* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 4 Chemical Quantities and Aqueous Reactions
Photoredox catalysis wikipedia , lookup
History of molecular biology wikipedia , lookup
Asymmetric induction wikipedia , lookup
Freshwater environmental quality parameters wikipedia , lookup
Radical (chemistry) wikipedia , lookup
Supramolecular catalysis wikipedia , lookup
Debye–Hückel equation wikipedia , lookup
Chemical equilibrium wikipedia , lookup
IUPAC nomenclature of inorganic chemistry 2005 wikipedia , lookup
Nucleophilic acyl substitution wikipedia , lookup
Electrolysis of water wikipedia , lookup
History of molecular theory wikipedia , lookup
Institute of Chemistry Ceylon wikipedia , lookup
Coordination complex wikipedia , lookup
California Green Chemistry Initiative wikipedia , lookup
Liquid–liquid extraction wikipedia , lookup
Transition state theory wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Chemical reaction wikipedia , lookup
Multi-state modeling of biomolecules wikipedia , lookup
History of chemistry wikipedia , lookup
Organic chemistry wikipedia , lookup
Acid dissociation constant wikipedia , lookup
Process chemistry wikipedia , lookup
Nuclear chemistry wikipedia , lookup
Molecular graphics wikipedia , lookup
Hypervalent molecule wikipedia , lookup
Equilibrium chemistry wikipedia , lookup
Drug discovery wikipedia , lookup
Stability constants of complexes wikipedia , lookup
Analytical chemistry wikipedia , lookup
Lewis acid catalysis wikipedia , lookup
Electrochemistry wikipedia , lookup
Bioorthogonal chemistry wikipedia , lookup
Biochemistry wikipedia , lookup
Metalloprotein wikipedia , lookup
Molecular dynamics wikipedia , lookup
Green chemistry wikipedia , lookup
Click chemistry wikipedia , lookup
Size-exclusion chromatography wikipedia , lookup
Acid–base reaction wikipedia , lookup
Molecular scale electronics wikipedia , lookup
Stoichiometry wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Inorganic chemistry wikipedia , lookup
Chemistry: A Molecular Approach, 1st Ed. Nivaldo Tro Chapter 4 Chemical Quantities and Aqueous Reactions Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2008, Prentice Hall Reaction Stoichiometry • the numerical relationships between chemical amounts • in a reaction is called stoichiometry the coefficients in a balanced chemical equation specify the 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, Chemistry: A Molecular Approach 2 Predicting Amounts from Stoichiometry • the amounts of any other substance in a chemical reaction can be determined from the amount of just one substance • How much CO2 can be made from 22.0 moles of C8H18 in the combustion of C8H18? 2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g) 2 moles C8H18 : 16 moles CO2 16 mol CO2 22.0 moles C8H18 176 moles CO2 2 mol C8H18 Tro, Chemistry: A Molecular Approach 3 Example – Estimate the mass of CO2 produced in 2004 by the combustion of 3.4 x 1015 g gasoline • assuming that gasoline is octane, C8H18, the equation • for the reaction is: 2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g) the equation for the reaction gives the mole relationship between amount of C8H18 and CO2, but we need to know the mass relationship, so the Concept Plan will be: g C8H18 mol C8H18 Tro, Chemistry: A Molecular Approach mol CO2 g CO2 4 Practice • According to the following equation, how many milliliters of water are made in the combustion of 9.0 g of glucose? C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(l) 1. 2. 3. 4. convert 9.0 g of glucose into moles (MM 180) convert moles of glucose into moles of water convert moles of water into grams (MM 18.02) convert grams of water into mL a) How? what is the relationship between mass and volume? density of water = 1.00 g/mL Tro, Chemistry: A Molecular Approach 5 Limiting Reactant • 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 the reactant that limits the amount of product is called the limiting reactant sometimes called the limiting reagent the limiting reactant gets completely consumed • reactants not completely consumed are called excess • reactants the amount of product that can be made from the limiting reactant is called the theoretical yield Tro, Chemistry: A Molecular Approach 6 Things Don’t Always Go as Planned! • many things can happen during the course of an experiment that cause the loss of product • the amount of product that is made in a