Chemical Bonds LACC Chem101

Chemical Bonds Attractive, intramolecular forces between nuclei Maintains a group of atoms together No definite time Generally stable Lowers electrical potential between nuclei Requires energy to break apart This may come from even lower potential in a new bond Three types Ionic Covalent Metallic LACC Chem101

Ionic Bonding Electron transfer Metal to nonmetal Results in two ions of opposite charges No overall charge Significant difference in electronegativity Generally higher than 1.6 Atoms arrange geometrically Crystal lattice Compound referred to as “salt” Very strong Solids at room temp Poor electrical conduction in solid state Electrons confined to nuclei LACC Chem101

Ionic Bonds Electron transfer causes opposite charges Bond is actually electrostatic force: Most commonly Main Group elements Cations I & II Anions VI & VII Large electronegativity difference allows for transfer LACC Chem101

Calculation of Attractive Energy Coulomb’s Law is simple to solve for a cation of charge +1 and anion of charge -1 (we use 3.5Å bond length here) The molar value (in kJ per mole) is: For comparison, this is approximately the ∆Hrxn for the formation of one mole of NaBr Not exact, so bond include more potentials than simple electrostatic force LACC Chem101

Lattice Energy Difference in molar enthalpy between the solid and a gas of widely separated ions Magnitude depends on charges of ionic species Low charge means low lattice energy This cannot be calculated directly Requires measurement of gaseous ions condensing into solid Using enthalpy (state function), we can construct several steps for a known equation Lattice energy is one of these steps! Born-Haber cycle Start with pure elements Atomize pure elements to form gaseous atoms Ionize the atoms Allow ions to form a solid Convert solid back to pure elements LACC Chem101

Born-Haber Cycle for NaCl Step 1: The transfer of an electron from an Na atom to a Cl atom is not in itself energetically favorable Requires +147 kJ/mol of energy Step 2a: -493 kJ of energy is released when oppositely charged ions come together to form ion pairs Step 2b: Additional energy 293 kJ is released when these ion pairs form the solid crystal The lattice energy released when 1 mol of Na+ & Cl- ions react to produce NaCl (s) is 786 kJ/mol The overall process of NaCl formation is energetically favorable, releasing 639 kJ/mol if gaseous Na & Cl atoms are initially used. Sublimation of Sodium Na(s) → Na(g) ∆H1 = 108 kJ Dissociation of Chlorine ½Cl2(g) → Cl(g) ∆H2 = 120 kJ Ionization of Sodium Na(g) → Na+(g) + e-(g) ∆H3 = 496 kJ Formation of Chloride ion Cl(g) + e-(g) → Cl-(g) ∆H4 = -349 kJ Formation of Sodium Chloride Na+(g) + Cl-(g) → NaCl(s) ∆H5 = -786 kJ Total Reaction Na(s) + ½Cl2(g) → NaCl(s) ∆Hof = LACC Chem101

Workshop on Born-Haber Process Problem #1: Devise and use a Born-Haber cycle to calculate the lattice enthalpy of potassium chloride. Problem #2: Calculate the lattice enthalpy of calcium chloride. Problem #3: Calculate the lattice enthalpy of magnesium bromide. LACC Chem101

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Ionic Radius Ionic Radius: measure of the size of the spherical region (electron probability) around the nucleus of an ion. Isoelectronic: species having the same electron configuration Periodic Trend ionic radius of cations decreases going left to right across a period until the ion becomes an anion (Group V) then there is an abrupt increase in ionic radius from Group V to Group VII the radius once again decreases. The ionic radius increases going down a group. LACC Chem101

Metallic Bonds Electron sea model Each metal atom is effectively a cation Donated electron(s) to the metal solid as a whole Electrons not localized LACC Chem101

Covalent Bonds LACC Chem101

Covalent bonding Sharing of electrons Low electronegativity difference Typically two or more nonmetals Compound is called a “molecule” Each component has a formal charge No individual charges, and no overall charge Electrons orbitals overlap into a molecular orbitals Multiple molecular orbitals may form (Ch10) Electrons localized between the two nuclei LACC Chem101

