HW #4: 4.34, 4.36, 4.38, 4.40, 4.46, 4.50, 4.56, 4.60, 4.62, 4.64, 4.70, 4.72, 4.76, 4.80, 4.88, 4.96, 4.106

Transcription

1 Chemistry 121 Lectures 9 & 10: Covalent Bonds and the Periodic Table; Molecular Formulas and Lewis Structures; Multiple Covalent Bonds; Coordinate Covalent Bonds; the Shapes of Molecules; Polar Molecules and Electronegativity Revisited; Naming Binary Molecules Chapter 4 in McMurry, Castellion, et. al. 7 th edition HW #4: 4.34, 4.36, 4.38, 4.40, 4.46, 4.50, 4.56, 4.60, 4.62, 4.64, 4.70, 4.72, 4.76, 4.80, 4.88, 4.96, Learning Objectives: 1. Relate the stability of obtaining a noble gas electronic configuration to bonding in covalent systems 2. Improve our ability to draw the Lewis structure of molecules, including the potential for double and triple bonds 3. Define coordinate covalent bond and show how the formation of coordinate covalent bond between N and H gives NH4OH when NH3 is bubbled into H2O 4. Introduce resonance to rationalize the stability of the polyatomic ions 5. Define VSEPR and use the theory to rationalize the geometry of molecules 6. Use the wedge and dash method to depict 3-D structure around a given atom 7. Define polarity in terms of electronegativity differences between bonding atoms 8. Introduce polarity as a way of rationalizing 2 major physical properties boiling point and water solubility 9. Describe and utilize the formal system for naming covalent compounds 10. Define haloacid and distinguish haloacids in the gas form and aqueous solutions of haloacids 1

2 4.2: Covalent Bonds and the Periodic Table The Problem: Atoms are most stable when they obtain a noble gas electronic configuration Nonmetals seek electrons to obtain a noble gas electronic configuration. As a result, when nonmetals bond together, neither will transfer an electron to the bonding partner selfish little Hobbitses The Solution: Sharing of electrons. The shared electrons count towards the noble gas electronic configuration for both atoms involved in the bond. As a result, to obtain a noble gas electronic configuration H C N O F Bonds Notice this equals the number of single valence electrons for each of the atoms shown. It should come as no surprise then that other group members form the same number of bonds (for bonding purposes, hydrogen should be grouped with the halogens) Excepting hydrogen, counting lone or non-bonding pairs of electrons and bonding electrons gives rise to the octet rule H C N O F Lone Pairs Bonds Total

3 A Review of Hydrogen Bonded to Period 2 Nonmetals 4.3: Multiple Covalent Bonds A Review of the Bonding Required for Generation of Stable Elemental Forms of the Period 2 Nonmetals F2, O2, and N2 Considering the elemental forms N 2 and O 2, the solution to the noble gas electronic configuration problem is one which requires the formation of more than 1 bond between the atoms these are referred to as multiple bonds. When there are 2 bonds between atoms we refer to this as a double bond, when there are 3 bonds we call it a triple bond 4.4: Coordinate Covalent Bonds Let s consider the generation of ammonium hydroxide formed by bubbling ammonia into water: We may interpret the resulting charge in 2 ways: 1. If we started with 2 neutral molecules and ammonia took the H + leaving the H electron behind, it must be positive. If the oxygen in water kept the electron that had belonged to H it must be negative. We have generated charged molecules from neutral ones polyatomic ions! 2. By introducing the concept of formal charge count 1 of the 2 electrons in a covalent bond and count all non-bonded (lone pair) electrons around a given atom. Then compare it to the Lewis dot structure to determine charge 3

