Lecture outline: Section 3. Law of conservation of mass: atoms are not created or. reactions. They simply rearrange. Mass before = mass after

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1 Lecture outline: Section 3 Chemical reactions: chemical changes that occur when substances react to form new substances 1. Chemical equations 2. Atomic and molecular 3. Chemical calculations Law of conservation of : atoms are not created or destroyed during chemical reactions. They simply rearrange. Mass before = after 1 2 Stoichiometry: the quantitative nature of chemical formulas and reactions - The study of quantities of materials consumed and produced in chemical reactions. + Chemical Equations Left of arrow: starting substances, reactants reacts with to produce + right of arrow: substances produced, products Conservation of : atoms, left side = atoms, right side 3 4

2 Chemical Equations + + Note the difference in meaning for a coefficient i in front of a formula vs. a subscript to a formula 2H 2 + O 2 2H 2 O Subscript after an element in a compound: number of atoms of that element in the compound Stoichiometric coefficients: the number of compounds involved in the reaction 5 6 Balancing a chemical Equation Sodium reacts with chlorine gas (Cl 2 ) to produce sodium chloride Identify the reactants and products Write the unbalanced chemical equation Potassium reacts with water to produce potassium hydroxide and hydrogen gas (H 2 ) Identify the reactants and products Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation Write the unbalanced chemical equation Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation Balance first the element(s) that t occur in only one substance on each side of the equation 7 8

3 Write a balanced chemical equation for the reaction of methane gas (CH 4 4) with molecular oxygen (O 2 ) to produce water and carbon dioxide Identify the reactants and products Write the unbalanced chemical equation Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation Balance first the element(s) that occur in only one substance on each side of the equation Additional information from balanced chemical equations Physical states of reactants and products Reactants or products that are dissolved in water 9 10 Common types of chemical reaction Combustion Combination decomposition Combustion: A chemical reaction where something burns in oxygen, producing heat Combustion of metals Combustion of diatomic molecules Combustion of nonmetal compounds Combustion of hydrocarbons Combustion of more complex organic molecules 11 12

4 Hydrocarbon combustion Reactants are a hydrocarbon (C x H y ) and O 2, products are CO 2 and H 2 O Write the balanced chemical equation for the combustion of butane (tetracarbon decahydride) Identify the reactants and products Write the unbalanced chemical equation Start by balancing an element that only occurs in one compound on each side of the equation Common types of chemical reaction Combustion Patterns of chemical reactivity Combination Alkali metal + water decomposition 0resources/media/CONTAC%7E1.MOV Alkali metal + halogen 15 16

5 Concept check Chemical calculations Chemical formulas subscripts Chemical equations coefficients Balancing equations Common reactions Combustion Combination Decomposition Atomic Formula The mole (mol) Empirical and molecular formulas Chemical equations and balancing Atomic Mass: The of an atom is proportional to the number of p +, n, and e - the atom has subatomic in grams in amu particle proton x g amu neutron x g amu electron 9.11 x g 5.49 x amu = x g 1 amu = x g 1 amu 1/12 the of 12 C the of 1 atom of 12 C 12 amu 19 20

6 By definition, the of 1 atom of 12 C 12 amu Sum the es of the subatomic particles of 12 C and what do you get? n p+ n n p+ n subatomic particle in grams in amu proton x g amu neutron x g amu electron 9.11 x g 5.49 x 10-4 The es of the elements are determined experimentally. They cannot be accurately predicted by the es of the constituent parts The greater the defect, the more stable an isotope of an element is Average atomic es Most elements in nature are mixtures of more than 1 isotope Average atomic incorporates the relative abundance of each isotope in nature n p+ n n p+ n n p+ n n n p+ n n n p+ n n n p+ n 6 protons 6 neutrons 6 protons 7 neutrons 6 protons 8 neutrons C C C % % Trace amount (<0.001%) (stable) (stable) (radioactive) 23 24

