Preparation for AP Exam Free Response Question #4

Writing Reactions

In this question, you will be given just the reactants of a reaction, described in words. You will have to write and balance the entire reaction (predicting

products) and also answer a question (????) about each reaction.This section was different a couple years ago, but it is still the hardest section

for chemists and students. We’ll do our best to prepare!

You need to be familiar with the various reaction types to help predict the products:

5 types of reactions- Double Replacement- Lewis Acid-Base- Organic- Redox- Complex Ion Formation

Precipitation Reactions(“double replacement”)

Description• two ionic compounds (or acids/bases) mix and

form two new compounds.• one of the new combinations of ions prevents the

ions from reforming the original compounds either:- because it forms a stable precipitate- one product breaks apart forming gas, which

leaves the solution

Preparation & Hints• Write water as HOH and acids with hydrogens out front and

recognize that it is H+ and OH-

• Memorize:•special metal & polyatomic ions and charges•solubility rules to predict precipitates•strong acids and bases so that you recognize the weak ones (write strong acids and bases as ions, weak in molecular form)•the double replacement products that break into gases

you can think of or write chemicals in different ways for different reactions. Ammonia may be NH3(aq) for complex ions or NH4OH for double replacement or acid-base reactions

A potassium chromate solution and barium nitrate solution are combined.molecular equation: K2CrO4(aq) + Ba(NO3)2 (aq) productcomplete ionic equation: 2K+ + CrO4

2- + Ba2+ + 2NO3- 2K+ + CrO4

2- + Ba2+ + 2NO3-

Net ionic equation: CrO42-(aq) + Ba2+(aq) BaCrO4(s)

Always write the net ionic equation! All strong electrolytes should be shown as free ions (eliminate spectators) and weak electrolytes shown as precipitates (solids).

You need to balance equations but you don’t need to include the phases of the reactants and products.

Solutions: split into ions (unless insoluble)Solids, crystals, chunks: these are not ions, even if they are things that should be soluble

A carbonic acid solution is combined with a calcium chloride solution.

H2CO3(aq) + Ca2+ --->

2 H+(aq) + CaCO3(s)

Neutralization

Solutions of ammonia and sulfuric acid are mixed.strA + wkB = salt + HOH + H+

H2SO4 + NH3 H+ + SO4

2- + NH4OH NH4+ +SO4

2- + H2OH+ + NH4OH NH4

+ + H2O

wkA + strB = salt + HOH + OH-

strA + strB salt + HOH (Net ionic always: H+ + OH- = H2O )wkA + wkB = depends

anhydridesSolid sodium oxide reacts with sulfuric acidNa2O + H2SO4 Na2SO4 + H2ONa2O + H+ Na+ + H2OSulfur trioxide gas is bubbled through a sodium hydroxide solution 2NaOH + SO3 Na2SO4 + H2OOH- + SO3 SO4

2- + H2O

Know the strong acids & bases!

Gas-Forming Reactions

Sulfuric acid erodes a limestone statue (solid calcium carbonate)CaCO3(s) + H2SO4(aq) ---> CaSO4(s) + H2CO3(aq) Carbonic acid is unstable: H2CO3(aq) ---> CO2 + waterCaCO3(s) + H+ (aq) ---> Ca2+ + CO2 + H2O

Description• Two species come together and share electrons in a coordinate covalent bond. The

species who donates the electrons is the Lewis base and the accepting species is the Lewis acid.

Preparation & Hints• “Have pair will share” = Lewis base; e- acceptors are Lewis acids• Aluminum ions and Fe3+ ions make a solution acidic because they are Lewis acids,

drawing e- away from O in H2O, making the O-H bond more polar, causing H+ to pull off H2O molecules

• Watch for: NO3-, NO2

-, CO32-, SO3

2-, SO42- ions in solids that are heated in a vacuum (not

air), this is not combustion. A gas (ex. CO2) and an anhydride (ex. CaO) are formed.• Two uncombined elements can form a salt. • A Lewis acid and base can also form a salt: CO2 + CaO CaCO3

• When metals and metal hydrides react with water, they usually form bases and hydrogen gas.

• If a gas compound is bubbled through a solution, water is acting like a Lewis base or acid.

