molecules and compounds-structure and reactivity
- 1. Compounds & Molecules NaCl, salt Buckyball, C60 Ethanol, C2H6O Molecule: The smallest identifiable unit that retains the physical and chemical properties of the pure substances.
- 2. Compounds & Molecules • COMPOUND is a combination of 2 or more elements in definite ratios by mass. • The character of each element is lost when forming a compound (e.g., think of NaCl). • MOLECULES are the smallest units of a compound that retains the characteristics of the compound.
- 3. MOLECULAR FORMULAS • Formula for glycine is C2H5NO2 (description of the composition) • In one molecule there are – 2 C atoms – 5 H atoms – 1 N atom – 2 O atoms
- 4. CONDENSED FORMULAS • Formula for glycine is NH2CH2CO2H (composition and functional groups) • In one molecule there are – 1 NH2 (amine group) – 1 CH2 group – 1 CO2H group
- 5. STRUCTURAL FORMULAS • Show how the atoms are attached within a molecule • The lines between atoms represent chemical bonds that hold the atoms together.
- 6. WRITING FORMULAS • Can also write glycine as the condensed formula H2NCH2COOH showing functional groups (atom ordering and connectivity) • or in the form of a structural formula showing how atoms are attached to each other (bond orders) C H H C H H O O H N
- 7. MOLECULAR MODELING C H H C H H O O H N Ball & stick Space-filling Drawing of glycine An even higher level of structural detail
- 8. Resources for Molecular Modeling • Modeling software –CAChe (General Chemistry Interactive CD-ROM) –Rasmol –Molden –Gaussview –Maestro
- 9. MOLECULAR WEIGHT AND MOLAR MASS Molecular weight = sum of the atomic weights of all atoms in the molecule. Molar mass = molecular weight in grams per mol.
- 10. What is the molar mass of ethanol, C2H6O? 1 mol contains 2 moles of C (12.01 g C/1 mol) = 24.02 g C 6 moles of H (1.01 g H/1 mol) = 6.06 g H 1 mol of O (16.00 g O/1 mol) = 16.00 g O TOTAL = molar mass = 46.08 g/mol
- 11. • Formula = • Molar mass = Tylenol C8H9NO2 151.2 g/mol
- 12. Molar Mass
- 13. How many moles of alcohol (C2H6O) are there in a “standard” can of beer if there are 21.3 g of C2H6O? (a) Molar mass of C2H6O = 46.08 g/mol (b) Calc. moles of alcohol 21.3 g • 1 mol 46.08 g = 0.462 mol
- 14. How many molecules of alcohol are there in a “standard” can of beer if there are 21.3 g of C2H6O? = 2.78 x 1023 molecules We know there are 0.462 mol of C2H6O. 0.462 mol • 6.022 x 1023 molecules 1 mol
- 15. How many atoms of C are there in a “standard” can of beer if there are 21.3 g of C2H6O? = 5.57 x 1023 C atoms There are 2.78 x 1023 molecules. Each molecule contains 2 C atoms. Therefore, the number of C atoms is 2.78 x 1023 molecules • 2 C atoms 1 molecule
- 16. Molecular & Ionic Compounds Heme NaCl Molecular compounds consist of discrete molecules Ionic compounds consist of discrete ions
- 17. IONS AND IONIC COMPOUNDS • IONS are atoms or groups of atoms with a positive or negative charge. • Taking away an electron from an atom gives a CATION with a positive charge • Adding an electron to an atom gives an ANION with a negative charge.
