Chemical Reactions Quiz 1 Common Problems….. Writing AP Equations for a Happier Life   While AP no longer has an equation writing section embedded in the test, understanding what reaction will occur is assumed throughout the content. The best way to prepare for this is to practice lots of equations. These equations will need to be balanced and written in net ionic form. All spectator ions must be left out and all ions must be written with correct charges. All molecular substances and non-soluble compounds must be written together (not ionized!). Weak electrolytes, such as acetic acid, are not ionized. Solids, gases and pure liquids are written together also. Equations are generally one of a limited number of types, the first challenge is to determine that type…. #1 Issue – Writing Reactants & Products in Proper Form

Solids, gases and pure liquids are written together…… Solid lithium dioxide (#2) – Li2O(s) Solid calcium fluoride (#3) - CaF2(s) Carbon disulfide vapor (#4) - CS2(g) Potassium Chloride (#10) - KCl(s) (No water, not in solution) Weak Acids HF(l) (#3) HNO2(l) (#9) (l) = (aq); (l) LIQUIDS = (aq) IONS in solution

Weak Acids are not ionized Weak electrolytes, such as acetic acid, are not ionized (but strong acids are)…… Weak Acids are not ionized HF(l) (#3) HNO2(l) (#9) Strong Acids and Bases ARE ionized Na+(aq) + OH-(aq) - Sodium hydroxide (#6) 2H+(aq) + SO42- (aq) – Sulfuric Acid (#3) Compounds that might be strong electrolytes are still solid if there is no water…. - (#10) - KCl(s)

All spectator ions must be left out and all ions written with correct charges…. Are IONS (in solution) at the beginning and end Do not react (they “watch” or “spectate”) Look for ions that are usually soluble (e.g. sodium, potassium, chloride, nitrate, sulfate) SO42- (aq) (#3) Na+(aq) & NO3-(aq) (#5) Cl-(aq) (#7)

Anhydrides (#2 & #9) Anhydride means “without water”. Water is a reactant, and there is NO oxidation state change (i.e. not redox) Metal oxide (basic anhydrides) plus water yields bases (#2) Solid lithium oxide is added to excess water Li2O(s) + H2O(l)  2Li+(aq) + 2OH-(aq) Non-metal oxides (acid anhydrides) plus water yields acids (#9) Dinitrogen trioxide gas is bubbled into water. N2O3(g) + H2O(l)  2HNO2(l) Metallic hydrides, phosphorous halides and group I & II nitrides on later problems

(free metals are strong reducers….) Strongly reducing metals (Li, K, Ba, Ca, Na) react with water to form metal ions, hydroxide and hydrogen (#6) Sodium metal is added to water. 2Na(s) + 2H2O(l)  2Na+(aq) + 2OH-(aq) + H2(g) Note – CHARGES must balance (because it is a redox equation)… Active metals replace less active metals (or hydrogen) (#7) Magnesium turnings are added to a solution of iron (III) chloride 3Mg(s) + 2Fe3+(aq)  3Mg2+(aq) + 2Fe(s)

Other stuff….. Redox: Oxidation and reduction BOTH occur….. Magnesium turnings are added to a solution of iron (III) chloride Mg(s) + 2Cl-(aq)  2MgCl2 has two problems: While Mg has oxidized, nothing has reduced Cl- is a spectator ion Metathesis: A conjugate base of a weak acid will attract protons in solution (#3) Dilute sulfuric acid is added to solid calcium fluoride 2H+(aq) + CaF2(s)  2HF(l) + Ca2+(aq) Calcium was part of the solid, so it is NOT a spectator Sulfate is a spectator (CaSO4 is soluble)