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Chapter 3 Chemical Compounds

Chapter 3 Chemical Compounds. General Chemistry Talia Ara. A. Chemical Formulas. The chemical formula of a substance is a notation using atomic symbols with subscripts to convey the relative proportions of atoms of the different elements in a substance.

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Chapter 3 Chemical Compounds

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  1. Chapter 3Chemical Compounds General Chemistry Talia Ara

  2. A. Chemical Formulas • The chemical formulaof a substance is a notation using atomic symbols with subscripts to convey the relative proportions of atoms of the different elementsin a substance. • For example, the chemical formula of carbon dioxide, CO2, indicates that the compound is composed of carbon and oxygen atoms in a 1:2 ratio.

  3. 1. Molecular Substances • A molecular substance is a substance that is composed of molecules, all of which are alike. • A moleculeis defined as a unit of matter in which two or more atoms are chemically bonded together (connected by very strong attractive forces – more on this later). eg. Water is a molecular substance.

  4. 1. Molecular Substances • a) For a molecular substance, the molecular formula gives the exact number of atoms in one molecule of that substance.

  5. b) Naming Binary Molecular Compounds • Binary molecules are molecules that contain two different nonmetallic elements. • Binary molecular compoundsare named in a systematic way such that every binary molecular compound has a unique name. eg. SiBr4 Silicon Tetrabromide CO2 Carbon Dioxide

  6. b)Naming Binary Molecular Compounds • For hydrogen compounds that contain oxygen, sulfur or a halogen: • Hydrogen is listed first • The other element is listed second, with the name changed to end in the suffix -ide

  7. b) Naming Binary Molecular Compounds • For binary molecular compounds containing elements from Groups 4A-8A, the two elements are listed in order of group number in both the formula and the name.

  8. b) Naming Binary Molecular Compounds • Prefixes are used to designate the number of each kind of atom. • The second element listed in the name is modified with the suffix-ide. • Exception: mono- is never used to describe the atom listed first in the name.

  9. b) Naming Binary Molecular Compounds

  10. b) Naming Binary Molecular Compounds • For example… • CS2: • SBr6: • OF2: • P4O10:

  11. b) Naming Binary Molecular Compounds • For example… • CS2: carbon disulfide • SBr6: • OF2: • P4O10:

  12. b) Naming Binary Molecular Compounds • For example… • CS2: carbon disulfide • SBr6: sulfur hexabromide • OF2: • P4O10:

  13. b) Naming Binary Molecular Compounds • For example… • CS2: carbon disulfide • SBr6: sulfur hexabromide • OF2: oxygen difluoride • P4O10:

  14. b) Naming Binary Molecular Compounds • For example… CS2: carbon disulfide SBr6: sulfur hexabromide OF2: oxygen difluoride P4O10: tetraphosphorus decoxide

  15. 2. Ionic Substances • Not all substances are made • up of molecules. • In general, binary compounds • composed of a metal and a • nonmetal are ionic. • Ionic substances are composed of a lattice of positive ions (cations) and negative ions (anions) – NO MOLECULES!

  16. 2. Ionic Substances (cont.) • As we saw previously: a) When an atom gains extra electrons, it becomes a negatively charged ion, called an anion – the charge equals the number of electrons gained by the atom. b) When an atom loses electrons, it becomes a positively charged ion, called a cation – the charge equals the number of electrons lost by the atom. eg. Salt (NaCl) is an ionic substance

  17. Sodium Chloride As you can see from these visual representations of sodium chloride, ionic compounds are not made up of distinct molecular units. Each sodium cation is surrounded by six chloride anions, and vice versa. The ions are held together by an electrostatic attraction.

  18. 2. Ionic Substances • Because ionic substances are not composed of molecules, they do not have molecular formulas. • Instead, the formula of an ionic compound is written by giving the smallest possible whole-number ratio of the different elements in the substance. (eg. NaCl, not Na4Cl4) • c) The formula unit of a substance is the group of atoms or ions explicitly symbolized by its formula. (eg. NaCl)

  19. e) Naming Binary Ionic Compounds • Binary ionic compounds are named with the metallic element listed first. • For transition metals that can form more than one kind of cation, a Roman numeral is included to indicate charge (eg. copper(I) for Cu+ or copper(II) for Cu2+). • The nonmetallic element is listed second and modified with the suffix –ide. • Prefixes are NOT used to indicate the number of atoms of each element.

