Chapter 3. Molecules, Compounds and Chemical Equations

Transcription

1 3. Molecules, Compounds and Chemical Equations Stoichiometry Mole concept and Avogadro s Number Determining Chemical Formulas Name Compound Balancing Chemical Reactions Yields Solutions and Stoichiometry Modified by Dr. Cheng-Yu Lai

2 An Atomic-Level View of Elements and Compounds Elements may be either atomic or molecular. Compounds may be either molecular or ionic. single atoms multiple atoms molecular or ionic compound. Chemical bonds between atoms 3/2

3 View of Elements Atomic elements exist in nature with single atoms as their basic units. Most elements fall into this category. Examples are Na, Ne, C, K, Mg, etc. Molecular elements do not normally exist in nature with single atoms as their basic units; instead, they exist as molecules two or more atoms of the element bonded together. There only seven diatomic elements and they are H2, N2, O2, F2, Cl2, Br2, and I2. Also, P4 and S8 are polyatomic elements Pearson Education, Inc. 3/3

4 An Atomic-Level View of Elements and Compounds Elements may be either atomic or molecular. Compounds may be either molecular or ionic. single atoms multiple atoms molecular or ionic compound. 3/ Pearson Education, Inc. Chemical bonds between atoms

5 Molecules and Chemical Bonds Based on atom- atom interactionschemical bonds are classified into two types: Covalent and Ionic, Covalent Bond: A bond that results when two atoms share several (usually two) electrons. Typically a nonmetal bonded to a nonmetal. Molecule: The unit of matter that results when two or more atoms are joined by covalent bonds Pearson Education, Inc.

6 Molecules Compounds, Covalent Bonds To visualize the molecules, it helps to imagine the individual atoms as spheres jointed together to form molecules with specific 3-D shapes shown below. A ball-and-stick molecular model represents atoms as balls and chemical bonds as sticks; how the two connect reflects a molecule s shape. The balls are typically color-coded to specific elements.

7 Which one is element? Molecular element? compound a single atom (of an element) a molecule (of an element) 2CO(g) + O2(g) a molecule (of a compound) 2CO2(l) 3/ Pearson Education, Inc.

8 Ionic Compound and Ionic Bonds Ionic Bond: Electrostatic attraction between the resulting charged particles after a complete transfer of one or more electrons from one atom to another; typically a metal bonded to a nonmetal. Ion: A charged particle, could be : 1. Cation: A positively charged particle. Metals tend to lose electron(s) and form cations. 2. Anion: A negatively charged particle. Nonmetals tend to gain electron(s) and form anions. 17 protons 11 protons 17 protons 11 protons 11 electrons 17 electrons 10 electrons 18 electrons Na 2014 Pearson Education, Inc. + Cl2 Na+ + Cl 3/8

9 Ions and Ionic Bonds In the formation of sodium chloride, one electron is transferred from the sodium atom to the chlorine atom. 1 Na + 2 Cl2 Na+ + Cl Cation: A positive ion -Na+ Anion: A negative ion -Cl, Ionic Bonding: Force of attraction between oppositely charged ions Pearson Education, Inc. 2/9

10 Predicting Ionic Charges Group 1: Lose 1 electron to form 1+ ions H+ Li+ Na+ K+ Rb+ Cs+ 3/10

11 Predicting Ionic Charges Group 2: Loses 2 electrons to form 2+ ions Be2+ Mg2+ Ca2+ Sr2+ Ba2+ 3/11

12 Predicting Ionic Charges B3+ Al3+ Ga3+ Group 13: Loses 3 electrons to form 3+ ions 3/12

13 Predicting Ionic Charges Group 14: Loses 4 electrons or gains 4 electrons 3/13

14 Predicting Ionic Charges N3- Nitride P3- Phosphide As3- Arsenide Group 15: Gains 3 electrons to form 3- ions 3/14

15 Predicting Ionic Charges O2- Oxide S2- Sulfide Se2- Selenide Group 16: Gains 2 electrons to form 2- ions 3/15

16 Predicting Ionic Charges F1- Fluoride Cl1- Chloride Br1- Bromide I1- Iodide Group 17: Gains 1 electron to form 1- ions 3/16

