Molecular Compounds. A molecule made of a central atom with four atoms bonded to it making a. an L shaped structural formula.

Transcription

1 Molecular Compounds Word Allotrope Bent Double bond Electronegativity difference Hydrogen bonds Line Of Symmetry Linear Molecule Network solid Nonpolar covalent bond Nonpolar molecule Polar Covalent bond Polar molecule Pyramidal Shared pair Tetrahedral Triple bond Unpaired Valence Electron Unshared Pair Definition Molecular form of a pure nonmetallic element. A molecule made of a central atom with two atoms bonded to it, making an L shaped structural formula. A bond where two pairs of electrons are shared between nonmetal atoms. The difference in electronegativity between two elements in a bond. A strong attractive force between polar molecules where one molecule contains H and the other molecule contains N, O or F. The H of one molecule attracts strongly (almost with ionic strength) to the N, O or F of the other molecule. A line that can be drawn through a shape that perfectly divides the two sides of the line into mirror images of each other. A molecule (usually made of 2 atoms) with a 180 o bond angle. A particle made of nonmetal atoms covalently bonded together to form a distinct particle. A crystal lattice formed from covalently bonded nonmetal atoms with no distinct molecules. A bond formed between two nonmetal atoms when unpaired electrons of two atoms are shared equally, with an electronegativity difference of 0 to 0.4. A molecule with symmetrical electron distribution resulting in any polar bonds canceling each other out to yield no partially charged ends. A bond formed between two nonmetal atoms when unpaired electrons of two atoms are shared unequally, with an electronegativity difference of 0.5 to 1.7. A molecule with asymmetrical electron distribution resulting in partially charged ends. A molecule made of a central atom with three atoms bonded to it making a T shaped structural formula. Two electrons that are being shared between two bonded nonmetal atoms, represents a single covalent bond. A molecule made of a central atom with four atoms bonded to it making a + shaped structural formula. A bond where three pairs of electrons are shared between nonmetal atoms. A valence electron that is available for bonding, it stands alone on one side of the dot diagram. The number of unpaired valence electrons equals the number of covalent bonds an atom can form. Electron pairs belonging only to one atom and not involved in any bond. 2008, Mark Rosengarten R 1

2 Topic 1: Covalent Bonding Objective: You will describe how covalent bonds form, and identify bonds as being either polar covalent or nonpolar covalent.. In the last unit, you learned how metals lose electrons to nonmetals and form + ions. The nonmetals form ions, which attract for form ionic bonds, which is how ionic compounds are made. In this unit you will see how nonmetal atoms share electrons to make covalent bonds, which make molecules! Molecules include water, plastics, DNA the very stuff of life and our existence. So now, with no further delay COVALENT BONDING!!!! COVALENT BONDING 1) Two NONMETAL atoms attempt to gain each other s valence electrons. They do not have enough difference in electronegativity to do so, therefore they share them. 2) The electrons shared are the unpaired valence electrons. 3) The bonded atoms actually become part of each other. This makes for a bond much stronger than an ionic bond. This bond can not be broken by dissolving in water or melting, so covalent compounds never conduct electricity, regardless of the phase (exception later this year). To determine if a bond is covalent, check the electronegativity difference. If it is below 1.7, the atom with the higher electronegativity does not have enough pull to remove the other atom s electrons, so they share them. Each bonded atom has 8 valence electrons after bonding, except for hydrogen (H), which is only large enough to have 2 valence electrons after bonding (because of H s single energy level, which can only hold up to 2 electrons. How many covalent bonds can a nonmetal atom form? Nonmetal Dot Diagram # unpaired e- # of covalent bonds Nonmetal Dot Diagram # unpaired e- N 3 3 S 2 2 O 2 2 Cl 1 1 F 1 1 P 3 3 C 4 4 Br 1 1 H 1 1 I 1 1 # of covalent bonds Making Molecules: Molecules are particles made from nonmetal atoms covalently bonding together. Each molecule of a substance has an identical molecular formula that tells you exactly how many atoms of each element are found in the molecule. H 2 O (water) is a molecule made of two atoms of hydrogen bonded to one oxygen atom. CH 4 (methane, natural gas) is a molecule made of one carbon atom with four hydrogen atoms bonded to it. NH 3 (ammonia) is made of one atom of nitrogen with three hydrogen atoms bonded to it. 2008, Mark Rosengarten R 2

