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10/10/2011 Principles of Chemistry: A Molecular Approach, 1st Ed. Nivaldo Tro Chapter 4 Chemical Quantities and Aqueous Reactions Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA Tro, Principles of Chemistry: A Molecular Approach 1 Quantities in Chemical Reactions stoichiometry - the study of the numerical relationship between chemical quantities in a chemical reaction - amount of every substance used and made in a chemical reaction is related to the amounts of all the other substances in the reaction. q specify p y the the coefficients in a balanced chemical equation relative amounts in moles of each of the substances involved in the reaction. 2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g) 2 molecules of C8H18 react with 25 molecules of O2 to form 16 molecules of CO2 and 18 molecules of H2O 2 moles of C8H18 react with 25 moles of O2 to form 16 moles of CO2 and 18 moles of H2O 2 mol C8H18 : 25 mol O2 : 16 mol CO2 : 18 mol H2O Tro, Principles of Chemistry: A Molecular Approach 2 Pizza/Recipes/Stiochiometry Consider the recipe for making pizza: 1 crust + 5 oz. tomato sauce + 2 cups cheese 1 pizza Expressed mathematically, 1 crust : 5 oz. sauce : 2 cups cheese : 1 pizza Note that the number of pizzas you can make depends on the amount of the ingredients you use: Consider a chemical reaction: 2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g) - the amounts of any other substance in a chemical reaction can be determined from the amount of just one substance. Tro, Principles of Chemistry: A Molecular Approach 3 1 10/10/2011 Stoichiometry Road Map Pure Substance Solution aAb B % A(aq) ppm A(aq) M A(aq) M = moles L Moles A MM equation 22.4 L mass A density M B(aq) Volume A(g) Moles B % B(aq) ppm B(aq) MM 22.4 L equation Volume B(g) volume A (l) mass B density volume B(l) Tro, Principles of Chemistry: A Molecular Approach 4 Stoichiometry Question: Sulfuric acid dissolves aluminum metal according to the following reaction: 2 Al (s) + 3 H2SO4 (aq) Al2(SO4)3 (aq) + 3 H2 (g) Suppose you wanted to dissolve an aluminum block with a mass of 15.2 g. What minimum mass of H2SO4 (in g) would you need? What mass of H2 gas (in g) would be produced by the complete reaction of the aluminum block? Tro, Principles of Chemistry: A Molecular Approach 5 Limiting Reactant, Theoretical and Actual Yield limiting reactant (limiting reagent) - reactant that limits the amount of product the limiting reactant gets completely consumed excess reactant(s) - reactants not completely consumed theoretical yield - the amount of product that can be made from the limiting reactant actual yield - the actual amount of product made in a chemical reaction percent yield - the percentage of the theoretical yield of a chemical reaction that is actually produces Tro, Principles of Chemistry: A Molecular Approach 6 2 10/10/2011 More Making Pizzas Recall the pizza recipe: 1 crust + 5 oz. tomato sauce + 2 cu cheese 1 pizza Consider using this recipe with 4 crusts, 15 oz. tomato sauce, and 10 cu cheese? Ingredient limiting Maximum Number of Pizzas it can make 4 Crusts 4 15 oz. Sauce 3 10 cu Cheese 5 limiting Note: - for reactions with multiple reactants, it is likely that one of the reactants will be completely used before the others. - when this reactant is used up, the reaction stops and no more product is made. Tro, Principles of Chemistry: A Molecular Approach 7 Limiting Reactant, Theoretical and Actual Yield Consider the combustion of Methane CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) recall: - our balanced equation for the combustion of methane implies that every 1 molecule of CH4 reacts with 2 molecules of O2. However, if we have 5 molecules of CH4 and 8 molecules of O2, which is the limiting g reactant? H H C H H H H H H H H H C C C H H + H H H H H C H H O O O O O O O O O O O O O O O O Tro, Principles of Chemistry: A Molecular Approach Since less CO2 can be made from the O2 than the CH4, the O2 is the limiting reactant. 