REPRESENTING CHEMICAL REACTIONS Representing Chemical Reactions with Equations

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 Na +1 • Magenta pink = basic solution high concentration of hydroxide ion OH -1 • Bubbles = gas • Possible gases oxygen or hydrogen

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) → Na. OH(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 • 2 Na(s) + 2 H 2 O(l) → 2 Na. OH(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 • 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 • 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

HYDROGEN PEROXIDE REACTS WITHMn. O 2 WHILE WATER DOES NOT!!VIDEO LINK Water + Mn. O 2 = NR Mn. O 2 water Hydrogen peroxide + Mn. O 2 =

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

2 Mg(s) + O 2(g) ∆ → 2 Mg. O(s)