Electronegativity Tendency of an atom (or functional group) to attract electrons to itself Usually given on the Pauling scale, with a maximum value of 4.0 All elements have an electronegativity, so none are zero Metals have low electronegativities Tend to lose electrons when ionized Nonmetals have comparatively higher electronegativities Tend to gain electrons when ionized Periodic Trend is increase across a period, and decrease down a group LACC Chem101

Covalent Bonds Aka: “Chemical Bonds” Generally between nonmetals Electron pairs are shared between nuclei Valence electron orbitals overlap in a molecular orbital Highest probability of find shared electron is in between the nuclei Similar (or same) electronegativities cause electron to be shared, not transferred Bond Order: Describes number of electron pairs being shared Typically 1, but can be 2 (double bond) or 3 (triple bond) LACC Chem101

Potential Energy between Atoms Balance between attractive and repulsive forces of particles Also consider kinetic energy of electrons Zero point occurs as the distance (r) approaches infinity No attraction or repulsion As distance approaches zero, potential is infinitely high Nuclei begin to repel Bond length is where the energy is at a minimum LACC Chem101

Bond Length Distance between nuclei at minimum potential energy Bond dissociation energy required to break covalent bond Larger energies required for more stable bonds Distance decreases as more bonds made (double or triple) LACC Chem101

Bond Energy Difference between the standard molar enthalpies of a molecule X-Y and the fragments X and Y on their own This is an approximation Energy required to overcome attractive force of a bond Related to bond strength and number of bonds Bond energy allows us to conclude: Endothermic Exothermic LACC Chem101

Bond Energies & Standard Enthalpy All bond enthalpies listed in your textbook are POSITIVE because energy must be supplied to break a bond. Bond breaking is always ENDOTHERMIC Bond formations is always EXOTHERMIC. Assume that the average bond energy applies regardless of the specific molecular environment intermolecular interactions are expected to be minimal and hence are NOT taken into account These calculations are limited to cases where ALL reactants/products are in the gas phase! LACC Chem101

Bond Energy Example CH3CH2I(g) + H2O(g)  CH3CH2OH(g) + HI(g) Estimate the enthalpy change of the reaction between gaseous iodoethane and water vapor: CH3CH2I(g) + H2O(g)  CH3CH2OH(g) + HI(g) Bond Energies (kJ/mol): C-O 360 H-O 463 C-I 238 H-I 299 LACC Chem101

Bond Energy Example Estimate the standard enthalpy of the following reaction: CCl3CHCl2(g) + 2HF(g)  CCl3CHF2(g) + 2HCl(g) Bond Energies (kJ/mol): C-F 485 C-Cl 339 H-F 565 H-Cl 431 LACC Chem101

Polarity Polarity: charge distribution about a chemical bond or a whole molecule Polarity helps to determine the equal or unequal distribution of an electron between two atoms Referring to probability of electron being near one or the other nucleus Nonpolar covalent bond Electrons shared with equal probability distribution Polar covalent bond Electrons shared, but unequal probability distribution between nuclei LACC Chem101

Dipole Moment Dipole moment: electrical vector indicating direction of electron density in a molecule Since it is an electrical moment, it has the formula: SI units are C-m, but most useful for physical chemists is the Debye: Arrow pointing towards more electronegative atom Positive “tail” typically drawn in for clarity We can find ionic character by comparing this to the actual dipole moment Actual will be much lower since an electron was not transferred LACC Chem101

Ionic Character example Hydrofluoric acid has an actual dipole moment of 1.82D. Calculate its ionic character. LACC Chem101

Polarity of a Bond Nonpolar covalent bonds occur between atoms of similar (same) electronegativity Ex Cl2 SCN- Polar covalent bonds occur due to a significant difference in dipole moments of atoms Ex H2O CO2 ICl LACC Chem101

Example: Ammonia Compare electronegativities of each atom Nitrogen is highest Not lessened by presence of three hydrogens Lone pair gives overall polarity See geometry LACC Chem101

Nonpolar vs Polar vs Ionic Bonds Note the overall dipole moments on each bond LACC Chem101

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