7 Average atomic of C found in nature n p+ n n p+ n n p+ n n n p+ n n n p+ n n n p+ n 6 protons 6 protons 6 protons 6 neutrons 7 neutrons 8 neutrons Naturally occurring bromine is composed of two isotopes which have atomic es of and amu. Isotope 1 is present at an abundance of 50.69% while isotope 2 is present at an abundance of 49.31%. Use this information to determine the average atomic of naturally occurring bromine 12 C % 12 amu 13 C % amu 14 C Trace amount (<0.001%) The atomic weights provided in a Periodic Table are the average atomic weights of the naturally occurring elements Formula Atoms have atomic es (atomic weights) Compounds have formula es (formula weights) To calculate l formula, sum the atomic es of all atoms in the molecule l 27 28

8 Molecular What is the formula weight of ammonium sulfate? Recall that the formulas for ionic compounds are always written as empirical formulas because there is no discrete molecular formula The es of molecular compounds with defined formulas may be referred to as either formula es or molecular es (molecular weights) The es of empirical compounds are always referred to as formula es (formula weights) 1 H (NH 4 ) 2 SO 4 6 N O S NaCl, FW = C 6 H 6, FW = C 2 H 2, FW = Determine the molecular of ethanol, C 2 H 6 O 1 H C A.About 29 amu 8 O Percent composition: the percentage, by, contributed by each element in a substance ( Atoms of Element)( AW) % Element e = 100 FW of Compound B.About 40 amu C.About 58 amu D.About 46 amu 31 32

9 What are the percentages of each element in ammonium sulfate? ( Atoms of Element)( AW) % Element = 100 FW of Compound (NH 4 4) 2 SO N FW = O S H How do we bridge the microscopic world of amu with the macroscopic world of grams? This week in chemistry Chapters 1 and 2 graded quizzes,chapter 3 graded quiz available Wednesday Recitations meet, graded, bring iclickers. Grading: two points participation, two points maximum for correct answers. I wrote this week s questions Office hours: Mon. and Wed., 2:30-4 PM, other times by appt. Supplemental instruction: M, T, W, R Read materials for chapters 3 and 4- overheads and text work problems in chapters 1-3 self tests, watch self test tutorials to help in developing problem-solving skills. Work old exam problems. 35 The mole (mol) The S.I unit of amount of a substance A collection of a very large number of particles The amount of a substance that contains as many objects (atoms, molecules, or other particles) as the number of atoms in exactly 12 grams of the 12 C isotope 36

10 1 mol = x particles One 12 C atom weighs 12 amu 602x C atoms weigh 12 grams One mol of 12 C atoms weigh 12 grams Avogadro s number 602,200,000,000,000,000,000,000 How big a quantity is the mol???? : the, in grams, of 1 mol (6.02 x particles) of a substance The more a particular particle weighs, the higher it s molar One mol of pennies has more than one mol of carbon atoms 1 molecule of H 2O: 1 dozen molecules of H 2O: 1moleofH of 2 O molecules: = 18.0 amu = 2.99 x = 216 amu = 3.59 x = 1.08 x amu = g g 18.0 g Mass = 18.0 amu/molecule Mass = 18.0 g/mole 39 40

11 Review some terms Atomic : of a single atom of a single isotope amu Average atomic : average of a collection of isotopes (as found in nature) amu Formula : the sum of the average atomic es of the elements in a compound amu Molecular : the same as formula weight, applied to covalent (molecular) compounds amu : the of a mol of particles g/mol Interconversion of, mols, and number of particles Avogadro s moles number molecules Using molar and Avogadro s number as conversion factors g C atoms g H 2 O molecules 1 mol C atoms 1 mol H 2 O molecules An experiment calls for 0.36 mol of Al. What of Al is needed?? amu or g/mol Al mol C atoms g C atoms 1molCatoms 6.02 x C atoms g C atoms 6.02 x C atoms 1molHO 2 molecules g H 2 O molecules 1 mol H 2 O molecules 6.02 x H 2 O molecules 6.02 x H 2 O molecules 602x Catoms 1 mol H 2O molecules g C atoms 43 44

