEO LINK Water + MnO 2 = NR Hydrogen peroxide + MnO 2 = MnO 2 water

Balance equation by using coefficients H 2 + O 2 → H 2 O 2 H 2 + O 2 → 2 H 2 O 2