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Chapter 4: Chemical Reactions Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor.

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Presentation on theme: "Chapter 4: Chemical Reactions Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor."— Presentation transcript:

1 Chapter 4: Chemical Reactions Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor

2 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

3 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

4 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. NH 3

5 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

6 Solubility rules Li +, Na +, K +, NH 4 +

7 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

8 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

9 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

10 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

11 Strong acids and strong bases Strong acids and bases ionize completely in water –Strong acids: HClO 4, H 2 SO 4, HI, HBr, HCl, HNO 3 –Strong bases: LiOH, NaOH, KOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2 Weak acids and bases only partly ionize in water

12 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

13 Acid-base reactions with gas formation Carbonates (CO 3 2- ) form H 2 O and CO 2 when reacted with acids Sufites (SO 3 2- ) form H 2 O and SO 2 when reacted with acids Sulfides (S 2- ) form H 2 S when reacted with acids

14 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

15 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

16 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 CO 2 and H 2 O Metals combust to form metal oxides

17 Molar concentration Molarity: measure of concentration = (moles of solute / liters of solution) –Unit: mol/L Diluting solutions: M i V i = M f V f

18 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

19 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

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