17 Some transition metals form more than one cation 2012 Pearson Education, Inc. 2/17

18 Representing Compounds with its Chemical Formulas Chemical formula indicates the elements present in the compound and the relative number of atoms or ions of each. Water is represented as H2O. Carbon dioxide is represented as CO2. Sodium Chloride is represented as NaCl. Carbon tetrachloride is represented as CCl Pearson Education, Inc. 3/18

19 Types of Chemical Formulas Chemical formulas can generally be categorized into three different types: Empirical formula Molecular formula Structural formula 2014 Pearson Education, Inc. 3/19

20 Types of Chemical Formulas An empirical formula gives the relative number of atoms of each element in a compound. A molecular formula gives the actual number of atoms of each element in a molecule of a compound. (a) (b) (c) 2014 Pearson Education, Inc. For C4H8, the greatest common factor is 4. The empirical formula is therefore CH2. For B2H6, the greatest common factor is 2. The empirical formula is therefore BH3. For CCl4, the only common factor is 1, so the empirical formula and the molecular formula are identical. 3/20

21 Types of Chemical Formulas A structural formula uses lines to represent covalent bonds and shows how atoms in a molecule are connected or bonded to each other. The structural formula for H2O2 is shown below ( seldom used in 100 level class ) 2014 Pearson Education, Inc. 3/21

22 Formulas Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio). Molecular: H2O C6H12O6 C12H22O11 Empirical: H2O CH2O C12H22O11 Empirical Formula Simplest, whole-number ratio of the atoms of elements in a compound 3/22

23 Formula Mass The mass of an individual molecule or formula unit also known as molecular mass or molecular weight Sum of the masses of the atoms in a single molecule or formula unit whole = sum of the parts! Mass of 1 molecule of H2O = 2(1.01 g/mole H) g/mole O = g/mole Molar mass = formula mass (in g/mole) Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

24 Calculating Formula Mass Its the sum of the individual atomic masses of each atom constituting the molecule. Calculate the formula mass of 1 mole of magnesium carbonate, MgCO g g + 3(16.00 g) = g/mole

25 Calculating Percentage Composition Calculate the percentage composition of magnesium carbonate, MgCO3. From previous slide: g g + 3(16.00 g) = g Mg % C % O %

26 Empirical Formula Determination Magnesium Carbonate contains 28.83% Mg, 14.24% C, and 56.93% O by mass. What is the empirical formula of magnesium carbonate? MgCO3 Empirical Formula Simplest, whole-number ratio of the atoms of elements in a compound Can be determined from elemental analysis

27 Empirical Formula Determination 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.

28 Empirical Formula Determination Magnesium Carbonate contains 28.83% Mg, 14.24% C, and 56.93% O by mass. What is the empirical formula of magnesium carbonate? 28.83g Base calculation on 100 grams of compound g 2. Determine moles of each C element in 100 grams of compound g O Divide each value of moles by 16 the smallest of the values. 4. Multiply each number by an Mg 1.185C1.185O3.558 integer to obtain all whole numbers. Mg Mg C O Mg1C1O 3

29 Empirical Formula Determination Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid? 49.32g C 1 mol C =4.107 mol C g C 6.85g H 1 mol H 6.78 mol H 1.01 g H g O 1 mol O 2.74 mol O g O 3/29

30 Empirical Formula Determination (part 2) Divide each value of moles by the smallest of the values. Carbon: Hydrogen: Oxygen: mol C mol O 6.78 mol 2.74 mol 2.74 mol 2.74 mol H 2.47 O O 1.00 O C1.5H2.47O1 3/30

31 Empirical Formula Determination (part 3) Multiply each number by an integer to obtain all whole numbers. Carbon: 1.50 x 2 3 Hydrogen: 2.50 x 2 5 Oxygen: 1.00 x 2 2 Empirical formula: C3H5O2 Empirical Formula Simplest, whole-number ratio of the atoms of elements in a compound Can be determined from elemental analysis 3/31

