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Chapter 4: Chemical Reactions. Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor. Chemical reactions. Ions in aqueous solution Molecular and ionic equations Types of reactions Precipitation reactions Acid-base reactions Oxidation-reduction reactions Solutions

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Chapter 4 chemical reactions
Chapter 4: Chemical Reactions

Chemistry 1061: Principles of Chemistry I

Andy Aspaas, Instructor

Chemical reactions
Chemical reactions

  • Ions in aqueous solution

    • Molecular and ionic equations

  • Types of reactions

    • Precipitation reactions

    • Acid-base reactions

    • Oxidation-reduction reactions

  • Solutions

    • Concentration and dilutions

  • Quantitative analysis

    • Gravimetric and volumetric analyses

Ions in aqueous solutions
Ions in aqueous solutions

  • Ionic theory of solutions: Arrhenius, 1884

    • When dissolved in water, the individual ions of ionic substances completely separate and enable the solution to conduct electricity

    • Pure water is a poor conductor of electricity

  • Electrolyte: substance that dissolves in water to give an electrically conducting solution

    • Generally, ionic solids that dissolve in water are electrolytes

    • A few molecular electrolytes, Ex. HCl (g)

    • Nonelectrolytes: dissolve in water, poorly conducting solution, usually neutral molecular substances

Strong and weak electrolytes
Strong and weak electrolytes

  • The extent to which a solution conducts electricity indicates the “strength” of the dissolved electrolyte

    • Strong electrolytes: exist in solution almost entirely as ions

      • Ex. NaCl

    • Weak electrolytes: dissolve in water to give only a small percentage of dissociated ions

      • Ex. NH3

Solubility rules
Solubility rules

  • Solubility: ability of a substance to dissolve completely in water

    • Ex. Sugar, NaCl, ethyl alcohol are soluble

    • Ex. Calcium carbonate, benzene are insoluble

  • Soluble ionic compounds are strong electrolytes

    • 8 solubility rules can determine whether an ionic compound is soluble or not

Solubility rules1
Solubility rules

Li+, Na+, K+, NH4+

Molecular and ionic equations
Molecular and ionic equations

  • Molecular equation: chemical equation in which reactants and products are written as if they were molecular substances, even if they exist as ions in solution

    • Explicit in the actual compounds added to a solution, and the products obtained

  • Complete ionic equation: all strong electrolytes are written as their dissociated ions (aq)

    • Insoluble compounds are written as a solid compound, not ions

Net ionic equations
Net ionic equations

  • Spectator ion: ion in an ionic equation that does not take part in the reaction

    • Appears in ionic form on both sides of a reaction

  • Net ionic equation: equation in which all spectator ions have been canceled

  • Several different reactions can have the same net ionic equation

Precipitation reactions
Precipitation reactions

  • Precipitate: insoluble compound formed during a chemical reaction in solution

  • Predicting precipitation reactions:

    • Exchange reaction most common, each compound “trades partners” to form products

    • Write molecular equation

    • Use solubility rules to determine phase lables for each product and reactant; (aq) if soluble, (s) if insoluble

    • If all components of reaction are soluble, no reaction occurs

    • If a product is insoluble, it forms as a precipitate

    • A net ionic equation shows the reaction at the ionic level

Acid base reactions
Acid-base reactions

  • Acids: vinegar (acetic acid), lemon juice (citric acid), Coca-Cola (phosphoric acid and carbonic acid), battery acid (sulfuric acid)

  • Bases: Drano (sodium hydroxide), ammonia, Milk of Magnesia (magnesium hydroxide)

  • Brønsted-Lowry acid: molecule or ion that donates a proton to another species in a proton-transfer reaction

  • Brønsted-Lowry base: molecule or ion that accepts a proton in a proton transfer reaction

Strong acids and strong bases
Strong acids and strong bases

  • Strong acids and bases ionize completely in water

    • Strong acids: HClO4, H2SO4, HI, HBr, HCl, HNO3

    • Strong bases: LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

  • Weak acids and bases only partly ionize in water

Neutralization reactions
Neutralization reactions

  • Reaction between acid and base to produce a salt and possibly water

    • Salt: ionic compound formed in neutralization reaction

  • Start by writing molecular equation

    • Acid anion and base cation form the salt

    • Water is usually a product

  • Net ionic equation: write any strong acid or base as its dissociated ions

Acid base reactions with gas formation
Acid-base reactions with gas formation

  • Carbonates (CO32-) form H2O and CO2 when reacted with acids

  • Sufites (SO32-) form H2O and SO2 when reacted with acids

  • Sulfides (S2-) form H2S when reacted with acids

Oxidation reduction reactions
Oxidation-reduction reactions

  • Oxidation-reduction reactions (redox) involve transfer of electrons

  • Oxidation number: actual charge of an atom if it exists as a monatomic ion, or a hypothetical charge assigned by a few rules

    • Elemental atoms always have ox. # 0

    • Oxygen is usually -2

    • Hydrogen is usually +1

    • Halogens usually -1 (unless bonded to another halogen or oxygen)

    • Sum of ox. #’s of atoms in a compound is 0, sum of ox #’s in a polyatomic ion is the charge on the ion

Describing oxidation reduction reactions
Describing oxidation-reduction reactions

  • If a species loses electrons, it is oxidized

  • If a species gains electrions, it is reduced

    • LEO, GER

  • Use oxidation numbers to determine this

  • Oxidizing agent: species that oxidizes another species, and is itself reduced

  • Reducing agent: species that reduces another species, and is itself oxidized

Combustion reaction
Combustion reaction

  • Reaction in which a substance reacts with oxygen, usually accompanied by release of heat and production of a flame

  • Organic compounds combust to form CO2 and H2O

  • Metals combust to form metal oxides

Molar concentration
Molar concentration

  • Molarity: measure of concentration

    = (moles of solute / liters of solution)

    • Unit: mol/L

  • Diluting solutions: MiVi = MfVf

Gravimetric analysis
Gravimetric analysis

  • Determination of amount of a species by precipitating that species out as an insoluble compound, and weighing the product

  • Mass precipitated product  moles product  moles unknown species  mass unknown species

Volumetric analysis
Volumetric analysis

  • Titration: method for determining amount of one substance by adding a precise volume of another substance until the two substances completely react

  • Colored pH indicator often used to detect endpoint

  • Volume added solution  moles added solution  moles unkn. solution  molarity or grams unkn. solution