reaction is called the actual yield generally less than the theoretical yield, never more! • the efficiency of product recovery is generally given as the percent yield actual yield Percent Yield 100% theoretica l yield Tro, Chemistry: A Molecular Approach 7 Limiting and Excess Reactants in the Combustion of Methane • CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) Our balanced equation for the combustion of methane implies that every 1 molecule of CH4 reacts with 2 molecules of O2 H H C H H O + O + O O C + H H + O O Tro, Chemistry: A Molecular Approach O O H H 8 Limiting and Excess Reactants in the Combustion of Methane CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) • If we have 5 molecules of CH4 and 8 molecules of O2, which is the limiting reactant? H C H H H H H H H C H H H C C H H H H H H H C + O O O O O O O O O O O O O O O O ? H H 9 Example 4.4 Finding Limiting Reactant, Theoretical Yield, and Percent Yield Example: • When 28.6 kg of C are allowed to react with 88.2 kg of TiO2 in the reaction below, 42.8 kg of Ti are obtained. Find the Limiting Reactant, Theoretical Yield, and Percent Yield. TiO 2(s) 2 C(s) Ti (s) 2 CO(g) Tro, Chemistry: A Molecular Approach 11 Practice – How many grams of N2(g) can be made from 9.05 g of NH3 reacting with 45.2 g of CuO? 2 NH3(g) + 3 CuO(s) → N2(g) + 3 Cu(s) + 3 H2O(l) Tro, Chemistry: A Molecular Approach 12 Solutions • when table salt is mixed with water, it seems to disappear, or become a liquid – the mixture is homogeneous the salt is still there, as you can tell from the taste, or simply boiling away the water • homogeneous mixtures are called solutions • the component of the solution that changes state is called • the solute the component that keeps its state is called the solvent if both components start in the same state, the major component is the solvent Tro, Chemistry: A Molecular Approach 13 Describing Solutions • since solutions are mixtures, 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 • so to describe solutions accurately, we must describe how much of each component is present we saw that with pure substances, we can describe them with a single name because all samples identical Tro, Chemistry: A Molecular Approach 14 Solution Concentration • qualitatively, solutions are often • • • described as dilute or concentrated dilute solutions have a small amount of solute compared to solvent concentrated solutions have a large amount of solute compared to solvent quantitatively, the relative amount of solute in the solution is called the concentration Tro, Chemistry: A Molecular Approach 15 Solution Concentration Molarity • moles of solute per 1 liter of solution • used because it describes how many molecules of solute in each liter of solution amount of solute (in moles) molarity, M amount of solution (in L) Tro, Chemistry: A Molecular Approach 16 Preparing 1 L of a 1.00 M NaCl Solution Tro, Chemistry: A Molecular Approach 17 Example 4.5 – Find the molarity of a solution that has 25.5 g KBr dissolved in 1.75 L of solution Using Molarity in Calculations • molarity shows the relationship between the moles of solute and liters of solution • If a sugar solution concentration is 2.0 M, then 1 liter of solution contains 2.0 moles of sugar 2 liters = 4.0 moles sugar 0.5 liters = 1.0 mole sugar • 1 L solution : 2 moles sugar 2 mol sugar 1 L solution Tro, Chemistry: A Molecular Approach 1 L solution 2 mol sugar 19 Example 4.6 – How many liters of 0.125 M NaOH contains 0.255 mol NaOH? Dilution • often, solutions are stored as concentrated stock • solutions to make solutions of lower concentrations from these 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, Chemistry: A Molecular Approach 21 Example 4.7 – To what volume should you dilute 0.200 L of 15.0 M NaOH to make 3.00 M NaOH? 