Reactions. Balancing Chemical Equations uses Law of conservation of mass: matter cannot be lost in any chemical reaction

Transcription

1 Reactions Chapter 8 Combustion Decomposition Combination Chapter 9 Aqueous Reactions Exchange reactions (Metathesis) Formation of a precipitate Formation of a gas Formation of a week or nonelectrolyte Single Displacement Reactions Redox reactions Oxidation numbers 1 Balancing Chemical Equations uses Law of conservation of mass: matter cannot be lost in any chemical reaction 2

2 Patterns of reactivity: these are a way of keeping reactions organized. They help us to predict products (and reactants). (Complete) Combustion reactions (Chapter 8) C 3 H 8 (g) + 5O 2 (g)! 3CO 2 (g) + 4H 2 O(g) Combination reactions (Chapter 8) 2Mg(s) + O 2 (!)! 2MgO(s) Decomposition reactions (Chapter 8) PbCO 3 (s)!pbo(s) + CO 2 (g) Know how to balance a chemical reaction. 3 Write balanced reactions for The combustion of ethanol CH 3 CH 2 OH The decomposition of calcium carbonate CaCO 3 The combination of oxygen and hydrogen to form water. 4

3 These are common patterns of reactivity for aqueous solutions Exchange reactions (Chapter 9) Precipitation Pb(NO 3 ) 2 (aq) + 2KI(aq)! PbI(s)! + 2KNO 3 (aq) Neutralization NaOH(aq) + HCl(aq)! NaCl(aq) + H 2 O(!) Gas formation 2HCl(aq) + Na 2 S(aq)! H 2 S(g) " + 2NaCl(aq) Single displacement reactions (Chapter 9) 2Ca(s) + O 2 (g)! 2CaO(s) Know how to balance a chemical reaction 5 Double Displacement is where we exchange positive ions to get products AD + XZ! AZ + XD Called Exchange reaction Example: Reactants: Pb(NO 3 ) 2 + KI Balance the reaction. Does anything happen? TRY! Do you have to do the reaction to know? 6

4 Metathesis reactions (or double displacement) requires a driving force (otherwise nothing happens) Given reactants, exchange positive ions to get products Driving Forces 1. Precipitate forms 2. Weak or nonelectrolyte forms 3. Gas forms How do you know what is happening? Net Ionic Equation 7 Formation of a precipitate is a driving force. Precipitation Reaction Mix silver nitrate and sodium chloride. What happens? Molecular Equation Ionic Equation Spectator ions: Net Ionic Equation 8

5 SOLUBILITY RULES FOR COMMON IONIC COMPOUNDS IN WATER 1. Almost all ammonium and alkali metal salts are soluble. 2. Most nitrates, acetates, chlorides, bromides, and sulfates are soluble. Exceptions: silver halides sulfates of Ca,Ba,Pb,Ag 3. Most sulfides, carbonates, phosphates and hydroxides are insoluble. Exceptions: alkali salts ammonium salts. (See #1 above.) See Table on your data sheet. 9 Formation of Weak or Nonelectrolyte is a driving force (neutralization reaction) Acid + base! salt + water Molecular Equation Ionic Equation Spectator ions: Net Ionic Equation 10

6 Practice What is the net ionic equation for the reaction between sodium hydroxide and acetic acid? What is the net ionic equation for the reaction between sodium chloride and potassium iodide? 11 Gas formation is a driving force. Direct production of a gas CO 2, H 2 S, NO 2, SO 2 Production of weak acid which decomposes. Example: Molecular Equation NaHCO 3 (aq) + HCl(aq)! H 2 CO 3 (aq) + NaCl(aq)! CO 2 (g) + H 2 O(l) Ionic Equation Na + (aq) + HCO 3! (aq) + H + (aq) + Cl! (aq)! H 2 CO 3 (aq)+ Na + (aq) + Cl! (aq) Net Ionic Equation H + (aq) + HCO 3" (aq)! CO 2 (g)+ H 2 O(l)! Na + (aq) + Cl! (aq) + CO 2 (g) + H 2 O(l) 12

