MASON CHEM 211 - CHAPTER 4 Chemical Reactions (22 pages)

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CHAPTER 4 Chemical Reactions



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CHAPTER 4 Chemical Reactions

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22
School:
George Mason University
Course:
Chem 211 - General Chemistry I
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CHAPTER 4 Chemical Reactions John A Schreifels Chemistry 211 notes 1 Overview Ions in Aqueous Solution Ion theory in solutions precipitation reactions Molecular and ionic equation Typical Reactions Precipitation Acid Base Oxidation Reduction Balancing Working with solutions Quantitative analysis John A Schreifels Chemistry 211 notes 2 Ionic Thory of Solutions Pure water is a very poor conductor of electricity Solutions from dissolving NaCl or KCl in water are very conductive Solutions from dissolving substances such as sugar sucrose C12H22O11 are non conductive Electrolyte substance that produces ions when dissolved in water Strong good electrical conductor when dissolved in water completely ionized E g NaCl KNO3 Mg NO3 2 etc H O NaCl s 2 Na aq Cl aq Weak poor conductor when dissolved in water partial ionization NH3 aq H 2 O l NH 4 aq OH Non electrolyte substance that does not produce ions when dissolved in water John A Schreifels Chemistry 211 notes 3 Aqueous Reactions and Net Ionic Equations Three forms for writing chemical reaction Molecular AgNO3 aq NaCl aq AgCl s NaNO3 aq Ionic Spectator ions are not directly involved in the reaction aq Na aq Cl aq Na aq NO aq AgCl s Ag aq NO 3 3 Net ionic exclude spectator ions Ag aq Cl aq AgCl s John A Schreifels Chemistry 211 notes 4 Precipitation Reactions Metathesis reaction reaction in which two substances react through exchange of their components Driving force is often a precipitation AX BY AY BX E g Predict if precipitation occurs for the mixture AgNO3 aq KI aq NaClO4 aq Pb NO3 2 aq Na2SO4 aq BaCl2 aq Ni NO3 2 aq NH4 2S aq Hint Use the solubility rules to determine if either product is insoluble John A Schreifels Chemistry 211 notes 5 Solubility Rules Rule Exception Soluble Group 1 elements NH4 NO3 ClO3 ClO4 Chlorides bromides iodides Ag Pb2 Hg22 Acetates Ag Hg22 Sulfates Sr2 Ba2 Pb2 Ca2 Insoluble Compounds Carbonates phosphates oxalates chromates sulfides Group 1 NH4 Hydroxides oxides Group 1 Ba2 John A Schreifels Chemistry 211 notes 6 Metathesis Reaction cont Driving force is sometimes formation of weak or non electrolyte E g acid base reactions CuO s 2HNO3 aq Cu NO3 2 aq H2O l CuO is normally insoluble in water but readily dissolves in aqueous nitric acid sometimes formation of gas most common is CO2 from carbonates or H2S from sulfides E g CuCO3 s 2HNO3 aq CO2 g Cu NO3 2 aq H2O l CuS s 2HNO3 aq Cu NO3 2 aq H2S g John A Schreifels Chemistry 211 notes 7 Acids Bases and Salts Arrhenius definition most often used Acid a hydrogen containing compound that releases hydrogen ions H in solution HA aq H2O l H3O aq A aq where HA HCl HNO3 etc and H3O hydronium ion often written as H Base compound that releases hydroxide ions OH in solution The general reaction for a base is MOH s OH aq M aq where M some metal such as Na K etc Acids and bases can be strong or weak electrolytes A base acid that is a strong electrolyte is a strong base acid John A Schreifels Chemistry 211 notes 8 Polyprotic acids and weak bases Some acids have more than one acidic proton Sulfuric H 2 SO 4 l H 2 O l HSO 4 aq H3 O aq HSO 4 aq H 2 O l SO 24 aq H3 O aq aq H O aq H PO l H O l H PO Phosphoric 3 4 2 2 4 3 H2PO 4 aq H2O l HPO 24 aq H3O aq HPO24 aq H2O l PO34 aq H3O aq Most weak bases produce hydroxide ions by reaction with water Ammonia NH3 aq H2O NH4 aq OH aq John A Schreifels Chemistry 211 notes 9 Strong and Weak Acids and Bases Organic acids are weak usually have COOH Amines containing nitrogen are weak In water they are completely dissociated HCl aq H2O l H3O aq Cl aq Strong Acids Strong Bases Chloric HClO3 Grp 1A hydroxides LiOH NaOH KOH RbOH CsOH Hydrobromic HBr Hydroiodic HI Perchloric HClO4 Grp 2A metal hydroxides Ca OH 2 Sr OH 2 Ba OH 2 Sulfuric H2SO4 Nitric HNO3 John A Schreifels Chemistry 211 notes 10 Neutralization Reaction Acids react with bases to form a salt and possibly water called the neutralization reaction HA aq MOH aq M aq A aq H2O l If either the acid or base is a strong electrolyte exclude spectator ions in the ionic form E g HCN weak acid NaOH strong base neutralization reaction is HCN aq OH aq CN aq H2O l Eg 2 HCl neutralized by NaOH net ionic equation H aq OH aq H2O l John A Schreifels Chemistry 211 notes 11 Oxidation Reduction Oxidation loss of at least one electron during a reaction Ni s H aq Ni2 aq H2 g Reduction gain of at least one electron during a reaction In above example H gains an electron to become reduced Every reaction must have an oxidation and reduction Metals react with acids to form salts and hydrogen gas Cu s 2HNO3 aq Cu NO3 2 aq H2 g Metals also oxidized with salts Fe s Cu NO3 2 aq Fe NO3 2 aq Cu s John A Schreifels Chemistry 211 notes 12 Oxidation Number Oxidation number state the charge on an atom in a substance or monatomic ion Rules Elemental form 0 Monatomic ions charge of ion Oxygen 2 except in H2O2 and other peroxides Hydrogen 1 except with metal hydrides when it is 1 Halogens 1 except when bound to oxygen or a halide above it Alkali and alkaline earth metal ions have a charge of 1 and 2 respectively Compounds and ions sum of the charges on the atoms in a compound add up to 0 and to the ion charge in the ion John A Schreifels Chemistry 211 notes Ca in CaO 2 Ca2 aq 2 Cl aq 1 Cr in CrO3 6 Fe in Fe2O3 3 Cr in K2Cr2O7 6 13 Displacement Reactions Activity series of the elements A relative reactivity scale allows us to predict if reaction will occur when two substances are mixed together E g Copper ions in solution are reduced to the metal when an iron nail is placed in the solution Cu2 aq Fe s Fe2 aq Cu s Iron displaces copper Fe2 aq Cu s NR copper will not displace iron Iron more reactive than copper E g Predict which reaction will occur when Li is mixed with K and Li is mixed with K E g In which of the following mixtures will reaction occur Li Mg Al Mn2 Fe Cd2 Cr Zn2 John A Schreifels Chemistry 211 notes Li K Ba Ca Na Mg Al Zn Cr Fe Cd Reacts vigorously with acids to give H2 Reacts with H2O to give H2 Reacts with acids to give H2 Reacts slowly with H2O to give H2 more vigorous with steam 14 Balancing Oxidation Number Method Determine oxidation for each atom both sides of equation Determine change in oxidation state for each atom Left side make loss of electrons gain Balance other side Insert …


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