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, Chemistry: A Molecular Approach 23 Example 4.8 – What volume of 0.150 M KCl is required to completely react with 0.150 L of 0.175 M Pb(NO3)2 in the reaction 2 KCl(aq) + Pb(NO3)2(aq) PbCl2(s) + 2 KNO3(aq) 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 25 Table Salt Dissolving in Water Tro, Chemistry: A Molecular Approach Each ion is attracted to the surrounding water molecules and pulled off and away from the crystal When it enters the solution, the ion is surrounded by water molecules, insulating it from other ions The result is a solution with free moving charged particles able to conduct electricity 26 Electrolytes and Nonelectrolytes • materials that dissolve in water to form a solution that will conduct electricity are called electrolytes • materials that dissolve in water to form a solution that will not conduct electricity are called nonelectrolytes Tro, Chemistry: A Molecular Approach 27 Molecular View of Electrolytes and Nonelectrolytes • in order to conduct electricity, a material must have • charged particles that are able to flow electrolyte solutions all contain ions dissolved in the water ionic compounds are electrolytes because they all dissociate into their ions when they dissolve • nonelectrolyte solutions contain whole molecules dissolved in the water generally, molecular compounds do not ionize when they dissolve in water the notable exception being molecular acids Tro, Chemistry: A Molecular Approach 28 Salt vs. Sugar Dissolved in Water ionic compounds dissociate into ions when they dissolve Tro, Chemistry: A Molecular Approach molecular compounds do not dissociate when they dissolve 29 Acids • acids are molecular compounds that ionize when they dissolve in water the molecules are pulled apart by their attraction for the water when acids ionize, they form H+ cations and anions • the percentage of molecules that ionize varies from one • • acid to another acids that ionize virtually 100% are called strong acids HCl(aq) H+(aq) + Cl-(aq) acids that only ionize a small percentage are called weak acids HF(aq) H+(aq) + F-(aq) Tro, Chemistry: A Molecular Approach 30 Strong and Weak Electrolytes • strong electrolytes are materials that dissolve completely as ions ionic compounds and strong acids their solutions conduct electricity well • weak electrolytes are materials that dissolve mostly as molecules, but partially as ions weak acids their solutions conduct electricity, but not well • when compounds containing a polyatomic ion dissolve, the polyatomic ion stays together Na2SO4(aq) 2 Na+(aq) + SO42-(aq) HC2H3O2(aq) H+(aq) + C2H3O2-(aq) Tro, Chemistry: A Molecular Approach 31 Classes of Dissolved Materials Tro, Chemistry: A Molecular Approach 32 Solubility of Ionic Compounds • some ionic compounds, like NaCl, dissolve very well in • • water at room temperature other ionic compounds, like AgCl, dissolve hardly at all in water at room temperature compounds that dissolve in a solvent are said to be soluble, while those that do not are said to be insoluble NaCl is soluble in water, AgCl is insoluble in water the degree of solubility depends on the temperature even insoluble compounds dissolve, just not enough to be meaningful Tro, Chemistry: A Molecular Approach 33 When Will a Salt Dissolve? • Predicting whether a compound will dissolve in water is not easy • The best way to do it is to do some experiments to test whether a compound will dissolve in water, then develop some rules based on those experimental results we call this method the empirical method Tro, Chemistry: A Molecular Approach 34 Solubility Rules Compounds that Are Generally Soluble in Water Compounds Containing the Exceptions Following Ions are Generally (when combined with ions on the Soluble 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+ Tro, Chemistry: A Molecular Approach 35 Solubility Rules Compounds that Are Generally Insoluble Exceptions Compounds Containing the (when combined with ions on the Following Ions are Generally left the compound is soluble or Insoluble slightly soluble) OH– S2– CO32–, PO43– Tro, Chemistry: A Molecular Approach Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+ Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+ Li+, Na+, K+, NH4+ 36 Precipitation Reactions • reactions between aqueous solutions of ionic compounds that produce an ionic compound that is insoluble in water are called precipitation reactions and the insoluble product is called a precipitate Tro, Chemistry: A Molecular Approach 37 2 KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2 KNO3(aq) 38 No Precipitate Formation = No Reaction KI(aq) + NaCl(aq) KCl(aq) + NaI(aq) all ions still present, no reaction Tro, Chemistry: A Molecular Approach 39 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 Balance charges of combined ions to get formula of each product 3. Determine Solubility of Each Product in Water Use the solubility rules If product is insoluble or slightly soluble, it will precipitate 4. If neither product will precipitate, write no reaction after the arrow Tro, Chemistry: A Molecular Approach 40 Process for Predicting the Products of a Precipitation Reaction 5. 