7 Single Displacement reactions involve Oxidation and Reduction Ionic equation: Zn(s) + CuSO 4 (aq)! ZnSO 4 (aq) +Cu(s) Net ionic equation What is oxidized? What is reduced? oxidation reduction 13 Rules for determining Oxidation States 1. Oxidation state of atom in elemental form is zero. e.g. Cl 2 O 2 P 4 C(s) S 8 2. The oxidation number of a monatomic ion equals its charge. 3. Some elements have common oxidation numbers that can be used as reference in determining the oxidation numbers of other atoms in the compound. Alkali metals +1 Alkaline earth metals +2 Fluorine 1 H usually +1 Hydrides: metal-h compounds ( 1) O usually 2 peroxides ( 1) & superoxides possible Cl, Br, I almost always 1 4. Sum of oxidation numbers is equal to overall charge of molecule or ion: For a neutral compound the sum of oxidation numbers equals zero. For a polyatomic ion, the sum of the oxidation numbers is equal to the charge on the ion. 5. Shared electrons are assigned to the more electronegative atom of the pair: more electronegative atom will have a negative oxidation number. 14

8 Activity Series Table 9.6 Will the metal element displace the atom from its compound in a reaction? Na(s) + H 2 O(l)! Au(s) + H 2 O(g)! Zn(s) + AgNO 3 (aq)! Active metals (groups IA, IIA, Al) very reactive! Inactive (inert metals): jewelry metals Do not react readily Other metals in between 15 Active Inert Activity Series Table 9.6 Element Oxidation half reactions Lithium Li! Li + + e " Potassium K! K + + e " Barium Ba! Ba e " Calcium Ca! Ca e " Sodium Na! Na + + e " Magnesium Mg! Mg e " Aluminum Al! Al e " Manganese Mn! Mn e " Zinc Zn! Zn e " Chromium Cr! Cr e " Iron Fe! Fe e " Cadmium Cd! Cd e " Cobalt Co! Co e " Nickel Ni! Ni e " Tin Sn! Sn e " Lead Pb! Pb e " Hydrogen H 2! 2H + + 2e " Copper Cu! Cu e " Silver Ag! Ag + + e " Mercury Hg! Hg e " Platinum Pt! Pt e " Mary J. Bojan Gold Chem 110Au! Au e " 16 Increasing ease of oxidation

9 Reactions: Stoichiometry Solving problems involving Chemical reactions Limiting Reactants Solution reactions Gas phase reactions 17 Basics: before you start! I will assume you know: Avogadro s number memorize it! definition of mole Formula weight (molar mass) the following connections (or conversions) gram! mole gram! molecules what is meant by: empirical formula molecular formula Use these along with the balanced chemical reaction to solve problems in chemistry 18

10 How to solve problems write the balanced chemical reaction (or process). make connections between experimentally measured properties and the balanced equation. In other words: Given information such as mass, volume, pressure and temperature, how can one determine quantities of moles/molecules? 19 Avogadro s number Connects molecules (or atoms) to moles Connects microscopic properties to macroscopic properties. Conservation of Mass Mass of Products = Mass of reactants Balance the reaction Balanced Chemical Equation Connects moles (molecules) of reactants with moles (molecules) of products (Think mole ratios ) Related to conservation of Mass Formula weight Connects mass to moles Connects a property that can be measured (determined) experimentally to moles (or molecules) of substance given in the balanced reaction. Empirical Formula Tells relative number of atoms in a molecule Obtained from %mass or molecular formula IMPORTANT CONNECTIONS 20

11 Problem Solving Template 1. Write balanced chemical equation 2. Make a table: Fill in given information 3. Note connections between measured quantities and moles Mass Volume (solutions) P, V, T (gases) M.W. concentration ideal gas law moles moles moles 4. Fill in table until you are able to solve the problem 5. Make sure your answer is REASONABLE 6. TRY things 21 Sample Problem 1 Methane reacts with water to produce hydrogen gas and carbon monoxide. If 8.0g of methane reacts with 9.0 g of water, how many grams of hydrogen gas will be produced? 22

12 Sample Problem 1 Methane reacts with water to produce hydrogen gas and carbon monoxide. If 8.0g of methane reacts with 9.0 g of water, how many grams of hydrogen gas will be produced? Mass (g) FW (g/mol) Moles CH 4 (g) + H 2 O(g) " 3H 2 (g) + CO(g) 23 Sample Problem 1 Methane reacts with water to produce hydrogen gas and carbon monoxide. If 8.0g of methane reacts with 9.0 g of water, how many grams of hydrogen gas will be produced? CH 4 (g) + H 2 O(g) " 3H 2 (g) + CO(g) Mass (g) FW (g/mol) Moles 24