Lewis Acid-Base Reactions

Sulfur dioxide gas is bubbled through waterSO2(g) + H2O(l) H2SO3(aq)

Nonmetal oxides (CO2 SO2 SO3 NO2 P4O10) form acids in water

A calcium oxide solution is prepared.CaO(s) + H2O(l) --> Ca(OH)2(aq)

Metal oxides (MgO, CaO, Na2O) form bases in water

Organic/Combustion Reactions

Description• Involves hydrocarbons, commonly combustion

but quite often other reactionsPreparation and Hints• Combustion produces carbon dioxide and water• If something reacts with an alkene or alkane, it

results in substitution for hydrogen• Combustion or burned: O2 is a reactant

Methane reacts with chlorine gasCH4 + Cl2 CH3Cl + HCl

cis-1,2 dichloroethene reacts with hydrogen gas

CH

Cl Cl

HC + H-H C

HCl

Cl

HCH

H

Methane gas is combusted in airproducts depend on how much oxygen is available

O2HCOCH :limited very OO4HCO23O2CH :limited O

O2HCO2OCH :plentiful O

2242

2242

22242

Alcohols:*Esterification with carb. acids*Oxidation (they lose e- when lose a H) to form

aldehydes, ketones, or carb acidsEsters: Hydrolysis to yield carb. acids and alcoholsKetones: Reduction to yield an alcoholAldehydes:*Carbonyl groups (ketones AND aldehydes) react with

PCl5 to make acid chlorides*Oxidation to form carb. acidsAmines: React with acids to form ammonium salts;

condensation to form amides

Complex Ion FormationDescriptionA transition metal (and some other metals) reacts and gets

surrounded by ligands, resulting in a charged compound formed of multiple ions (ex Cu(NH3)4

2+)Ligand: has lone pair to act as lewis base and bond with metal

ion( lewis acid)Preparation and Hints• A ligand can be a polar molecule (H2O, NH3) or a negative anion:

Cl-, OH-, SCN-, CN- , S2O32-

• ‘Excess’ and ‘concentrated’ may indicate complex ions, not precipitates: AgNO3 + HCl forms a white precipitate but concentrated HCl results in AgCl2

- formation and the solution clears.

• Watch for FeSCN2+

Central Metal Ion Coordination numberAg+ 2Cu2+ 4Hg2+ 4 Generally, you place twiceCd2+ 4 the number of ligands aroundSn2+ 4 or 6 the central ion as the

chargeFe3+ 6 on that central ion.Al3+ 4 This is not always correct

but it will do.

Silver chloride dissolves in ammonia solutionAgCl(s) + 2NH3 [Ag(NH3)2]+ (aq) + Cl-

Aluminum hydroxide dissolves in a concentrated NaOH solutionAl(OH)3(s) + OH- Al(OH)4

- (aq)

Redox Reactions (“Oxidation-Reduction”)Description•An oxidizing species accepts electrons from a reducing species. The oxidation number of one element increasing as the other decreases.Preparation & Hints• Memorize:

-the strong oxidizers (ions with lots of oxygens) and what they turn in to (ex HNO3 may be an acid or an oxidizer forming NO2,)- the strong reducers and what they turn in to

• Look for:- free neutral element in the reaction.-battery reactions. Metal with the greatest E⁰ will reduce- key words like ‘acidified solution’ or an acid included in the reactants. The H+ will form H2O with oxygens from the oxidizer

• LeO says GeR• When the hydrides of an alkali metal, Ca, Ba, or Sr dissolve in water, hydroxides

form and H2 gas is released (redox)• When a hydroxide dissolves, hydroxides form but no gas is released (not redox)

Redox ReactionsMg + 2H+ Mg2+ + H2

Combination: sulfur reacts with oxygenS + O2 SO2

Decomposition: potassium chlorate undergoes decomposition 2KClO3 2KCl + 3O2

0 0 +4 -2

+1 +5 -2 +1 -1 0

Single Replacement: Magnesium metal reacts with hydrochloric acid

Disproportionation Reaction: Element is simultaneously oxidized and reduced

Chlorine gas is bubbled through a strongly basic solution

Cl2 + 2OH- ClO- + Cl- + H2O0

+1 -1