8 More Pizza Making actual and percent yield - assume that as we are making pizzas, we burn a pizza, drop one on the floor, or other uncontrollable events happen so that we make only 2 pizzas - we determine the efficiency of making pizzas by calculating the percentage of the maximum number of pizzas we actually make. Actual yield percentt yield i ld Theoretical yield Things to Note: - in order to determine the theoretical yield, we should use reaction stoichiometry to determine the amount of product each of our reactants could make. - theoretical yield will always be the least possible amount of product and will be based on the limiting reactant - the actual yield will always be less than the theoretical yield Tro, Principles of Chemistry: A Molecular Approach 9 3 10/10/2011 Limiting Reagent and Percent Yield Question: For the reaction shown, compute the theoretical yield of the product (in moles) and identify the limiting reagent. Ti (s) + 2 Cl2 (g) TiCl4 (s) (12.4 mol) (18.8 mol) Tro, Principles of Chemistry: A Molecular Approach 10 Limiting Reagent and Percent Yield Question: Urea (CH4N2O) is a common fertilizer that can be synthesized by the reaction of ammonia (NH3) with carbon dioxide as follows: 2 NH3 (aq) + CO2 (aq) CH4N2O (aq) + H2O (l) In an industrial synthesis of urea, a chemist combines 136.4 kg of ammonia with 211.4 kg of carbon dioxide and obtains 168.4 kg of urea. Determine the limiting reagent, theoretical yield of urea and percent yield of the reaction. Tro, Principles of Chemistry: A Molecular Approach 11 Solutions solutions - homogeneous mixtures eg. when table salt is mixed with water, it seems to disappear, or become a liquid - the composition can vary from one sample to another. Pure substances have constant composition. Salt water samples from different seas or lakes have different amounts of salt solute - the component of the solution that changes state solvent - the component that keeps its state Note: - if both components start in the same state, the major component is the solvent. Tro, Principles of Chemistry: A Molecular Approach 12 4 10/10/2011 Solutions Describing Solutions - to describe solutions accurately, we must describe how much of each component is present. - qualitatively, solutions are often described as: i.) dilute solutions - solutions with a small amount of solute compared to solvent ii.) concentrated solutions - solutions with a large amount of solute compared to solvent Note: - a more precise method for describing a solution is to quantify the amount of solute in a given amount of solution. Tro, Principles of Chemistry: A Molecular Approach 13 Solutions Concentrations—Quantitative Descriptions of Solutions Concentration - the amount of solute in a given amount of solution occasionally the amount of solvent - moles of solute per 1 liter of solution - used because it describes how many molecules of solute in each liter of solution molarity - the relationship between the moles of solute and liters of solution. Tro, Principles of Chemistry: A Molecular Approach 14 Solutions Preparing a Solution of a particular molarity: - when you need to know amount of solution and concentration of solution - calculate the mass of solute needed 1.) start with the amount of solution 2.) use concentration as a conversion factor. - “Dissolve the grams of solute in enough solvent to total the total amount of solution.” Solution Preparation: - preparing 1 L of a 1.00 M NaCl Solution Tro, Principles of Chemistry: A Molecular Approach 15 5 10/10/2011 Solutions Dilution stock solutions - a highly concentrated form of a solution used in laboratories - used to make less concentrated solutions by dilution dilution - to make solutions of lower concentrations from stock solutions - more solvent is added. the amount of solute doesn’t change, just the volume of solution. moles solute in solution 1 = moles solute in solution 2 - the concentrations and volumes of the stock and new solutions are inversely proportional M1·V1 = M2·V2 Tro, Principles of Chemistry: A Molecular Approach 16 Dilution Question: To what volume should you dilute 25 mL of a 10.0 M H2SO4 solution to obtain a 0.150 M H2SO4 solution? Tro, Principles of Chemistry: A Molecular Approach 17 Solutions What happens when a solute dissolves? - there are attractive forces between the solute particles holding them together. - there are also attractive forces between the solvent molecules. - when we mix the solute with the solvent, there are attractive forces between the solute particles and the solvent molecules. - if the attractions between solute and solvent are strong enough, the solute will dissolve. Tro, Principles of Chemistry: A Molecular Approach 18 6 10/10/2011 