32 Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 1. Find the formula mass of C3H5O2 3(12.01 g) + 5(1.01) + 2(16.00) = g 3/32

33 Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 2. Divide the molecular mass by the mass given by the emipirical formula. 3(12.01 g) + 5(1.01) + 2(16.00) = g The molecular formula is a multiple of the empirical formula. 3/33

34 Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 3. Multiply the empirical formula by this number to get the molecular formula. 3(12.01 g) + 5(1.01) + 2(16.00) = g (C3H5O2) x 2 = C6H10O4 Molecular formula = (empirical formula)n, where n is a positive integer. 3/34

35 Empirical Formula Determination Adipic acid contains 26.0% N and 74.0% O by mass. What is the empirical formula of a compound? (26.0 g N )( 1 mole N) = mole N g N (74.0 g O )( 1 mole O) = mole O g O Divide each value of moles by the smallest of the values. N1.OO2.5 = N2O5 Multiply each number by an integer to obtain all whole numbers. 3/35

36 Moles and Formula Mass compound mole

37 Worked Example 3.6 Converting Moles To Mass How many grams are in mol of NaHCO3, the main ingredient in Alka-Seltzer tablets? Strategy The problem gives the number of moles of NaHCO3 and asks for a mole-to-mass conversion. First, calculate the molar mass of NaHCO3. Then use molar mass as a conversion factor, and set up an equation so that the unwanted unit cancels. Solution Formula mass of NaHCO3 = 23.0 amu amu amu + ( amu) = 84.0 amu Molar mass of NaHCO3 = 84.0 g/mol Instructor Resource DVD for Chemistry, 6th Edition John McMurry & Robert C. Fay 2012 Pearson Education, Inc.

38 Example 3.13 The Mole Concept Converting between Mass and Number of Molecules An aspirin tablet contains 325 mg of acetylsalicylic acid (C9H8O4). How many acetylsalicylic acid molecules does it contain? Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

39 Example 3.14 Mass Percent Composition Calculate the mass percent of Cl in Freon-112 (C2Cl4F2), a CFC refrigerant. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

40 Example 3.18 Obtaining an Empirical Formula from Experimental Data A laboratory analysis of aspirin determined the following mass percent composition: C 60.00% H 4.48% O 35.52% Find the empirical formula. Given: In a 100 g sample: g C, 4.48 g H, g O Find: empirical formula Step 3 Write down a pseudoformula for the compound using the number of moles of each element (from step 2) as subscripts. C4.996H4.44O2.220 The correct empirical formula is C9H8O4. What is empirical formula of a compound that is 30.4 % of nitrogen and 69.6 % of oxygen? Exam! Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

41 Example 3.19 Calculating a Molecular Formula from an Empirical Formula and Molar Mass Butanedione a main component responsible for the smell and taste of butter and cheese contains the elements carbon, hydrogen, and oxygen. The empirical formula of butanedione is C2H3O, and its molar mass is g/mol. Find its molecular formula. Sort You are given the empirical formula and molar mass of butanedione and asked to find the molecular formula. Given: Empirical formula = C2H3O molar mass = g/mol Find: molecular formula Strategize Solve Calculate the empirical formula mass. Divide the molar mass by the empirical formula mass to find n. Multiply the empirical formula by n to obtain the molecular formula. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

42 Chemical Equations Provide information about the reaction Formulas of reactants and products States of reactants and products Relative numbers of reactant and product molecules that are required Can be used to determine weights of reactants used and products that can be made 2014 Pearson Education, Inc. 3/42

43 How to Balance Chemical Equations Chemical Equation - Shorthand way of describing a reaction 2Na(s) + Cl2(g) 2NaCl(s) left side: right side: 2 Na 2 Cl 2 Na 2 Cl A balanced chemical equation shows that the law of conservation of mass is adhered to. In a balanced chemical equation, the numbers and kinds of atoms on both sides of the reaction arrow are identical. 3/43