  20. e) Naming Binary Ionic Compounds eg. LiI: BeCl2:

  21. e) Naming Binary Ionic Compounds eg. LiI: lithium iodide BeCl2:

  22. e) Naming Binary Ionic Compounds eg. LiI: lithium iodide BeCl2: beryllium chloride

  23. e) Naming Binary Ionic Compounds eg. LiI: lithium iodide BeCl2: beryllium chloride Without using prefixes, how do you know the relative numbers of atoms of each element?

  24. e) Naming Binary Ionic Compounds eg. LiI: lithium iodide BeCl2: beryllium chloride Without using prefixes, how do you know the relative numbers of atoms of each element? eg. Magnesium Bromide – Is it MgBr, MgBr2, MgBr3, MgBr4, Mg2Br3, Mg3Br4?

  25. e) Charges on Common Monatomic Ions • In a binary ionic compound, the negative charge on the anion is canceled out by the positive charge on the cation. • There are equal and opposite charge so that the overall compound is neutral. • If you know the charge of each ion, you can figure out the ionic formula.

  26. e) Charges on Common Monatomic Ions (The charge on a transition metal cation will be indicated with a Roman numeral.)

  27. e) Charges on Common Monatomic Ions • Some general trends: • Main group metallic elements form cations with the charge equal to the group number (Group 1A – 1+, Group 2A – 2+) • Most nonmetallic elements form anions with the charge equal to the group number minus eight. (Group 5A – 3-, Group 6A – 2-, Group 7A – 1-)

  28. e) Charges on Common Monatomic Ions • What is the formula of aluminum oxide? • Aluminum (Group 3A): • Oxygen (Group 6A):

  29. e) Charges on Common Monatomic Ions • What is the formula of aluminum oxide? • Aluminum (Group 3A): 3+ • Oxygen (Group 6A):

  30. e) Charges on Common Monatomic Ions • What is the formula of aluminum oxide? • Aluminum (Group 3A): 3+ • Oxygen (Group 6A): 6 – 8 = 2-

  31. e) Charges on Common Monatomic Ions • What is the formula of aluminum oxide? • Aluminum (Group 3A): 3+ • Oxygen (Group 6A): 6 – 8 = 2- • What is the smallest whole number ratio that will balance these charges?

  32. e) Charges on Common Monatomic Ions • What is the formula of aluminum oxide? • Aluminum (Group 3A): 3+ • Oxygen (Group 6A): 6 – 8 = 2- • What is the smallest whole number ratio that will balance these charges? Total Positive Charge: 2 x (3+) = 6+ Total Negative Charge: 3 x (2-) = 6-

  33. e) Charges on Common Monatomic Ions • What is the formula of aluminum oxide? • Aluminum (Group 3A): 3+ • Oxygen (Group 6A): 6 – 8 = 2- • What is the smallest whole number ratio that will balance these charges? Total Positive Charge: 2 x (3+) = 6+ Total Negative Charge: 3 x (2-) = 6- ________________________________ Overall Charge: 6 – 6 = 0

  34. e) Charges on Common Monatomic Ions • What is the formula of aluminum oxide? Aluminum (Group 3A): 3+ Oxygen (Group 6A): 6 – 8 = 2- What is the smallest whole number ratio that will balance these charges? Total Positive Charge: 2 x (3+) = 6+ Total Negative Charge: 3 x (2-) = 6- ________________________________ Overall Charge: 6 – 6 = 0

  35. e) Charges on Common Monatomic Ions • What is the formula of magnesium bromide? • Magnesium (Group 2A): • Bromine (Group 7A):

  36. e) Charges on Common Monatomic Ions • What is the formula of magnesium bromide? • Magnesium (Group 2A): 2+ • Bromine (Group 7A):

  37. e) Charges on Common Monatomic Ions • What is the formula of magnesium bromide? • Magnesium (Group 2A): 2+ • Bromine (Group 7A): 7 – 8 = 1-

  38. e) Charges on Common Monatomic Ions • What is the formula of magnesium bromide? • Magnesium (Group 2A): 2+ • Bromine (Group 7A): 7 – 8 = 1- • It takes two bromide anions (2 x 1-) to match the charge on one magnesium cation (1 x 2+).