Solutions Electrolytes and Nonelectrolytes: electrolytes - materials that dissolve in water to form a solution that will conduct electricity - all contain ions dissolved in the water - includes ionic compounds y nonelectrolytes - materials that dissolve in water to form a solution that will not conduct electricity - contain whole molecules dissolved in the water - includes molecular compounds exception is molecular acids Tro, Principles of Chemistry: A Molecular Approach 19 Solutions Molecular View of Electrolytes and Nonelectrolytes - in order to conduct electricity, a material must have charged particles that are able to flow Ionic compounds dissociate into ions when they dissolve. Tro, Principles of Chemistry: A Molecular Approach Molecular compounds do not dissociate when they dissolve. 20 Solutions Acids - molecular compounds that ionize when dissolved in water molecules are pulled apart by their attraction for the water and as a result form corresponding cations and anions when acids ionize, they form H+ cations and anions. - percentage of molecules that ionize varies from one acid to another i.) strong acid - ionize virtually 100% - strong electrolytes HCl(aq) H+(aq) + Cl−(aq) ii.) weak acid - only ionize a small percentage - weak electrolytes HF(aq) H+(aq) + F−(aq) Tro, Principles of Chemistry: A Molecular Approach 21 7 10/10/2011 Solutions dissociation and ionization dissociation - when ionic compounds dissolve in water, the anions and cations are separated from each other Na2S(aq) 2 Na+(aq) + S2–(aq) Note: - when compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion. Na2SO4(aq) 2 Na+(aq) + SO42−(aq) ionization - when a strong acid dissolves in water, the molecule ionizes into H+ and anions. H2SO4(aq) 2 H+(aq) + SO42–(aq) Tro, Principles of Chemistry: A Molecular Approach 22 Solutions solution stoichiometry - since molarity relates the moles of solute to the liters of solution, it can be used to convert between amount of reactants and/or products in a chemical reaction. Tro, Principles of Chemistry: A Molecular Approach 23 Solution Stoichiometry Question: A student dissolved 3.00 g of Co(NO3)2 in enough water to make 100. mL of stock solution. He took 4.00 mL of the stock solution and then diluted it with water to give 275. mL of a final solution. What is the final concentration of NO3- in the final solution? Tro, Principles of Chemistry: A Molecular Approach 24 8 10/10/2011 Solution Stoichiometry Question: Consider the following precipitation reaction. An aqueous solution of sodium phosphate is reacted with an aqueous solution of copper (II) chloride to produce solid copper (II) phosphate and aqueous sodium chloride. What volume (in mL) of 0.175 M sodium phosphate solution is necessary to completely react with 95 95.4 4 mL of 0 0.102 102 M copper (II) chloride? Tro, Principles of Chemistry: A Molecular Approach 25 Solubility of Ionic Compounds - degree of dissolution of ionic compounds varies i.) soluble compounds - dissolve well in a solvent e.g. ionic compound such as NaCl ii.) insoluble compounds - dissolve hardlyy at all in water e.g. ionic compounds such as AgCl Note: - degree of solubility depends on temp. - even insoluble compounds dissolve, just not enough to be meaningful - predicting dissolution most easily determined using empirical method Tro, Principles of Chemistry: A Molecular Approach 26 Solubility Rules Compounds that Are Generally Soluble in Water Compounds Containing the Following Ions Are Generally Soluble Exceptions (when combined with ions on the left the compound is insoluble) Li+, Na+, K+, NH4+ none NO3–, C2H3O2– none Cl–, Br–, I– Ag+, Hg22+, Pb2+ SO42– Ag+, Ca2+, Sr2+, Ba2+, Pb2+ Compounds that Are Generally Insoluble in Water Compounds Containing the Following Ions Are Generally Insoluble Exceptions (when combined with ions on the left the compound is soluble or slightly soluble) OH– Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+ S2– Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+ CO32–, PO43– Li+, Na+, K+, NH4+ Tro, Principles of Chemistry: A Molecular Approach 27 9 10/10/2011 Precipitation Reactions precipitation reactions - reactions between aqueous solutions of ionic compounds that produce an ionic compound that is insoluble in water precipitate - the insoluble product formed in a precipitation reaction molecular equations - equations that