4 Result: A coordinate covalent bond arises when an atom that has a non-bonded pair of electrons uses them to form a covalent bond. If the bond is one that transfers a hydrogen between atoms, then an acid-base reaction has occurred, with the acid being the molecule that gave up the H +, and the base being the molecule with a nonbonded pair of electrons that took it Bear in mind that if a pair of electrons attacks an orbital already containing 2 electrons, those electrons must leave. Put another way a pair of electrons in, a pair of electrons out A pair of electrons holding 2 atoms together is a covalent bond, whether the electrons were donated one from each atom or not. As a result, a pair of electrons in, a pair of electrons out may be [usefully] viewed as a bond made, a bond broken with the transfer of the central atom from one molecule to another A coordinate covalent bond does not always give rise to polyatomic ions. Consider the reverse reaction between ammonium and hydroxide: Given the reaction between ammonia and water and ammonium and hydroxide are occurring inside the same container, we will eventually arrive at a state of equilibrium, with the more energetically favored compounds predominating (much more on this later): When considering coordinate covalent bond formation, it is important to bear in mind all bonding atoms retain a noble gas electronic configuration. The question then becomes how do we know which atoms are likely to act as acids and donate H + and which are likely to act as bases and accept H +. The key is electronegativity if you are going to place a negative charge on an atom it would make sense to place it on an atom of high electronegativity. 4

5 That placing a negative charge on an atom of high electronegativity is a good option and placing a positive charge on an atom of high electronegativity is a poor option may be seen by examining the tendencies of the period 2 hydrides to act as acids or bases: +H + +H + +H + +H + EN = 2.5 CH4 EN = 3.0 NH3 H2O EN = 3.5 HF EN = 4.0 -H + -H + -H + -H + Notice that the steepest drop in electronegativity values on the periodic table is from F to C To summarize Carbon always forms 4 bonds - period Nitrogen forms 3 bonds when neutral o N with 4 bonds positive (common) o N with 2 bonds negative (very uncommon) Oxygen forms 2 bonds when neutral o O with 3 bonds positive (common) The ability of a compound, let s call it AH, to protonate water defines it as an acid AH + H2O A - + H3O + o O with 1 bond negative (common) The ability of a compound, let s call it B, to remove a proton from water defines it as a base B + H2O BH + + HO - 5

6 Fluorine forms 1 bond when neutral o F with 2 bonds positive (extremely uncommon) o F with 0 bonds negative (common) : Molecular Formulas and Lewis Structures Revisited Lewis Structures, an Abbreviated How To: 1. Draw the Lewis dot structure for each element, showing the valence electrons 2. Using those atoms that can form more than 1 bond, pair unpaired electrons to form a single bonds between the atoms, bearing in mind the geometric restrictions indicated on page 8 (e.g. 3 membered rings are difficult to form) a. If the number of single electrons remaining = the number of hydrogens and halogens (which only require 1 bond to achieve a noble gas configuration) add them, forming bonds to complete the structure b. If there are more unpaired electrons then hydrogen or halogens, pair them to generate double or triple bonds until # of single electrons remaining = # of H and halogens. Finish as in (a) A couple of pointers C is generally found in the middle since it forms 4 bonds. Place H or halogens last around the outside of the backbone structure Every double bond or ring formed reduces the number of H required by 2 While non-bonded electrons may be used in coordinate covalent bond formation between existing molecules, do not use them when making the initial [electrically neutral] structures 1 1 There are occasions where an electron must be transferred from a non-bonded pair to allow all atoms to have a noble gas electronic configuration. Carbon monoxide (CO) is one such molecule, as is nitric acid (HNO 3). Try it if you dare, noting N is the central atom in nitric acid 6

7 Draw the Lewis structures for the general anesthetic agent halothane, C2HBrClF3, noting that all F are on the same C Draw the Lewis structure for methanol, CH4O Draw the Lewis Structure for formaldehyde, CH2O Draw the Lewis Structure for formic acid, CH2O2 Given the possibility of isomers, drawing the correct structural formulas for butyric acid would be difficult based on its molecular formula, C4H8O2. The task becomes easier with the condensed structural formula, CH3CH2CH2CO2H 7

8 4.8: 3-D Structure Valence Shell Electron Pair Repulsion (VSEPR) is simply the idea that electrons, being of like charge, will get as far away from one another as possible, even when bonded in a molecule INCLUDING PAIRS OF NONBONDED ELECTRONS, the consequence is All single bonds tetrahedral, with ca. 109 o bond angles about the central atom o Example: ethane 1 multiple bond trigonal planer, with 120 o bond angles about the central atom o Example: ethene (ethylene) 2 multiple bonds linear, with 180 o bond angles about the central atom o Example: ethyne (acetylene) Notice that the fundamental geometry does not change between methane, ammonia, and water the only reason we give a different name to the geometries is when the non-bonding electron pairs are not considered Molecule e - pairs not considered e - pairs considered CH4 tetrahedral tetrahedral NH3 pyramidal tetrahedral H2O angular tetrahedral 2 Isomers exist for the molecular formula C3H6 what are they? Of the 2 isomers which is least stable? 8