  39. e) Charges on Common Monatomic Ions • What is the formula of magnesium bromide? Magnesium (Group 2A): 2+ Bromine (Group 7A): 7 – 8 = 1- It takes two bromide anions (2 x 1-) to match the charge on one magnesium cation (1 x 2+). MgBr2

  40. 3. Molar Mass of Compounds • When working with molecular or ionic compounds on the macroscale, it is most convenient to deal with quantities in moles. • The molar mass of a substance is the mass of one mole of that substance. • The molar mass of a molecular compound is the mass of one mole of molecules. • The molar mass of an ionic compound (also called the formula weight) is the mass of one mole of the formula unit.

  41. 3. Molar Mass of Compounds • Calculate the molar mass of aspirin (C9H8O4).

  42. 3. Molar Mass of Compounds • Calculate the molar mass of aspirin (C9H8O4). C: 12.011 g/mol x 9 = 108.10 g/mol H: 1.008 g/mol x 8 = 8.064 g/mol O: 15.999 g/mol x 4 = 63.996 g/mol _______________________________________ C9H8O4 : 180.16 g/mol (The molecular mass is the same - in atomic mass units.)

  43. 3. Molar Mass of Compounds • Calculate the molar mass (or formula weight) of potassium sulfide.

  44. 3. Molar Mass of Compounds • Calculate the molar mass (or formula weight) of potassium sulfide. • K (Group 1A): +1 • S (Group 6A): -2 • Formula: K2S • K: 39.098 g/mol x 2 = 78.196 g/mol • S: 32.065 g/mol x 1 = 32.065 g/mol • _______________________________________ • K2S : 110.261 g/mol

  45. B. Determining Chemical Formulas • The composition of any compound can be expressed in two distinct ways: • the number of atoms of each element per molecule • the mass of each element in a mole of the compound. • For example: • There are two hydrogen atoms and one oxygen atom in each molecule of water (H2O). • There are 2.0158 g of hydrogen and 15.9994 g of oxygen in one mole of water – you can use this information to calculate the percent compositionof water.

  46. B. Determining Chemical Formulas • 1. The percent composition of a compound is the mass percentage (mass %) of each element in the compound. • Mass% of A in AxBy= mass of A in 1 mol x 100% • mass of AxBy in 1 mol • The molecular formula of a compound can be used to calculate its percent composition. • In many cases, an estimate of the percent composition for an unknown compound can be helpful in determining its molecular formula.

  47. B. Determining Chemical Formulas • Calculate the percent composition of ethane from the molecular formula (C2H6). • The molecular formula gives us the molar ratio of C and H (2 : 6). • To find mass percentages for C and H, we need to calculate the mass ratio. • How many grams of C are there in 1 mol of ethane? How many grams of H?

  48. B. Determining Chemical Formulas • Percent Composition - Ethane (C2H6)

  49. B. Determining Chemical Formulas • Percent Composition - Ethane (C2H6) • C: (12.011 g/mol) x 2 = 24.02 g/mol • H: (1.008 g/mol) x 6 = 6.048 g/mol • ______________________________________________________________________________________________________________________________ • Total: 30.07 g/mol • Mass % C = (24.02/30.07) x 100% = 79.89% • Mass % H = (6.048/30.07) x 100% = 20.11% • (Notice: 79.89% + 20.11% = 100%!!)

  50. B. Determining Chemical Formulas • If you do not know the molecular formula of a compound, knowing the percent composition allows you to calculate the empirical formula. 2. Empirical Formula: formula showing the simplest possible ratio of atoms of different elements in a compound eg. The molecular formula of hydrazine is N2H4. The empirical formula is NH2.

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