3 THE TWO TYPES OF COVALENT BONDING Nonpolar Covalent formed between nonmetal atoms with an electronegativity difference of 0 to 0.4. The electrons are being shared equally in the bond. Examples include the diatomic molecules (Br 2, I 2, N 2, Cl 2, H 2, O 2 and F 2 ) molecules, which form when nonmetal atoms that are unstable by themselves bond together to form more stable molecules, because each bonded atom now has a stable octet of 8 valence electrons. Polar Covalent - formed between nonmetal atoms with an electronegativity difference of 0.5 to about 1.7. The electrons are being shared unequally in the bond. The electrons spend more time with the more electronegative atom, giving it a slight negative charge, and the less electronegative atom becomes slightly positive. The charged ends of the bonds form POLES (oppositely charged ends), which is why the bond is called polar. Writing partial all the time is a pain, so use the lower-case Greek letter delta instead: δ Partially positive = δ + Partially negative = δ- Bonding Nonmetal Atoms Electronegativity Of Each Atom (END) Which Pole Is δ+ and which is δ-? H and Cl H: 2.2 Cl: 3.2 (1.0) δ+ H-Cl δ- H and O H: 2.2 O: 3.5 (1.3) δ+ H-O δ- O and N O: 3.5 N: 3.0 (0.5) δ- O-N δ+ C and F C: 2.6 F: 4.0 (1.4) δ+ C-F δ- 2008, Mark Rosengarten R 3

4 How Atoms Covalently Bond To Form Molecules In each case below, notice how each bonding atom s unpaired valence electrons pair up, with the atoms becoming part of one another. Notice how this is different than how ionic bonds work, where electrons are transferred, and not shared. The valence electrons of one bonding atom are shown as dots, the valence electrons of the other bonding atom are shown as circles, so you can clearly see where the electrons in each bond are coming from. The particle formed by the bonding atoms is called a MOLECULE. Molecules can only be made of nonmetal atoms bonding to each other. Metals cannot be in molecules OK, that is not the whole truth (your blood s hemoglobin is a huge molecule with an iron atom at the center), but as far as this course is concerned, only nonmetal atoms will be in molecules. 2008, Mark Rosengarten R 4