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 6. Balance the equation Tro, Chemistry: A Molecular Approach 41 Example 4.10 – Write the equation for the precipitation reaction between an aqueous solution of potassium carbonate and an aqueous solution of nickel(II) chloride 1. Write the formulas of the reactants K2CO3(aq) + NiCl2(aq) 2. Determine the possible products a) Determine the ions present (K+ + CO32-) + (Ni2+ + Cl-) b) Exchange the Ions (K+ + CO32-) + (Ni2+ + Cl-) (K+ + Cl-) + (Ni2+ + CO32-) c) Write the formulas of the products cross charges and reduce K2CO3(aq) + NiCl2(aq) KCl + NiCO3 Example 4.10 – Write the equation for the precipitation reaction between an aqueous solution of potassium carbonate and an aqueous solution of nickel(II) chloride 3. Determine the solubility of each product KCl is soluble NiCO3 is insoluble 4. If both products soluble, write no reaction does not apply since NiCO3 is insoluble Tro, Chemistry: A Molecular Approach 43 Example 4.10 – Write the equation for the precipitation reaction between an aqueous solution of potassium carbonate and an aqueous solution of nickel(II) chloride 5. Write (aq) next to soluble products and (s) next to insoluble products K2CO3(aq) + NiCl2(aq) KCl(aq) + NiCO3(s) 6. Balance the Equation K2CO3(aq) + NiCl2(aq) 2 KCl(aq) + NiCO3(s) Tro, Chemistry: A Molecular Approach 44 Ionic Equations • equations which describe the chemicals put into the water and the product molecules are called molecular equations 2 KOH(aq) + Mg(NO3)2(aq) 2 KNO3(aq) + Mg(OH)2(s) • equations which describe the actual dissolved species are called complete ionic equations aqueous strong electrolytes are written as ions soluble salts, strong acids, strong bases insoluble substances, weak electrolytes, and nonelectrolytes written in molecule form solids, liquids, and gases are not dissolved, therefore molecule form 2K+1(aq) + 2OH-1(aq) + Mg+2(aq) + 2NO3-1(aq) 2K+1(aq) + 2NO3-1(aq) + Mg(OH)2(s) Tro, Chemistry: A Molecular Approach 45 Ionic Equations • ions that are both reactants and products are called spectator ions 2K+1(aq) + 2OH-1(aq) + Mg+2(aq) + 2NO3-1(aq) 2K+1(aq) + 2NO3-1(aq) + Mg(OH)2(s) • an ionic equation in which the spectator ions are removed is called a net ionic equation 2OH-1(aq) + Mg+2(aq) Mg(OH)2(s) Tro, Chemistry: A Molecular Approach 46 Acid-Base Reactions • also called neutralization reactions because the acid and base neutralize each other’s properties 2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l) • the net ionic equation for an acid-base reaction is H+(aq) + OH(aq) H2O(l) as long as the salt that forms is soluble in water Tro, Chemistry: A Molecular Approach 47 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 Tro, Chemistry: A Molecular Approach 48 Common Acids Chemical Name Formula Uses Strength Perchloric Acid HClO4 explosives, catalyst Strong Nitric Acid HNO3 explosives, fertilizer, dye, glue Strong Sulfuric Acid H2SO4 Hydrochloric Acid HCl Phosphoric Acid H3PO4 Chloric Acid HClO3 Acetic Acid HC2H3O2 Hydrofluoric Acid HF Carbonic Acid Hypochlorous Acid H2CO3 HClO soda water sanitizer Weak Weak Boric Acid H3BO3 eye wash Weak explosives, fertilizer, dye, glue, Strong batteries metal cleaning, food prep, ore Strong refining, stomach acid fertilizer, plastics & rubber, Moderate food preservation explosives Moderate plastics & rubber, food Weak preservation, vinegar metal cleaning, glass etching Weak Common Bases Chemical Name sodium hydroxide potassium hydroxide calcium hydroxide sodium bicarbonate magnesium hydroxide ammonium hydroxide Formula NaOH Common Name lye, caustic soda Uses soap, plastic, petrol refining soap, cotton, electroplating Strength Strong KOH caustic potash Ca(OH)2 slaked lime cement Strong NaHCO3 baking soda cooking, antacid Weak Mg(OH)2 milk of magnesia antacid Weak NH4OH, {NH3(aq)} ammonia water detergent, fertilizer, explosives, fibers Weak Tro, Chemistry: A Molecular Approach Strong 50 HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Tro, Chemistry: A Molecular Approach 51 Example - Write the molecular, ionic, and netionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide 1. Write the formulas of the reactants 2. HNO3(aq) + Ca(OH)2(aq) Determine