12 How many moles of silicon correspond to 454 g Si?? amu or g/mol Si You have a graduated cylinder containing 25.4 ml Hg. How many moles of Hg are in the cylinder? 80 How many atoms of Hg are in the cylinder? Hg d Hg = g/ml Stoichiometric relations of atoms in a molecule Sucrose is C 12 H 22 O 11 1 molecule of sucrose contains 12 atoms of C 11molecule l of sucrose contains 22 atoms of fh 1 molecule of sucrose contains 11 atoms of O 1 mol of sucrose contains 12 mols of C 11 mol of sucrose contains 22 mols of H 1 mol of sucrose contains 11 mols of O Wit Write these relations as conversion factors 47 Which conversion factor is incorrect for sucrose, C 12 H 22 O 11 : A. 12 atoms C 1 molecule sucrose B. C. D. 1molecule l sucrose 22 atoms H 1molecule sucrose 22 mol H 1mol sucrose 11mol O 48

13 Interconversion of, mols, and number of particles Stoich. Avog. moles a relation moles b # molecules l How many C atoms are present in 46.0 grams of sucrose, C 12 H 22 O 11? 1 6 H C O You have 23.6 grams of ethanol, C 2 H 6 O. How many moles of ethanol do you have? How many molecules of ethanol do you have? How many atoms of carbon do you have? What is the of one molecule of ethanol in grams? You have 23.6 grams of ethanol, C 2 H 6 O. How many moles of ethanol do you have? How many molecules of ethanol do you have? How many atoms of carbon do you have? Stoich. Avog. relation # molecules moles a moles b or atoms What is the of one molecule of ethanol in grams? 51 52

14 Problem solving strategy for stoichiometric calculations Stoich. Avog. relation # molecules moles a moles b or atoms Stoich. relation molecules or atoms is a conversion factor (g/mol) For a compound, the relationship between atoms and molecules is a conversion factor For a compound, the relation between numbers of mols is a conversion factor Avogradro s number can be used as a converion foactor to convert mols and molecules (or atoms) Example to illustrate problem solving strategy for stoichiometric calculations By just observing this bike, no conversion factors can be deduced equating the of the bike, the bike frame, the wheels and pedals By just observing the bike, we can deduce conversion factors relating the numbers of the components of the bike Scale these relationships up to a collection of parts equal to a mol Empirical formula from experimental analysis 1. Base calculation on 100 grams of compound. 2. Determine moles of each element in 100 grams of compound. 3. Divide each value of moles by the smallest of the values. 4. Multiply each number by an integer to obtain all whole numbers. The compound hydrazine (FW = 32) consists of fn and dh. The percentages of fn and H in hydrazine are 87.42% and 12.58%, respectively. el What is the empirical formula for hydrazine? What is the molecular formula for hydrazine? 55 56

15 The compound hydrazine (FW = 32) consists of N and H. The percentages of N and H in hydrazine are 87.42% and 12.58%, respectively. What is the empirical formula for hydrazine? What is the molecular formula for hydrazine? 1. Base calculation on 100 grams of compound. 2. Determine moles of each element in 100 grams of compound. 3. Divide each value of moles by the smallest of the values. 4. Multiply each number by an integer to obtain all whole numbers. What is the molecular formula for hydrazine? Determine true molar of hydrazine experimentally Compare true molar to the molar based on empirical formula The true molar will be the empirical multiplied by some number n Multiply n by the empirical formula to get the molecular formula Mothballs are made of napthalene, a C and H containing compound. The % of C in naphthalene is 93.71%, while the % of H is 6.29%. Naphthalene has a true molar of 128 amu. What is the empirical formula for naphthalene? What is the molecular formula for naphthalene? Mothballs are made of napthalene, a C and H containing compound. The % of C in naphthalene is 93.71%, while the % of H is 6.29%. Naphthalene has a true molar of 128 amu. What is the empirical formula for naphthalene? What is the molecular formula for naphthalene? 59 60

