Representing Chemical Reactions

Representing Chemical Reactions with Equations Reaction of sodium with water ((Phenolphthalein added) 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 Na +1 OH -1

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

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) + H2O(l) → Na+1 OH-1 + H2 Na(s) + H2O(l) → NaOH(aq) + H2(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 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. + +

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 Br2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br2 = 2 Br2 = 3 Br2 =

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

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

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

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

2 Mg(s) + O2(g) → 2 MgO(s) ∆