- 18. Forming Cations & Anions A CATION forms when an atom loses one or more electrons. An ANION forms when an atom gains one or more electrons Mg --> Mg2+ + 2 e- F + e- --> F- oxidation reduction
- 20. PREDICTING ION CHARGES In general • metals (Mg) lose electrons ---> cations • nonmetals (F) gain electrons ---> anions
- 21. Charges on Common Ions +3 -4 -1 -2 -3 +1 +2 By losing or gaining e-, atom has same number of electrons as nearest Group 8A atom. Cation charge=group # Anion charge=group #-8
- 22. Predicting Charges on Monatomic Ions
- 23. METALS M ---> n e- + Mn+ where n = periodic group Na+ sodium ion Mg2+ magnesium ion Al3+ aluminum ion Transition metals --> M2+ or M3+ are common Fe2+ iron(II) ion Fe3+ iron(III) ion
- 24. NONMETALS NONMETAL + n e- ------> Xn- where n = 8 - Group no. C4-,carbide N3-, nitride O2-, oxide S2-, sulfide F-, fluoride Cl-, chloride Group 7A Group 6A Group 4A Group 5A Br-, bromide I-, iodide
- 26. POLYATOMIC IONS CD Screen 3.6 Groups of atoms with a charge. MEMORIZE the names and formulas of common polyatomic ions listed in Table 3.1, page 107 (next slide)
- 28. Polyatomic Ions NH4 + ammonium ion One of the few common polyatomic cations
- 29. Polyatomic Ions (oxoanions) HNO3 nitric acid NO3 - nitrate ion Prefix per- and suffix –ate: largest # Suffix -ate : greater # of oxygen atoms Suffix -ite : smaller # of oxygen atoms Prefix hypo- and suffix –ite: smallest #
- 32. Polyatomic Ions CO3 2- carbonate ion HCO3 - bicarbonate ion hydrogen carbonate
- 34. CATION + ANION ---> COMPOUND A neutral compd. requires equal number of + and - charges. COMPOUNDS FORMED FROM IONS Na+ + Cl- --> NaCl
- 36. Some Ionic Compounds Mg2+ + NO3 - ----> Mg(NO3)2 magnesium nitrate Fe2+ + PO4 3- ----> Fe3(PO4)2 iron(II) phosphate calcium fluoride Ca2+ + 2 F- ---> CaF2
- 37. Properties of Ionic Compounds Forming NaCl from Na and Cl2 • A metal atom can transfer an electron to a nonmetal. • The resulting cation and anion are attracted to each other by electrostatic forces.
- 38. Electrostatic Forces The oppositely charged ions in ionic compounds are attracted to one another by ELECTROSTATIC FORCES. These forces are governed by COULOMB’S LAW.
- 39. Electrostatic Forces COULOMB’S LAW Force of attraction = (charge on +)(charge on -) (distance between ions)2 As ion charge increases, the attractive force _______________. As the distance between ions increases, the attractive force ________________. This idea is important and will come up many times in future discussions!
- 40. Importance of Coulomb’s Law NaCl, Na+ and Cl-, m.p. 804 oC MgO, Mg2+ and O2- m.p. 2800 oC
- 41. ELEMENTS THAT EXIST AS MOLECULES See SCREEN 3.2 on the CD-ROM Allotropes of C
- 42. Screen 3.2
- 43. ELEMENTS THAT EXIST AS DIATOMIC MOLECULES (gases)
- 44. ELEMENTS THAT EXIST AS POLYATOMIC MOLECULES White P4 and polymeric red phosphorus S8 sulfur molecules
- 45. Molecular Compounds Compounds without Ions CH4 methane CO2 Carbon dioxide BCl3 boron trichloride
- 46. Naming Molecular Compounds CH4 methane BCl3 boron trichloride CO2 Carbon dioxide All are formed from two or more nonmetals. Ionic compounds generally involve a metal and nonmetal (NaCl)
- 47. Empirical & Molecular Formulas A pure compound always consists of the same elements combined in the same proportions by weight. Therefore, we can express molecular composition as PERCENT BY WEIGHT Ethanol, C2H6O 52.13% C 13.15% H 34.72% O
- 48. Percent Composition Consider some of the family of nitrogen- oxygen compounds: NO2, nitrogen dioxide and closely related, NO, nitrogen monoxide (or nitric oxide) Structure of NO2 Chemistry of NO, nitrogen monoxide
- 49. Percent Composition Consider NO2, Molar mass = ? What is the weight percent of N and of O? Wt. % O 2 (16.0 g O per mole ) 46.0 g x 100% 69.6% Wt. % N = 14.0 g N 46.0 g NO2 • 100% = 30.4 % What are the weight percentages of N and O in NO?