13 If a clean burning engine burns 1 gallon of gasoline (assume this is 2600 g of octane), how many kg of CO 2 will it produce? Octane is C 8 H 18 Assume: Gasoline is all octane Density of octane = g/ml 1 gal = 3.79 L = 3790 ml 3790 ml # g/ml = g # 2600 g of octane 25 If a clean burning engine burns 1 gallon of gasoline (assume this is 2600 g of octane), how many kg of CO 2 will it produce? Octane is C 8 H 18 2 C 8 H 18 (g) + 25 O 2 (g) # 16 CO 2 (g) + 18 H 2 O(g) Mass (g) 2600 excess??? FW (g/mol) Moles 26

14 Empirical formulas are determined by combustion analysis. Fuel + O 2! CO 2 + H 2 O C x H y O z Approach: Find moles of C from moles of CO 2. Find moles of H from moles of H 2 O. (N!N 2, S!SO 2, Cl! Cl 2 ) If oxygen is present in the fuel, all products and reactants will have O# Use conservation of mass to find moles of O in sample (fuel). Mass of sample = mass of C + mass of H + mass of O MJ Bojan Chem g sample of an alcohol produces 7.65 g of CO 2 and 4.70 g of H 2 O upon combustion. C x H y O z (g) + O 2 (g) # CO 2 (g) + H 2 O(g) Mass (g) 4g excess 7.65g 4.70g FW (g/mol)??? Moles What is the empirical formula of the alcohol? To get empirical formula need mole ratio( x:y:z) or Moles C = Moles H = Moles O = 28

15 You try! If 36.6 g of C 2 H 5 OH reacts with 63.8 g of O 2, how many grams of CO 2 will be produced? g g g g g 29 How do you know which reactant is the limiting reactant? Try to connect chemical reactions to something you know: making a sandwich. Example: 1 lb. of bread, $ lb of Ham, % lb. cheese How many sandwiches can I make? Bd + Hm + Ch! Bd Hm C bread + ham + cheese = sandwich Reactants How many Products? slices of bread slices of ham slices of cheese What is the limiting reagent? What is left over? 30

16 Review limiting reagent problem 1. Balance the reaction 2. Test both reactants. 3. Different ways to get the limiting reagent. A. Determine quantity of product B. Use mole ratio of reactants. If 36.6 g of C 2 H 5 OH reacts with 63.8 g of O 2, how many grams of CO 2 will be produced? C 2 H 5 OH(g) + O 2 (g) " CO 2 (g) + H 2 O(g) Mass (g) 36.6 g 63.8g FW (g/mol) 46.1 g/mol 32.0 g/mol 44.0 g/mol 18.0 g.mol Moles 31 Not all reactions go to completion. Theoretical yield = maximum amount of product that can be obtained under the experimental conditions. Actual yield = amount of product obtained. % yield = actual yield theoretical yield! 100% 32

17 A 20.2 g sample of magnesium nitride is reacted with 9.00 g of water and 15.3 g of MgO is recovered. What is the percent yield of the reaction? Mg3N2 (s) + Mass (g) FW (g/mol) 20.2 g 101. g/mol 3 H2O(!) " 2NH3(aq) g 18.0 g.mol 3MgO(s) 15.3 g 17.0 g/mol 40.3 g/mol Moles Mary J. Bojan Chem Problem Solving with solutions: Titration Molarity is the connection between moles and volume in solutions. If 46.0ml of 0.100M NaOH is needed to neutralize 20.0 ml of a H2SO4 solution what is the concentration of H2SO4? Mary J. Bojan Chem

18 Problem Solving with solutions: Titration If 46.0ml of 0.100M NaOH is needed to neutralize 20.0 ml of a H 2 SO 4 solution what is the concentration of H 2 SO 4? Molarity (M) Volume (L) Moles H 2 SO 4 (aq) + NaOH(aq) " Na 2 SO 4 (aq) + H 2 O(!) 35 Problem Solving with gases For a gas, PV=nRT is the link between the # of moles (n) and the experimental quantities P,V, and T of gas. Example: How many L of N 2 at 735 mm Hg and 26 C are produced from 126g of NaN 3 (sodium azide)? sodium azide gas in an airbag 2NaN 3 (s) " 2 Na (s) + 3 N 2 (g) Mass (g) 126 g P = 735 mmhg T = 26 C V =??? P,V,T FW (g/mol) 65 PV = nrt Moles moles 36