describe the chemicals put into the water and the product molecules 2 KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2 KNO3(aq) Tro, Principles of Chemistry: A Molecular Approach 28 Precipitation Reactions Process for Predicting the Products of a Precipitation Reaction 1. Determine what ions each aqueous reactant has. 2. Determine formulas of possible products. Exchange ions. (+) ion from one reactant with (–) ion from other product. Balance charges of combined ions to get formula of each product Determine solubility of each product in water. Use the solubility rules. If product is insoluble or slightly soluble, it will precipitate. If neither product will precipitate, write no reaction after the arrow. If either product is insoluble, write the formulas for the products after the arrow—writing (s) after the product that is insoluble and will precipitate, and (aq) after products that are soluble and will not precipitate. Balance the equation. 3. 4. 5. 6. Tro, Principles of Chemistry: A Molecular Approach 29 Precipitation Reactions Ionic Equations Consider the following molecular equation: 2 KOH(aq) + Mg(NO3)2(aq) 2 KNO3(aq) + Mg(OH)2(s) complete ionic equations - equations that describe the actual dissolved species 1.) aqueous strong electrolytes are written as ions. soluble salts, strong acids, strong bases 2.) insoluble substances, weak electrolytes, and nonelectrolytes are written in molecule form. solids, liquids, and gases are not dissolved, therefore, molecule form 2 K+(aq) + 2 OH−(aq) + Mg2+(aq) + 2 NO3−(aq) 2 K+(aq) + 2 NO3−(aq) + Mg(OH)2(s) Tro, Principles of Chemistry: A Molecular Approach 30 10 10/10/2011 Precipitation Reactions ionic equations: spectator ions - ions that are both reactants and products 2 K+(aq) + 2 OH−(aq) + Mg2+(aq) + 2 NO3−(aq) 2 K+(aq) + 2 NO3−(aq) + Mg(OH)2(s) net ionic equations - an ionic equation in which the spectator ions are removed 2 OH−(aq) + Mg2+(aq) Mg(OH)2(s) Tro, Principles of Chemistry: A Molecular Approach 31 Precipitation Reactions Question: Consider the reaction between sodium phosphate (aq) and nickel (II) chloride (aq). Predict the products of the reaction, write a balanced molecular reaction, write complete and net ionic equations for the reaction. Tro, Principles of Chemistry: A Molecular Approach 32 Acid–Base Reactions acid – base reactions (neutralization reactions) - a reaction in which an acid reacts with a base and the two neutralize each other, producing water 2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l) net ionic equation for an acid–base reaction: H+(aq) + OH(aq) H2O(l) Note: - this is the case as long as the salt that forms is soluble in water Tro, Principles of Chemistry: A Molecular Approach 33 11 10/10/2011 Acids and Bases in Solution - acids ionize in water to form H+ ions. More precisely, the H from the acid molecule is donated to a water molecule to form hydronium ion, H3O+. Most chemists use H+ and H3O+ interchangeably. - bases dissociate in water to form OH ions. Bases, like NH3, that do not contain OH ions, produce OH by pulling H off water molecules. - in the reaction of an acid with a base, the H+ from the acid combines with the OH from the base to make water. - the cation from the base combines with the anion from the acid to make the salt. acid + base salt + water HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Tro, Principles of Chemistry: A Molecular Approach 34 Common Acids Tro, Principles of Chemistry: A Molecular Approach 35 Common Bases Tro, Principles of Chemistry: A Molecular Approach 36 12 10/10/2011 Acid/Base Reactions Question: Write an net ionic equation for the reaction between nitric acid and calcium hydroxide. Tro, Principles of Chemistry: A Molecular Approach 37 Gas-Evolution Reactions - some reactions form a gas directly from the ion exchange. K2S(aq) + H2SO4(aq) K2SO4(aq) + H2S(g) - other reactions form a gas by the decomposition of one of the ion exchange products into a gas and water. K2SO3(aq) + H2SO4(aq) K2SO4(aq) + H2SO3(aq) H2SO3 H2O(l) + SO2(g) Reactant Type Reacting With Ion Exchange Product Decom -pose? Gas Formed Example metalnS, metal HS acid H2S no H2S K2S(aq) + 2 HCl(aq) 2 KCl(aq) + H2S(g) metalnCO3, metal HCO3 acid H2CO3 yes CO2 K2CO3(aq) + 2 HCl(aq) 2 KCl(aq) + CO2(g) + H2O(l) metalnSO3 metal HSO3 acid H2SO3 yes SO2 K2SO3(aq) + 2 HCl(aq) 2 KCl(aq) + SO2(g) + H2O(l) (NH4)nanion