5 Topic 2: Molecular Substances Objective: You will describe the properties of molecular and network substances, draw dot diagrams for covalent compounds, You will differentiate between hydrogen bond, dipole and London dispersion forces and explain how the properties of melting point, boiling point and vapor pressure relate to attractive force strength. TYPES OF COVALENT SUBSTANCES Molecular Compounds: formed by covalent bonding, either polar or nonpolar. These form individual particles called molecules which can attract to each other to form the solid or liquid phase. Molecules can have oppositely charged ends, which allow them to attract to one another. These are called intermolecular attractive forces, and are weaker than ionic attractions. Therefore, molecular compounds are more easily melted and boiled, so their melting and boiling points are low compared to ionic compounds. They also tend to evaporate more quickly. Their solids are soft (think wax or water ice, compared to something like steel!) Dissolving in water and melting do not break the covalent bond. There are no ions to carry electrical current, so molecules do not conduct electricity. Acids are the exception to this, but for now we will turn a blind eye to them Network Solids: also formed by covalent bonding (usually nonpolar), but it doesn t form separate molecules. Instead, it forms one single crystal made of nonmetal atoms connected with a continuous network of covalent bonds with no areas of weakness that can break apart. Molecules can be separated from each other, but network solids have no such weakness. They are among the hardest substances known to science. They also occupy the top of the Mohs hardness scale (talc at 1, diamond at 10). Quartz (SiO 2 ) has a hardness of 7, corundum (Al 2 O 3, rubies and sapphires) have a hardness of 9, and diamond (pure crystalline carbon) has a hardness of 10, the top of the scale. Being made only of nonmetals, network solids are nonconductors of electricity and poor conductors of heat. They are also quite brittle. Allotropes: molecular forms of pure nonmetal atoms. Nonmetal atoms are more stable when they are bonded, so when you see a pure sample of nonmetal (especially the more reactive ones), it is usually in molecular form. Examples include: Oxygen: O 2 (diatomic oxygen, what you breathe to live) and O 3 (ozone, air pollution at ground level, but protects us from UV rays up in the ionosphere) Carbon: Coal (amorphous carbon, no crystal structure), graphite (soft crystalline carbon), diamond (hard crystalline solid), fullerenes (hollow, ball-shaped carbon molecules, nanotubes (conductive hollow carbon tubes, might someday be used in making computers) MOLECULAR POLARITY How can you tell if a molecule is polar or nonpolar? The polarity of the molecule is different than the polarity of the bond. A bond is polar if the END between the bonding atoms is 0.5 or higher. A molecule can have polar bonds and still be a nonpolar molecule. Why is it important to know the polarity of a molecule? The polarity can tell you many things: 1) How high or low the melting and boiling point of the substance is 2) How easily the liquid form of the substance evaporates 3) Whether the substance will dissolve in water, or in some other solvent Molecular polarity causes intermolecular attractive forces, which is what allows certain adhesives (tapes, glues) to hold things together. Attractive forces also hold the two twisted strands of RNA together to form DNA. Attractive forces allow insects and geckos to walk up walls, even glass walls. Think of attractive forces as molecular Velcro you can attach and detach the molecules from each other without doing any damage to the molecules themselves. Some molecules have stronger Velcro than others. How do you know which ones these are? Read on! 2008, Mark Rosengarten R 5

6 First thing you need to know is what is the shape of the molecule? Its shape determines its properties. Molecules can be represented three ways, using molecular or structural formulas, or by using dot diagrams. Nonmetal atoms bond in such a way that their unpaired valence electrons pair up so that both atoms end up with a stable octet (or, in the case of hydrogen, 2 electrons), including the ones being shared. The structural formula is just the dot diagram, without the dots. Each pair of shared electrons gets replaced with a dash to represent the bond. Two pairs of shared electrons are replaced with two dashes (=). That s it! Structural formulas are very useful in showing what the molecule actually looks like. We will use them rather a lot through the rest of the course. 2008, Mark Rosengarten R 6

7 Molecular Polarity POLAR MOLECULES: Nonmetals in a molecule share electrons, sometimes equally (nonpolar bond) or unequally (polar covalent bond) when forming molecules. If the molecule does not have a symmetrical shape (asymmetrical), then there is a greater concentration of electrons on one side of the molecule compared to the other side. This makes one side charged partially negative and the other side partially positive. The oppositely charged ends of these molecules are poles, making the molecule polar. Polar molecules can attract each other, δ+ end of one molecule attracting to the δ- end of the other molecule. These are the intermolecular attractive forces mentioned on page 5! 2008, Mark Rosengarten R 7

8 NONPOLAR MOLECULES ). If the molecule has a symmetrical shape, then the electrons are distributed evenly through the molecule, and the whole molecule is nonpolar (even if it contains polar bonds). Nonpolar molecules have equal pull of electrons on all sides of the molecule, so no side develops a pole. Since the molecule lacks oppositely charged ends, any attractive forces will be extremely weak. Small nonpolar molecules are usually found in the gaseous state at room temperature (CH 4, methane, also known as natural gas; C 3 H 8, propane, also known as bottled gas and C 4 H 8, butane, also known as lighter fuel), larger molecules can be liquids (C 8 H 18, octane, also known as gasoline and C 6 H 6, benzene, which is a liquid capable of dissolving plastic) and huge nonpolar molecules can be found in the solid phase (C 6 H 4 Cl 2, paradichlorobenzene is one molecule that mothballs have been made of). Attractive Forces Polar Molecules form DIPOLE attractions. There are two types of this, normal dipole and HYDROGEN BOND dipole, which is formed between molecules containing H bonded to a highly electronegative atom, specifically N, O or F. Hydrogen bond dipole attraction is much stronger than normal dipoles, since the molecules are much more polar. 2008, Mark Rosengarten R 8

