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PCC CH 100 - Lecture Notes

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1Ch 100: Fundamentals for ChemistryChapter 8: Chemical EquationsLecture NotesChemical Equations (Intro)1. Chemical equations are used to symbolically describe chemical reactions2. In a chemical equation (or reaction for that matter) the substances that undergo chemical change(s) are called the reactants3. The resulting substances formed are called the products4. The standard representation of a chemical equation:Reactant(s) →→→→ Product(s)Example: The production of water2H2(g) + 1O2(g) →→→→ 2H2O (g)The underlined numbers are called coefficients.a. The number of each molecule for each reactant & product in the chemical reactionb. They are always whole numbers2Chemical Equations (cont.)Balanced chemical equations indicate the ____1. identity of each reactant & product involved in the reaction2. phase of each reactant and product involved in the reaction (i.e. solid (s), liquid (l)or gas (g))3. relative quantity of each reactant and product involved in the reaction (the coefficients!)4. relative molar quantity of each reactant and product involved in the reaction (the coefficients!)Balancing Chemical EquationsAccording to the Law of Mass Conservation(& John Dalton!)matter is never created nor destroyed during chemical reactions– All of the atoms in the reactants of a chemical reaction must be accounted for in the productsThe Basic Process of Balancing Chemical Equations:1. Identify all reactants & products in the reaction & write out their formulas (this is the unbalanced chemical equation)2. Count the number of each atom for each compound for each reactant & product(these values must be the same for both reactants & products when the reaction is balanced!)4. Starting with the most “complicated” molecule, systematically adjust the coefficients to balance # of the atoms on each side of the reaction (balance one atom at a time)5. Repeat until all atoms are balanced for the reaction6. Now you should have a balanced chemical equation!3Balancing Chemical Equations (example)When sodium metal is added to water a violent reaction takes place producing aqueous sodium hydroxide and releasing hydrogen gas.1. Write out the unbalanced chemical reaction:2. Now, balance the chemical reaction:Balancing Chemical Reactions (Hint)When a polyatomic ion(s) appears on both the reactant & product side of the reaction unchanged, treat the whole ion as a “unit” when balancing the reactionExample:1. Note the nitrate ion (NO3-) gets swapped between the Ag+and the Ca2+ions in this reaction2. So NO3-can be treated as a whole unit when balancing this reaction3. Balance it!AgNO3(aq) + CaCl2(aq)→ AgCl(s) + Ca(NO3)2(aq)4Common Classifications for Chemical Reactions1. Combination (or Synthesis): reactions in which reactants combine to make one product2. Decomposition: reactions in which one reactant breaks down into smaller products3. Single Displacement: reactions where a part of one reactant is displaced and combined with another reactant 2Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)4. Double Displacement: reactions where a part of two reactants is displaced and exchangedAgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)Examples:a. Acid-base neutralizationb. Formation of insoluble products (Precipitation reactions)c. Metal oxide + acidd. Gas formation5. Oxidation-Reduction Reactions: reactions involving the transfer or rearrangement of electronsCombination & Decomposition Reactions1. Reactions in which chemicals combine to make one product are called Combination or Synthesis Reactionsa. Metal + Nonmetal reactions can be classified as Combination Reactions2 Na(s) + Cl2(g) → 2 NaCl(s)b. Reactions between Metals or Nonmetals with O2can be classified as Combination ReactionsN2(g) + O2(g) → 2 NO(g)Note: these two types of Combination Reactions are also subclasses of Oxidation-Reduction Reactions2. Reactions in which one reactant breaks down into smaller molecules are called Decomposition Reactionsa. Decomposition reactions are generally initiated by the addition of energy (via electric current or heat)b. Decomposition reactions are the opposite of Combination Reactions:2 NaCl(l) → 2 Na(l) + Cl2(g)5Single Displacement ReactionsSingle displacement reactions involve one part of a reactant being transferred to anotherThe basic pattern of the single displacement reaction:XY + A →→→→X + AYExample 1: Metal + Acid → Salt + HydrogenZn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)Example 2: Metal + Water → Hydrogen + Metal Oxide (or metal hydroxide)3 Fe(s) + 4 H2O(l) → 4 H2(g) + Fe2O3(s)Example 3: Metal + Salt → Metal + Salt2 Al(s) + Fe2O3(s) → 2 Fe(s) + Al2O3(s)Example 4: Halogen + Halide Salt → Halogen + Halide SaltCl2(g) + 2 NaBr(s) → Br2(g) + 2 NaCl(s)Double Displacement ReactionsDouble Displacement Reactions involve the double exchange of a component (such as ions) between two reactantsThe basic form of double displacement reactions is:XY + AB→→→→XB + AYwhere X, Y, A, and B are the components of the reactantsExample 1: Acid Base NeutralizationH2SO4(aq) + Ca(OH)2(aq) → CaSO4(aq) + 2 H2O(l)Example 2: Metal Oxide + AcidCaO(s) + 2HCl(aq) → CaCl2(aq) + H2O(l)Example 3: Formation of an Insoluble Precipitate (Precipitation)KCl(aq) + AgNO3(aq) → KNO3(aq) + AgCl(s)Example 4: Formation of a GasHCl (aq) + ZnS (s) → ZnCl2(aq) + H2S (g)or 2HOHor HOH6Solubility & Precipitation Reactions1. When 2 solutions are combined and result in the formation of an insoluble product:a. The product will not dissolve in the solventb. The product will form a precipitate2. Solubility is an intrinsic physical property and a measure of how well a substance (solute) will dissolve in another substance (solvent)a. Solubility is temperature dependentb. Solid solubility increases with increased temperature (i.e. you can dissolve more sugar in hot water than in cold water)c. Gas solubility increases with decreased temperature (i.e. you can dissolve more CO2in cold water than hot water)3. A solute is soluble if any of it will dissolve in a solventEg. NaCl is soluble in water4. A solute is insoluble if no appreciable amount of it will dissolve in solvent Eg. AgCl is insoluble in water5. Precipitation (formation of an insoluble solid) is one indication that a chemical change has occurred!General Rules for Solubility1. Most compounds that contain NO3-ions are soluble2. Most compounds that contain Na+, K+, or NH4+ions are soluble3. Most compounds that contain Cl-ions are soluble, except AgCl,


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