Transcript Chapter 9
Chapter 9: Basic Concepts of Chemical Bonding NaCl versus C12H22O11. Types of Bonds We can classify bonds based on the kinds of atoms that are bonded together. Types of Atoms metals to nonmetals nonmetals to nonmetals metals to metals Tro: Chemistry: A Molecular Approach, 2/e Type of Bond Ionic Covalent Metallic Bond Characteristic electrons transferred electrons shared electrons pooled 2 Types of Bonding 3 Tro: Chemistry: A Molecular Approach, 2/e Lewis Dot Symbols Combines the element symbol plus the valence electrons as dots placed around symbol. Dots are first placed on each of the four sides (N-S-EW), then paired up after that. Symbols for period 2. Octet Rule Atoms tend to gain, lose, or share electrons until they are surrounded by eight electrons. Ionic = gain or lose Covalent = share While there are exceptions to this rule, it is the important driving force for the formation of compounds. Ionic Bonding When Na(s) and Cl2(g) are combined, a very violent, exothermic reaction results. LEP #1 Ionic Bonding Ionic Bonding The energetics of ionic bond formation can be explained by a series of steps. Step 1: Loss of electron by Na Na(s) Na(g) ; Na(g) Na+(g) + 1e- ; DH = +108 kJ DH = +496 kJ Step 2: Gain of electron by Cl ½ Cl2(g) Cl(g) ; Cl(g) + 1e- Cl-(g) ; DH = +122 kJ DH = -349 kJ Ionic Bonding Lattice energy is the energy required to completely separate a mole of a solid ionic compound into gaseous ions. NaCl(s) Na+(g) + Cl-(g) ; DH = 788 kJ Reverse this AND add it to the four previous reactions yields: Na(s) + ½ Cl2(g) NaCl(s) ; DH = -411 kJ Lattice Energy The electrostatic attraction of two charged particles is ruled by the equation: E Q1Q 2 d Q1, Q2 are the magnitudes of the charges and d is the distance between the two nuclei. Lattice Energy As the magnitude of the charge increases, the lattice energy will increase. Na+1 – Cl-1 U = +788 kJ Sr+2 – Cl-1 U = +2127 kJ Sr+2 – O-2 U = +3217 kJ Lattice Energy As the distance between the ions increases, the lattice energy decreases. Summary of Lattice Energies The force of attraction between oppositely charged particles is directly proportional to the product of the charges Larger charge means the ions are more strongly attracted larger charge = stronger attraction Lattice Energy = −910 kJ/mol stronger attraction = larger lattice energy Of the two factors, ion charge is generally more important LEP #2 13 Tro: Chemistry: A Molecular Approach, 2/e Lattice Energy = −3414 kJ/mol Covalent Bonding When two electrons are shared by two atoms, this is known as a covalent bond. Formation of H2 Covalent Bonding Formation of Cl2 – each Cl atom has seven valence electrons. Each has one unpaired electron that can pair up to make a bond. Lewis Structures 1. 2. For molecules or ions containing three or more atoms, we can follow a set of rules to guide in the process. Add up the total valence electrons from all the atoms in the compound. Ex) CF4 Make a skeleton structure – the first element in the formula is usually the central atom – all others are then placed around this atom and connected with a bond. Lewis Structures Fill the external atoms until they have an octet. 4. Compare total number of electrons used to step 1. If all are used, then go to step 5. If some are left, place on central atom as lone pair(s). 5. Check the central atom for an octet. If no octet, may need multiple bond(s) by moving nonbonding pair(s). 3. Lewis Structures General Guidelines Group 7A as an external atom will NOT do multiple bonds. Carbon will almost always have four bonds. Group 2A, 3A as a central atom may be deficient of octet. Group 5A, 6A, 7A, and Xe as a central atom may exceed the octet. Oxygen will do up to two bonds. Electronegativity Non-polar covalent bond – the electrons are shared equally. Ex) F2 , Br2 , I2 , O2 , etc. Polar covalent bond – electrons are not shared equally – one atom has a greater desire for the electron pair. Electronegativity Scale Electronegativity Bond Polarity If the electronegativity difference is zero, then the bond is non-polar covalent. If there is a difference AND the two elements are non-metals, then the bond is polar covalent. If there is a difference AND one element is a metal and the other is a non-metal, then the bond is ionic. Bond Polarity Simple molecules like HCl have a polar covalent bond. The more electronegative element will have a partial negative charge and the less electronegative element will have a partial positive charge. Dipole Moment This is the quantitative measurement of the polar bond. m=Qxd m is measured in a unit called the Debye or Coulomb x meter. LEP #4 Resonance Structures In some cases, a molecule or ion may be described by more than one Lewis Structure. Ex) O3 LEP #5 Bond Strength The energy required to break a covalent bond is its strength. Some are fairly simple. Cl2(g) 2 Cl(g) ; DH = 242 kJ Others are more complicated. CH4(g) C(g) + 4 H(g) ; DH = 1660 kJ Note: Energy is ALWAYS required to break a bond. Using Bond Enthalpies One method for estimating the enthalpy of a reaction is: DH = S(Bonds Broken) – S(Bonds Made) LEP #6 Chemistry of Explosives Many explosives are solids or liquids that contain the nitro (NO2) or nitrate (NO3) group. The nitrogen atoms are typically weak (singly) bonded to carbon (293 kJ). They then become N-N strong triple bonds (941 kJ). Chemistry of Explosives Bond Order Bond order is the number of bonds connecting two atoms. Can be 1, 2, or 3. C2H2 Bond Length As bond order increases, the bond lengths decrease. Ex) N-N N=N 1.47Å 1.24Å Ex) C-C C=C 1.54Å 1.34Å NN 1.10Å CC 1.20Å Resonance Structures The bond orders and lengths for resonance structures must be averaged. Ex) SO2 , NO3-1 , C6H6 Formal Charges Formal Charge is a fictitious charge assigned to each atom in a Lewis Structure. It helps to evaluate the validity of competing structures. Formal Charge = #Valence Electrons – #Nonbonding electrons – ½ #Bonding Electrons Formal Charges General Rules: The sum of all formal charges in a neutral molecule will equal zero. 2. The sum of all formal charges in an ion must equal the charge of the ion. 3. Small (+1 or -1 or 0) formal charges are preferred over larger ones. 4. When formal charges cannot be avoided, a negative formal charge will preferentially go on the more electronegative atom. 1. Formal Charges HCN molecule = 10 electrons. Is skeleton structure H – C – N or H – N – C? Which Lewis Structure for the cyanate (OCN-) ion is best?