16 Quantitative information from balanced chemical equations 2H 2 + O 2 2H 2 O Quantitative information from balanced chemical equations 2H 2 + O 2 2H 2 O Stoichiometrically equivalent quantities are used as conversion factors Molecules relate to molecules as mols relate to mols How many mols of H 2 O are produced from the reaction of excess H 2 with 3.5 mols of O 2? A camping propane cylinder contains one pound of propane (C 3 H 8 ). What of CO 2 is produced from the complete combustion of all the propane in the cylinder? 1 lb = 454 g A camping propane cylinder contains one pound of propane (C 3 H 8 ). What of CO 2 is produced from the complete combustion of all the propane in the cylinder? 1lb= 454g a moles a Stoich. relation moles b b Write a chemical equation for this reaction Balance the equation Use the proper stoichiometric relationships from the balanced equation Stoich. a moles a relation moles b b 63 64

17 A camping propane cylinder contains one pound of propane (C 3 H 8 ). What of CO 2 is produced from the complete combustion of all the propane in the cylinder? 1lb= 454g a moles a Stoich. relation moles b b Limiting reactants 2H 2 + O 2 2H 2 O No limiting reactant Limiting reactants 2H 2 + O 2 2H 2 O Limiting reactants 2H 2 + O 2 2H 2 O No limiting reactant O 2 is the limiting reactant (H 2 is in excess) 67 68

18 Limiting reactants 2H 2 + O 2 2H 2 O Silicon is used in computer chips and solar cells. The final step in the manufacturing of silicon metal is SiCl 4(l) +2M 2Mg (s) Si (s) +2M 2MgCl 2(s) 225 g of SiCl 4 is mixed with 225 g Mg. H 2 is the limiting reactant (O 2 is in excess) 69 Is one reagent present in a limiting amount, and if so, which one? What t of Si(s) is formed if the reaction goes to completion? What of the excess reagent is left over after the reaction goes to completion? Stoich. a moles a relation moles b b 70 SiCl 4(l) + 2Mg (s) Si (s) + 2MgCl 2(s) 225 g of SiCl 4 is mixed with 225 g Mg. Is one reagent present in a limiting iti amount, and if so, which h one? What of Si(s) is formed if the reaction goes to completion? What of the excess reagent is left over after the reaction goes to completion? Stoich. relation a moles a moles b b SiCl 4(l) + 2Mg (s) Si (s) + 2MgCl 2(s) 225 g of SiCl 4 is mixed with 225 g Mg. Is one reagent present in a limiting iti amount, and if so, which h one? What of Si(s) is formed if the reaction goes to completion? What of the excess reagent is left over after the reaction goes to completion? Stoich. relation a moles a moles b b 71 72

19 Methanol can be synthesized from carbon monoxide and Methanol can be synthesized from carbon monoxide and dihydrogen (a combination reaction): CO (g) + 2H 2(g) CH 3 OH (l) 365gofCOaremixedwith650gH 65.0 g 2. dihydrogen (a combination reaction): CO (g) + 2H 2(g) CH 3 OH (l) 365gofCOaremixedwith650gH 65.0 g 2. Which is the limiting reagent? What is the maximal of methanol that can be formed?? What of the excess reagent is left over after the reaction goes to completion? Which is the limiting reagent? What is the maximal of methanol that can be formed?? What of the excess reagent is left over after the reaction goes to completion? Theoretical vs. actual yield Theoretical yield is the quantity of product calculated to form when all of the limiting reagent reacts The actual yield is the amount of product actually obtained experimentally Actual yield theoretical ti yield actual yield %ild %yield = x100 theoretical yield Theoretical vs. actual yield actual yield %yield = x100 theoreticalti yield ild 75 76