9 Nonpolar Molecules form LONDON DISPERSION FORCE attractions. Since there are no permanent positive or negative ends, these attractions are extremely weak. The attractions are a combination of temporary poles due to electron movement around the molecule or, in the case of huge molecules, they actually get tangled up with each other like sticky strands of spaghetti or yarn. Attractive Force Type, Strength And Resulting Molecular Properties Lines Of Symmetry Molecule Polarity Attractive Force Type 0 or 1 Polar (if H end of one molecule is attracted to N,O or F end of the other molecule) HYDROGEN BOND 0 or 1 Polar (any other polar molecule) DIPOLE Attractive Force Strength Melting & Boiling Points Strong High Poor Moderate Moderate Fair Vapor Pressure (ability to evaporate) 2 or more nonpolar LONDON DISPERSION FORCES Weak Low Good 2008, Mark Rosengarten R 9

10 SUMMING IT ALL UP: A FLOWCHART So, what should you be able to do now? 1) Identify whether a compound is molecular, ionic, metallic or network based on its properties 2) Draw dot diagrams of simple molecules 3) Draw structural formulas of simple molecules 4) Determine the shape of simple molecules 5) Determine if simple molecules are polar or nonpolar 6) If polar, draw the dipole moment and identify the partially charged ends 7) Determine the attractive force type that attracts specific simple molecules to each other. 2008, Mark Rosengarten R 10

11 Topic 3: Molecular Formulas and Naming Objective: You will calculate the molecular formula for a compound given its empirical formula and molecular mass and you will name and write formulas for molecular substances. MOLECULAR FORMULAS Molecular Formulas tell you the actual number of atoms of each nonmetal element in a molecule. They work the same way as empirical formulas, but they often can be simplified into empirical formulas. Molecular formulas are whole-number multiples of empirical formulas. Molecular formulas are simply whole-number multiples of empirical formulas. The chart below shows how! Molecular Formula Divide by to simplify to empirical formula Corresponding Empirical Formula CH 4 1:4 ratio cannot be simplified. CH 4 C 2 H 6 Divide by 2 CH 3 N 2 O 4 Divide by 2 NO 2 H 2 O 2:1 ratio cannot be simplified H 2 O C 8 H 16 Divide by 8 CH 2 C 6 H 6 Divide by 6 CH Multiply the empirical formula by a whole number, and you will get the molecular formula. Same thing goes for formula mass multiple the empirical formula s mass by that same whole number and you will get the molecular formula s mass. Empirical Formula, which has an Empirical Formula Mass of Multiply by and get the Molecular formula with a molecular mass of CH CH 2 is not a possible molecular formula, as carbon forms 4 bonds. CH C 2 H CH C 3 H CH C 4 H CH C 5 H , Mark Rosengarten R 11

12 To determine the molecular formula of a compound given its molecular mass and empirical formula: a) Determine the formula mass of the empirical formula. b) Divide the molecular mass by the empirical mass. This will give you a whole-number multiple that tells you how many times larger the molecular formula is than the empirical formula c) Multiply the whole number by the empirical formula. This will give the molecular formula. The empirical formula of a compound is C 2 H 3, and the molecular mass is 54.0 grams/mole. What is the molecular formula? a) Determine the formula mass of the empirical formula. C 2 H 3 = (2 C X 12.0 g/mol) + (3 H X 1.0 g/mol) = 27.0 g/mole b) Divide the molecular mass by the empirical mass. This will give you a whole-number multiple that tells you how many times larger the molecular formula is than the empirical formula (54.0 g/mol) / (27.0 g/mol) = 2.00 c) Multiply the whole number by the empirical formula. This will give the molecular formula X C 2 H 3 = C 4 H 6 What is the molecular formula of a compound whose empirical formula is CH with a molecular mass of 52.0 grams/mole? a) Determine the formula mass of the empirical formula. CH = (1 C X 12.0 g/mol) + 1 H X 1.0 g/mol) = 13.0 g/mole b) Divide the molecular mass by the empirical mass. This will give you a whole-number multiple that tells you how many times larger the molecular formula is than the empirical formula (52.0 g/mol) / (13.0 g/mol) = 4.00 c) Multiply the whole number by the empirical formula. This will give the molecular formula X CH = C 4 H , Mark Rosengarten R 12