base NH4OH yes NH3 KOH(aq) + NH4Cl(aq) KCl(aq) + NH3(g) + H2O(l) Tro, Principles of Chemistry: A Molecular Approach 38 Gas Evolution Reactions Question: Write the net ionic equation for the reaction between aqueous magnesium carbonate and nitric acid. Tro, Principles of Chemistry: A Molecular Approach 39 13 10/10/2011 Oxidation–Reduction Reactions oxidation–reduction reactions (redox reactions) - reactions that involve transferring electrons from one atom to another many involve the reaction of a substance with O2(g) 4 Fe(s) + 3 O2(g) 2 Fe2O3(s) - types of redox reactions: i ) combustion as redox i.) 2 H2(g) + O2(g) 2 H2O(g) 4 Na(s) + O2(g) → 2 Na2O(s) ii.) redox without combustion 2 Na(s) + Cl2(g) 2 NaCl(s) Note: - both reactions may involve metals reacting with nonmetals and, in this case, result in the conversion of free elements into ions Tro, Principles of Chemistry: A Molecular Approach 40 Oxidation–Reduction Reactions Oxidation and Reduction - to convert a free element into an ion, the atoms must gain or lose electrons. Of course, if one atom loses electrons, another must accept them. oxidation–reduction reactions (redox reactions) - reactions that involve transferring electrons from one atom to another i.) oxidized - atoms that lose electrons Ger ii.) reduced - atoms that gain electrons 2 Na(s) + Cl2(g) → 2 Na+Cl–(s) oxidation Na → Na+ + 1 e– Cl2 + 2 e– → 2 Cl– reduction Tro, Principles of Chemistry: A Molecular Approach Leo 41 Electron Bookkeeping oxidation state - a number assigned to each element in a reaction that allows them to determine the electron flow in the reaction Even though they look like them, oxidation states are not ion charges! Oxidation states are imaginary charges assigned based on a set of rules. Ion charges are real, measurable charges. Tro, Principles of Chemistry: A Molecular Approach 42 14 10/10/2011 Rules for Assigning Oxidation States Rules are in order of priority. 1. Free elements have an oxidation state = 0. Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g) 2. Monatomic ions have an oxidation state equal to their charge. Na = +1 and Cl = –1 in NaCl 3 (a) 3. ( ) The Th sum off the th oxidation id ti states t t off allll th the atoms t in i a compound is 0. Na = +1 and Cl = –1 in NaCl, (+1) + (–1) = 0 (b) The sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion. N = +5 and O = −2 in NO3–, (+5) + 3(−2) = −1 Tro, Principles of Chemistry: A Molecular Approach 43 Rules for Assigning Oxidation States 4. (a) Group I metals have an oxidation state of +1 in all their compounds. Na = +1 in NaCl (b) Group II metals have an oxidation state of +2 in all their compounds. Mg = +2 in MgCl2 5. In their compounds, nonmetals have oxidation states according to the table below. Note: Nonmetals higher on the table take priority. Nonmetal Oxidation State F −1 Example CF4 H +1 CH4 O −2 CO2 Group 7A −1 CCl4 Group 6A −2 CS2 Group 5A −3 NH3 Tro, Principles of Chemistry: A Molecular Approach 44 Oxidation–Reduction Reactions oxidation - occurs when an atom’s oxidation state increases during a reaction reduction - occurs when an atom’s oxidation state decreases during a reaction CH4 + 2 O2 → CO2 + 2 H2O –4 +1 0 +4 –2 +1 –2 oxidation reduction Note: - oxidation and reduction must occur simultaneously. (if an atom loses electrons another atom must take them) Tro, Principles of Chemistry: A Molecular Approach 45 15 10/10/2011 Oxidation–Reduction Reactions reducing agent - the reactant that reduces an element in another reactant the reducing agent contains the element that is oxidized oxidizing agent - the reactant that oxidizes an element in another reactant the oxidizing g agent g contains the element that is reduced. 2 Na(s) + Cl2(g) → 2 Na+Cl–(s) Na is oxidized, Cl is reduced Na is the reducing agent, Cl2 is the oxidizing agent Tro, Principles of Chemistry: A Molecular Approach 46 Combustion Reactions combustion reactions - reactions in which O2(g) is a reactant - release lots of energy - are a subclass of oxidation–reduction reactions 2 C8H18(g) + 25 O2(g) 16 CO2(g) + 18 H2O(g) - to predict the products of a combustion reaction, combine each element in the other reactant with oxygen. Reactant Combustion Product contains C CO2(g) contains H H2O(g) contains S SO2(g) contains N NO(g) or NO2(g) M2On(s) contains metal Tro, Principles of Chemistry: A Molecular Approach 47 16