44 Count Atoms 3H2O subscripts little numbers that tell how many atoms there are (ex: In 3H2O, the 2 is the subscript) coefficients regular-sized numbers that tell how many molecules there are (ex: In 3H2O, the 3 is the coefficient) 2012 Pearson Education, Inc. 3/44

45 Example 3.22 Balancing Chemical Equations Balancing Equations Worksheet N2 + H2 NH3 1 N2 + 3 H2 2 NH3 CH4 + O2 CO2 + H2O 1 CH4 + 2 O2 1 CO2 + 2 H2O C3H7OH + O2 CO2 + H2O 2C3H7OH + 9_ O2 6_ CO2 + _8 H2O How many carbon atoms are found in 10 g of C2H5OH? Molar Mass of C2H5OH=46, Exam 10.0 g C2H5OH 1 mol C2H5OH 46.0 g C2H5OH Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2 mole Carbon 6.02X1023 atoms 1 mol C2H50H = 1 mol 2.62X1023 atoms 2014 Pearson Education, Inc.

46 Ionic Compound Formulas Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio). Examples: NaCl MgCl2 Al2(SO4)3 K2CO3 3/46

47 Cations 3/47

48 Anions 3/48

49 Writing Ionic Compound Formulas Example: Barium nitrate 1. Write the formulas for the cation and anion, including CHARGES! 2. Check to see if charges are balanced. 3. Balance charges, if necessary, using subscripts. Use parentheses if you need more than one of a polyatomic ion. Ba2+ ( NO3- ) 2 Not balanced 3/49

50 Writing Ionic Compound Formulas Example: Iron(III) chloride 1. Write the formulas for the cation and anion, including CHARGES! 2. Check to see if charges are balanced. 3. Balance charges, if necessary, using subscripts. Use parentheses if you need more than one of a polyatomic ion. Fe3+ Cl- 3 Not balanced 3/50

51 Writing Ionic Compound Formulas Example: Magnesium carbonate 1. Write the formulas for the cation and anion, including CHARGES! 2. Check to see if charges are balanced. 2+ Mg CO32- They are balanced 3/51

52 Writing Ionic Compound Formulas Example: Zinc hydroxide 1. Write the formulas for the cation and anion, including CHARGES! 2. Check to see if charges are balanced. 3. Balance charges, if necessary, using subscripts. Use parentheses if you need more than one of a polyatomic ion. Zn2+ ( OH- ) 2 Not balanced 3/52

53 Writing Ionic Compound Formulas Example: Aluminum phosphate 1. Write the formulas for the cation and anion, including CHARGES! 2. Check to see if charges are balanced. Al3+ PO43They ARE balanced 3/53

54 Naming Binary Molecular Compounds Compounds between two nonmetals First element in the formula is named first. Keeps its element name Gets a prefix if there is a subscript on it Second element is named second Use the root of the element name plus the -ide suffix Always use a prefix on the second element 3/54

55 Naming Ionic Compounds Cation first, then anion Monatomic cation = name of the element Ca2+ = calcium ion Monatomic anion = root + -ide Cl- = chloride CaCl2 = calcium chloride 3/55

56 Naming Ionic Compounds Metals with multiple oxidation states some metal forms more than one cation use Roman numeral in (in parentheses) that indicates the charge of the metal in that particular compound. PbCl2 Pb2+ is cation PbCl2 = lead(ii) chloride For example, we distinguish between Fe2+ and Fe3+ as follows: Fe2+ Iron(II) Fe3+ Iron(III) Cu3(PO4)2 is copper(ii) phosphate3/56

57 Naming Chemical Compounds Binary Molecular Compounds Because nonmetals often combine with one another in different proportions to form different compounds, numerical prefixes are usually included in the names of binary molecular compounds Pearson Education, Inc. 2/57

58 Naming Binary Compounds P2O5 = diphosphorus pentoxide CO2 = carbon dioxide N2O = dinitrogen monoxide SO3 = Sulfur trioxide PbO = Lead(II) monoxide PbO2 = Lead(II) dioxide 3/58