13 NAMING AND FORMULA WRITING FOR MOLECULAR COMPOUNDS Molecular formulas can be named in two ways: 1) The Stock System This works the same as with ionic compounds. Use a Roman numeral to represent the charge of the first atom written in the formula (the one with lower electronegativity). Molecular Formula Name (Stock System) Name (Stock System) Molecular Formula CO 2 Carbon (IV) oxide Sulfur (II) oxide SO CO Carbon (II) oxide Sulfur (IV) oxide SO 2 NO 2 Nitrogen (IV) oxide Carbon (IV) chloride CCl 4 NO 3 Nitrogen (VI) oxide Nitrogen (III) chloride NCl 3 N 2 O 5 Nitrogen (V) oxide Phosphorous (III) oxide P 2 O 3 2) The Prefix System This system uses prefixes to describe how many atoms of each element are found in the molecule: 1 atom 2 atoms 3 atoms 4 atoms 5 atoms 6 atoms mono- (or none) di- tri- tetra- (or tetr-) penta (or pent-) hexa- (or hex-) Molecular Formula Name (Prefix System) Name (Prefix System) Molecular Formula CO 2 Carbon dioxide Sulfur trioxide SO 3 CO Carbon monoxide Sulfur dioxide SO 2 NO 2 Nitrogen dioxide Carbon tetrachloride CCl 4 NO 3 Nitrogen trioxide dinitrogen tetroxide N 2 O 4 N 2 O 5 Dinitrogen pentoxide diphosphorous trioxide P 2 O , Mark Rosengarten R 13

14 Student Name: Grades:,, Topic 1: Covalent Bonding Homework A) Multiple Choice Questions: Place your answer in the space in front of each question. 1) When two atoms of nitrogen bond, how many pairs of electrons will be shared between them? a) 1 b) 2 c) 3 d) 4 2) When two atoms of fluorine bond, how many electrons will be shared between them? a) 1 b) 2 c) 3 d) 4 3) When an atom of H and an atom of F bond together: a) The H will be partially positive, because it has higher electronegativity than F. b) The H will be partially negative, because it has higher electronegativity than F. c) The F will be partially positive, because it has higher electronegativity than H. d) The F will be partially negative, because it has higher electronegativity than H. 4) Which of the molecules listed below has the most polar bond between the bonded atoms, in terms of greatest END? a) HF b) HCl c) HBr d) HI 5) Which of the following compounds is formed by covalent bonding? a) Na 2 S b) AlCl 3 c) C 6 H 12 O 6 d) LiH 6) Which of the following molecules contains a nonpolar covalent bond? a) H 2 O b) HF c) F 2 d) NH 3 7) Which of the following molecules contains a polar covalent bond? a) H 2 b) PH 3 c) F 2 d) NH 3 B) Complete the following chart, drawing the dot diagram of each element in the molecule and then the dot diagram of the molecule. If the formula is H 2 O, make sure you have two atoms of H and one of O in your dot diagram of the molecule. Formula Dot diagram for: Dot diagram for: Dot Diagram of Molecule F F F 2 HBr H Br H 2 O H O N H NH , Mark Rosengarten R 14

15 C) Identify the following bonds as being polar covalent or nonpolar covalent. For the polar covalent bonds, label the δ + and δ - ends. Bond END Polar or Nonpolar? If polar, label the δ + and δ - ends H H H C H Cl C Cl P Cl Cl Cl H P H O O O H H H C H Cl C Cl P Cl Cl Cl H P H O O O D) Explain, in terms of electronegativity difference, why Cl 2 contains nonpolar covalent bonds. E) Explain, in terms of electronegativity difference, why H 2 O contains polar covalent bonds. 2008, Mark Rosengarten R 15