- 50. Determining Formulas In chemical analysis we determine the % by weight of each element in a given amount of pure compound and derive the EMPIRICAL or SIMPLEST formula. PROBLEM: A compound of B and H is 81.10% B. What is its empirical formula?
- 51. • Because it contains only B and H, it must contain 18.90% H. • In 100.0 g of the compound there are 81.10 g of B and 18.90 g of H. • Calculate the number of moles of each constitutent. A compound of B and H is 81.10% B. What is its empirical formula?
- 52. Calculate the number of moles of each element in 100.0 g of sample. 81.10 g B • 1 mol 10.81 g = 7.502 mol B 18.90 g H • 1 mol 1.008 g = 18.75 mol H A compound of B and H is 81.10% B. What is its empirical formula?
- 53. Now, recognize that atoms combine in the ratio of small whole numbers Find the ratio of moles of elements in the compound. A compound of B and H is 81.10% B. What is its empirical formula?
- 54. But we need a whole number ratio. 2.5 mol H/1.0 mol B = 5 mol H to 2 mol B EMPIRICAL FORMULA = B2H5 Take the ratio of moles of B and H. Always divide by the smaller number. 18.75 mol H 7.502 mol B = 2.499 mol H 1.000 mol B = 2.5 mol H 1.0 mol B A compound of B and H is 81.10% B. What is its empirical formula?
- 55. A compound of B and H is 81.10% B. Its empirical formula is B2H5. What is its molecular formula ? Is the molecular formula B2H5, B4H10, B6H15, B8H20, etc.? B2H6 is one example of this class of compounds. B2H6
- 56. A compound of B and H is 81.10% B. Its empirical formula is B2H5. What is its molecular formula? We need to do an EXPERIMENT to find the MOLAR MASS. Here experiment gives 53.3 g/mol Compare with the mass of B2H5 = 26.66 g/unit Find the ratio of these masses. 53.3 g/mol 26.66 g/unit of B2H5 = 2 units of B2H5 1 mol Molecular formula = B4H10
- 57. How to Determine the molar mass? Mass spectrometer
- 58. Mass Spectrum of Ethanol Mass Spectrum of Ethanol (from the NIST site) 46 45 CH3CH2OH+ CH3CH2O+ 31 CH2O+
- 59. Determine the formula of a compound of Sn and I using the following data. • Reaction of Sn and I2 is done using excess Sn. • Mass of Sn in the beginning = 1.056 g • Mass of iodine (I2) used = 1.947 g • Mass of Sn remaining = 0.601 g
- 60. Find the mass of Sn that combined with 1.947 g I2. Mass of Sn initially = 1.056 g Mass of Sn recovered = 0.601 g Mass of Sn used = 0.455 g Find moles of Sn used: 0.455 g Sn • 1 mol 118.7 g = 3.83 x 10-3 mol Sn Tin and Iodine Compound
- 61. Tin and Iodine Compound Now find the number of moles of I2 that combined with 3.83 x 10-3 mol Sn. Mass of I2 used was 1.947 g. 1.947 g I2 • 1 mol 253.81 g = 7.671 x 10-3 mol I2 How many moles of iodine atoms? = 1.534 x 10-2 mol I atoms 7.671 x 10-3 mol I2 2 mol I atoms 1 mol I2
- 62. Tin and Iodine Compound Now find the ratio of number of moles of moles of I and Sn that combined. 1.534 x 10-2 mol I 3.83 x 10-3 mol Sn = 4.01 mol I 1.00 mol Sn Empirical formula is SnI4