59 Practice Write the Formula Compound Name Compound Formula Carbon dioxide Carbon monoxide Diphosphorus pentoxide Sulfur trioxide Copper(II) oxide Carbon tetrabromide Lead (II) Oxide Lead (IV) dioxide Iodine trichloride Sodium nitride copper(ii) phosphate Check next slide for answers3/59

60 Answers Write the Formula Compound Name Compound Formula Carbon dioxide CO2 Carbon monoxide CO Diphosphorus pentoxide P2O5 Sulfur trioxide SO3 Copper(II) oxide CuO Carbon tetrabromide CBr4 Lead (II) oxide PbO Lead (IV) dioxide PbO2 Iodine trichloride ICl3 Sodium nitride copper(ii) phosphate NaN3 Cu3(PO4)2

61 Hydrated Ionic Compounds Hydrates are ionic compounds containing a specific number of water molecules associated with each formula unit. For example, the formula for epsom salts is MgSO4 7H2O. Its systematic name is magnesium sulfate heptahydrate. CoCl2 6H2O is cobalt(ii)chloride hexahydrate Pearson Education, Inc. 3/61

62 Hydrates Common hydrate prefixes hemi = ½ mono = 1 di = 2 tri = 3 tetra = 4 penta = 5 hexa = 6 hepta = 7 octa = Pearson Education, Inc. Other common hydrated ionic compounds and their names are as follows: CaSO4 1/2H2O is called calcium sulfate hemihydrate. BaCl2 6H2O is called barium chloride hexahydrate. CuSO4 6H2O is called copper sulfate hexahydrate. 3/62

63 Acids Acids are molecular compounds that release hydrogen ions (H+) when dissolved in water. Acids are composed of hydrogen, usually written first in their formula, and one or more nonmetals, written second. HCl is a molecular compound that, when dissolved in water, forms H+(aq) and Cl (aq) ions, where aqueous (aq) means dissolved in water Pearson Education, Inc. 3/63

64 Acids Binary acids have H+1 cation and nonmetal anion. Oxyacids have H+ cation and oxyanion ( an anion contaning a nonmetal and oxygen) Pearson Education, Inc. 3/64

65 Naming Binary Acids Write a hydro- prefix. Follow with the nonmetal name. Change ending on nonmetal name to ic. Write the word acid at the end of the name. HF hydrofluoric acid HCl hydrochloric acid HBr hydrobrmoic acid 2014 Pearson Education, Inc. HI hydroiodic acid 3/65

66 Naming Oxyacids If polyatomic ion name ends in ate, then change ending to ic suffix. HNO3 (oxyanion is nitrate, NO3 - ) = Nitric acid H2SO4 (oxyanion is sulfate, SO4 2- ) = Sulfuric acid If polyatomic ion name ends in ite, then change ending to ous suffix. HNO2 (oxyanion nitrite, NO2 - ) = Nitrous acid H2SO3 (oxyanion is sulfite, SO3 2- ) = Sulfurous acid Write word acid at the end of all names Pearson Education, Inc.

67 Name the Following Acids 1. H2CO3- Oxyacids Carbonic acid 2. HCl -Binary acids Hydrochloric acid 3. HC2H3O2- Oxyacids acetic acid 2014 Pearson Education, Inc. 3/67

68 Acid Rain and Climate Change Certain pollutants such as NO, NO2, SO2, SO3 form acids when mixed with water, resulting in acidic rainwater. Acid rain can fall or flow into lakes and streams, making these bodies of water more acidic. The details on this marble statue have been eaten over the years by acid rain. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

69 Organic Compounds Early chemists divided compounds into two types: organic and inorganic. Compounds from living things were called organic; compounds from the nonliving environment were called inorganic. Organic compounds are easily decomposed and could not be made in the lab. Inorganic compounds are very difficult to decompose, but are able to be synthesized. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

70 Modern Organic Compounds Today organic compounds are commonly made in the lab and we find them all around us. Organic compounds are mainly made of C and H, sometimes with O, N, P, S, and trace amounts of other elements The main element that is the focus of organic chemistry is carbon. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.