9 On depicting the 3-D structure of molecules The tetrahedral structure gives rise to 3 dimensional possibilities. To include these we adopt the convention that our usual line indicating a covalent bond lies in the plane of the paper, a solid wedge indicates the bond is moving towards the viewer from the plane of the paper, and a dashed wedge is receding away from the viewer Problem: Draw the 3-D structure for CH3CHClOH 4.9, 4.10: Electronegativity and Polarity in Bonding: As a loose rule of thumb, if the electronegativity difference is > 1.9 the bond formed is ionic. If we are dealing with molecular compounds, the question then becomes do I have a polar covalent bond or a nonpolar covalent bond? To answer this question, we must consider the electronegativity difference (EN) between the atoms participating in the bond If EN 0.5 the bond is polar 2 Chemical bonding is a continuum between an ionic bond and a nonpolar covalent bond 2 Such an abrupt distinction between a polar bond and non-polar bond is an approximation for the purposes of this course 9

10 If the electronegativity difference is 0, a truly nonpolar covalent bond is formed; diatomic molecules such as N2, O2, & Cl2 are examples of this, but the hydrocarbons are the standard for non-polar molecules. The lack of polarity is reflected in their boiling points Molecule MW EN BP ( o C) N O Cl Molecule MW EN BP ( o C) CH C2H C3H C4H C5H As electronegativity differences increase between bonding atoms, polarity increases in the resulting bond (and molecule overall if there is no offsetting symmetry). Let s look again at H bonded to C, N, and O: Molecule MW EN BP ( o C) CH NH H2O

11 Showing polarity in covalent structures: (1) H2O via (2) H2O via l We must be careful to distinguish bond polarity from overall molecular polarity, since there are occasions where the pull for electrons in one direction is offset by equal pull in exactly the opposite direction. CO2 is one such molecule: based on the tables above, we might expect CO2 (EN = 1.0, MW = 44) to be liquid at room temperature. Not only is CO2 not liquid, but it has a boiling point = - 78 o C! Even though molecular polarity can be important, in the vast majority of cases where small molecules are concerned if there is a single polar bond the molecule will be polar Even though CO2 has no net dipole 3, there are consequences due to bond polarity CO2 dissolves in water, reacting to form carbonic acid (H2CO3). There is great concern amongst environmental chemists that the increasing levels of CO2 in the atmosphere will lead to an acidification of the oceans Normally, one would not expect a non-polar molecule to interact with a polar molecule like CO2 and H2O do, but CO2 contains polar bonds that are offset by its symmetrical shape Clearly, it is important to able to identify polar bonds within a molecule that s where the action is! Start by looking for high electronegativity atoms 3 CO 2 will not orient in any particular fashion in an electric field 11

12 Aside from the boiling point behavior, a major result of polarity considerations is simply oil and water don t mix. Think of it as non-polar molecules lacking the cache to get into Club Agua: Oil and oil do mix (non-polar mixed with non-polar), just as water and water-like (polar mixed with polar) also mix. This leads to one of the most useful rules of thumb in chemistry Like Dissolves Like Question: Methanol (CH3OH) and ethanol (CH3CH2OH) are freely soluble in water, but octanol (CH3(CH2)6CH2OH) is not. What limits the solubility of octanol? Question: Given water beads up on freshly waxed vehicle, what can be said about the chemical bonding in wax? Question: Petrolatum (petroleum jelly) may be used to remove very nonpolar road tar from your vehicle. What might be a drawback to this approach? 12

13 4.11: Naming Binary Molecular Compounds The formal system for binary molecular compounds Notice however, that common names that have replaced formal nomenclature for most common molecules Gaseous vs. aqueous mineral acids If we only assign different names to indicate different compounds, something must change when a hydrogen halide comes in contact with water Questions: What is the structural formula for HCl in the gas state? If hydrogen must form covalent bonds, what is the structural formula for aqueous HCl? 13

14 Notice this is again an example of an acid-base reaction with HCl completely donating the H + to water, owing to the stability/electron greediness ( selfish Hobbitses ) of chloride. Since HCl completely donates H + to water it is a strong acid 14