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 Concentration and dilutions Quantitative analysis Gravimetric and volumetric analyses

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 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: 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 rules Li+, Na+, K+, NH4+

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 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 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 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 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 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 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 (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 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 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 Molarity: measure of concentration = (moles of solute / liters of solution) Unit: mol/L Diluting solutions: MiVi = MfVf

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 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