16 Topic 2: Molecular Substances Homework A) Multiple Choice Questions: Place your answer in the space in front of each question. 1) Which of the following molecules is polar? a) F 2 b) NH 3 c) O 2 d) Cl 2 2) Which of the following molecules has the strongest hydrogen-bond attractions? a) HF b) HCl c) HBr d) HI 3) Which of the following nonpolar molecules has the highest boiling point? a) CH 4 b) C 2 H 6 c) C 3 H 8 d) C 4 H 10 4) Which of the following molecules is a liquid at STP? a) N 2 b) H 2 c) Br 2 d) I 2 5) Which of the following molecules is linear? a) N 2 b) H 2 O c) NH 3 d) CCl 4 6) Which of the following molecules is bent? a) N 2 b) H 2 O c) NH 3 d) CCl 4 7) Which of the following molecules is pyramidal? a) N 2 b) H 2 O c) NH 3 d) CCl 4 8) Which of the following molecules is tetrahedral? a) N 2 b) H 2 O c) NH 3 d) CCl 4 9) Which of the following substances is molecular? a) NaCl b) CO 2 c) K 2 O d) C 10) Which of the following substances has a very high melting point and does not conduct electricity in the liquid phase? a) NaCl b) CO 2 c) CH 4 d) SiO 2 11) Which of the following substances is a poor conductor and melts at a relatively low temperature? a) Li 2 O b) CH 4 c) W d) Cu 2008, Mark Rosengarten R 16

17 B) For each of the following molecules represented by structural formulas indicate a) if the molecule is polar or nonpolar. b) If polar, draw the dipole moment and which side is partially positive and which side is partially negative. If nonpolar, then skip this step and move on to c). c) Identify the shape of the molecule d) Identify the type of attractive force that will hold molecules of this substance together in the liquid and solid phase. Molecule (do B in this box) H Cl Polar or Nonpolar Shape IMAF Type Dot Diagram H H H O H H - N H H Cl Cl C Cl Cl O = C = O H - Br 2008, Mark Rosengarten R 17

18 Topic 3: Molecular Formulas and Naming Homework A) Multiple Choice Questions: Place your answer in the space in front of each question. 1) Which of the following is an empirical formula, only? a) H 2 O 2 b) CaCO 3 c) C 2 H 4 d) C 6 H 12 O 6 2) Which of the following is a molecular formula, only? a) H 2 O 2 b) CaCO 3 c) CH 4 d) CO 2 3) What is the empirical formula of C 4 H 10? a) C 4 H 10 b) C 2 H 5 c) CH 5 d) CH 2 4) How many moles of oxygen atoms are there in 2 moles of C 6 H 12 O 6? a) 6 b) 12 c) 24 d) 48 B) Determine the molecular formulas for each question below, showing all work. 1) The empirical formula of a compound is found to be CH, and the molecular mass has been determined to be 78.0 g/mole. What is the molecular formula of this compound? 2) The empirical formula of a compound is found to be HO, and the molecular mass has been determined to be 34.0 g/mole. What is the molecular formula of this compound? 3) The empirical formula of a compound is found to be NO 2, and the molecular mass has been determined to be 92.0 g/mole. What is the molecular formula of this compound? 4) The empirical formula of a compound is found to be CH 2 O, and the molecular mass has been determined to be g/mole. What is the molecular formula of this compound? 5) The empirical formula of a compound is found to be CH 3 O, and the molecular mass has been determined to be 62.0 g/mole. What is the molecular formula of this compound? 2008, Mark Rosengarten R 18

19 C) Identify each of the following as an empirical or molecular formula. If a formula is molecular, write its empirical formula. Formula Empirical or Molecular? Simplify if Molecular Formula NaCl N 2 O 4 Empirical or Molecular? Simplify if Molecular C 2 H 6 Ra(CN) 2 Ba(NO 3 ) 2 C 6 H 12 O 6 C) Fill in the blanks for each molecular compound, using the clue given for each: Formula Name (Stock System) Name (Prefix System) SO 3 H 2 O HCl Phosphorous (V) oxide Hydrogen nitride Hydrogen sulfide Phosphorous trichloride Carbon tetrabromide 2008, Mark Rosengarten R 19