the possible products a) Determine the ions present when each reactant dissociates (H+ + NO3-) + (Ca+2 + OH-) b) Exchange the ions, H+1 combines with OH-1 to make H2O(l) (H+ + NO3-) + (Ca+2 + OH-) (Ca+2 + NO3-) + H2O(l) c) Write the formula of the salt cross the charges (H+ + NO3-) + (Ca+2 + OH-) Ca(NO3)2 + H2O(l) Tro, Chemistry: A Molecular Approach 52 Example - Write the molecular, ionic, and netionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide 3. Determine the solubility of the salt Ca(NO3)2 is soluble 4. Write an (s) after the insoluble products and a (aq) after the soluble products HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + H2O(l) 5. Balance the equation 2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l) Tro, Chemistry: A Molecular Approach 53 Example - Write the molecular, ionic, and netionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide 6. Dissociate all aqueous strong electrolytes to get complete ionic equation not H2O 2 H+(aq) + 2 NO3-(aq) + Ca+2(aq) + 2 OH-(aq) Ca+2(aq) + 2 NO3-(aq) + H2O(l) 7. Eliminate spectator ions to get net-ionic equation 2 H+1(aq) + 2 OH-1(aq) 2 H2O(l) H+1(aq) + OH-1(aq) H2O(l) Tro, Chemistry: A Molecular Approach 54 Titration • often in the lab, a solution’s concentration is • determined by reacting it with another material and using stoichiometry – this process is called titration in the titration, the unknown solution is added to a known amount of another reactant until the reaction is just completed, at this point, called the endpoint, the reactants are in their stoichiometric ratio the unknown solution is added slowly from an instrument called a burette a long glass tube with precise volume markings that allows small additions of solution Tro, Chemistry: A Molecular Approach 55 Acid-Base Titrations • the difficulty is determining when there has been just enough titrant added to complete the reaction the titrant is the solution in the burette • in acid-base titrations, because both the reactant and product solutions are colorless, a chemical is added that changes color when the solution undergoes large changes in acidity/alkalinity the chemical is called an indicator • at the endpoint of an acid-base titration, the number of moles of H+ equals the number of moles of OH aka the equivalence point Tro, Chemistry: A Molecular Approach 56 Titration Tro, Chemistry: A Molecular Approach 57 Titration The base solution is the titrant in the burette. As the base is added to the acid, the H+ reacts with the OH– to form water. But there is still excess acid present so the color does not change. At the titration’s endpoint, just enough base has been added to neutralize all the acid. At this point the indicator changes color. Tro, Chemistry: A Molecular Approach 58 Example 4.14: The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution? Tro, Chemistry: A Molecular Approach 59 Gas Evolving 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) Tro, Chemistry: A Molecular Approach 60 NaHCO3(aq) + HCl(aq) NaCl(aq) + CO2(g) + H2O(l) Tro, Chemistry: A Molecular Approach 61 Compounds that Undergo Gas Evolving Reactions Reactant Type Reacting With Ion Exchange Product Decompose? Gas Formed Example metalnS, metal HS acid H2S no H2S K2S(aq) + 2HCl(aq) 2KCl(aq) + H2S(g) metalnCO3, metal HCO3 acid H2CO3 yes CO2 K2CO3(aq) + 2HCl(aq) 2KCl(aq) + CO2(g) + H2O(l) metalnSO3 metal HSO3 acid H2SO3 yes SO2 K2SO3(aq) + 2HCl(aq) 2KCl(aq) + SO2(g) + H2O(l) (NH4)nanion base NH4OH yes NH3 KOH(aq) + NH4Cl(aq) KCl(aq) + NH3(g) + H2O(l) Tro, Chemistry: A Molecular Approach 62 Example 4.15 - When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves 1. Write the formulas of the reactants Na2CO3(aq) + HNO3(aq) 2. Determine the possible products a) Determine the ions present when each reactant dissociates (Na+1 + CO3-2) + (H+1 + NO3-1) b) Exchange the anions (Na+1 + CO3-2) + (H+1 + NO3-1) (Na+1 + NO3-1) + (H+1 + CO3-2) c) Write the formula of compounds cross the charges Na2CO3(aq) + HNO3(aq) NaNO3 + H2CO3 Tro, Chemistry: A Molecular Approach 63 Example 4.15 - When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves 3. Check to see either product H2S - No 4. Check to see if either product decomposes – Yes H2CO3 decomposes into CO2(g) + H2O(l) Na2CO3(aq) + HNO3(aq) NaNO3 + CO2(g) + H2O(l) Tro, Chemistry: A Molecular Approach 64 Example 4.15 - When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves 5. Determine the solubility of other product NaNO3 is soluble 6. Write an (s) after the insoluble products and a (aq) after the soluble products Na2CO3(aq) + 2 HNO3(aq) 2 NaNO3(aq) + CO2(g) + H2O(l) 7. Balance the equation Na2CO3(aq) + 2 HNO3(aq) 2 NaNO3 + CO2(g) + H2O(l) Tro, Chemistry: A Molecular Approach 65 Other Patterns in Reactions • the precipitation, acid-base, and gas evolving reactions all involved exchanging the ions in the solution • other kinds of reactions involve transferring electrons from one atom to another – these are called oxidation-reduction reactions also known as redox reactions many involve the reaction of a substance with O2(g) 4 Fe(s) + 3 O2(g) 2 Fe2O3(s) Tro, Chemistry: A Molecular Approach 66 Combustion as Redox 2 H2(g) + O2(g) 2 H2O(g) Tro, Chemistry: A Molecular Approach 67 Redox without Combustion 2 Na(s) + Cl2(g) 2 NaCl(s) 2 Na 2 Na+ + 2 e Tro, Chemistry: A Molecular Approach Cl2 + 2 e 2 Cl 68 Reactions of Metals with Nonmetals • consider the following reactions: • • 4 Na(s) + O2(g) → 2 Na2O(s) 2 Na(s) + Cl2(g) → 2 NaCl(s) the reaction involves a metal reacting with a nonmetal in addition, both reactions involve the conversion of free elements into ions 4 Na(s) + O2(g) → 2 Na+2O– (s) 2 Na(s) + Cl2(g) → 2 Na+Cl–(s) Tro, Chemistry: A Molecular Approach 69 Oxidation and Reduction • in order 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 • reactions where electrons are transferred from one • atom to another are redox reactions atoms that lose electrons are being oxidized, atoms that gain electrons are being reduced Ger Na+Cl–(s) 2 Na(s) + Cl2(g) → 2 Na → Na+ + 1 e– oxidation Cl2 + 2 e– → 2 Cl– reduction Tro, Chemistry: A Molecular Approach Leo 70 Electron Bookkeeping • for reactions that are not metal + nonmetal, or do • not involve O2, we need a method for determining how the electrons are transferred chemists assign a number to each element in a reaction called an oxidation state 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, Chemistry: A Molecular Approach 71 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) the sum of the oxidation states of all the atoms in a compound is 0 Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0 Tro, Chemistry: A Molecular Approach 72 Rules for Assigning Oxidation States 3. (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 4. (a) Group I metals have an oxidation state of +1 in all their compounds Na = +1 in NaCl 4. (b) Group II metals have an oxidation state of +2 in all their compounds Mg = +2 in MgCl2 Tro, Chemistry: A Molecular Approach 73 Rules for Assigning Oxidation States 5. in their compounds, nonmetals have oxidation states according to the table below nonmetals higher on the table take priority Nonmetal Oxidation State Example F -1 CF4 H +1 CH4 O -2 CO2 Group 7A -1 CCl4 Group 6A -2 CS2 Group 5A -3 NH3 Tro, Chemistry: A Molecular Approach 74 Practice – Assign an Oxidation State to Each Element in the following • • • • • • Br2 K+ LiF CO2 SO42- Na2O2 Tro, Chemistry: A Molecular Approach 75 Oxidation and Reduction Another Definition • 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 Tro, Chemistry: A Molecular Approach 76 Oxidation–Reduction • oxidation and reduction must occur simultaneously if an atom loses electrons another atom must take them • the reactant that reduces an element in another reactant is called the reducing agent the reducing agent contains the element that is oxidized • the reactant that oxidizes an element in another reactant is called the oxidizing agent the oxidizing agent 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, Chemistry: A Molecular Approach 77 Identify the Oxidizing and Reducing Agents in Each of the Following 3 H2S + 2 NO3– + 2 H+ 3 S + 2 NO + 4 H2O MnO2 + 4 HBr MnBr2 + Br2 + 2 H2O Tro, Chemistry: A Molecular Approach 78 Combustion Reactions • Reactions in which O2(g) is a • • reactant are called combustion reactions Combustion reactions release lots of energy Combustion reactions are a subclass of oxidationreduction reactions 2 C8H18(g) + 25 O2(g) 16 CO2(g) + 18 H2O(g) Tro, Chemistry: A Molecular Approach 79 Combustion Products • 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) contains metal M2On(s) Tro, Chemistry: A Molecular Approach 80 Practice – Complete the Reactions • combustion of C3H7OH(l) • combustion of CH3NH2(g) Tro, Chemistry: A Molecular Approach 81