Presentation on theme: "Ionic Bonding – the Born Haber Cycle Insight into the stability of ionic compounds can be obtained if we imagine breaking a reaction forming a binary ionic."— Presentation transcript:
Ionic Bonding – the Born Haber Cycle Insight into the stability of ionic compounds can be obtained if we imagine breaking a reaction forming a binary ionic compound (from a metal and a nonmetal) into several steps. We’ll look at this for the formation of NaCl(s). In the next slide we will identify ΔH’s for familiar processes and introduce a new ΔH – the enthalpy of crystallization (lattice energy).
Born Haber Cycle - Comments We consider a binary ionic substance being formed from its constituent elements in their standard states. Along the way we first form neutral gaseous atoms of each element (a metal and a nonmetal) in the gas phase. We next form a metal ion (Na + (g), Mg 2+ (g)……) and a non-metal ion (Cl - (g), O 2- (g)…..). Finally we combine the two metal ions to form an ionic crystal.
Born Haber Cycle For the case of NaCl(s) formation you should be able to identify the signs of ∆H 1, ∆H 2, ∆H 3 and ∆H 5. (∆H 4 is “trickier”?). You also should be able to see what physical or chemical process is occurring at each step. If ∆H 5 were not a highly exothermic step would ionic compounds be as stable?
Born Haber Cycle for NaCl(s) Step or ∆H Value Description of Physical/Chemical Change ∆H 1 Enthalpy of sublimation of Na(s) ∆H 2 ½ x (Bond energy of Cl 2 ) ∆H 3 1 st ionization energy of Na(g) ∆H 4 Electron affinity of Cl(g) ∆H 5 Lattice energy of NaCl(s)
Class Examples 1. How would the Born Haber cycle for the formation of NaBr(s) differ from the Born Haber cycle already considered for NaCl(s) formation? 2. How would the Born Haber cycle for the formation of MgO(s) and MgCl 2 (s) differ from the Born Haber cycle already considered for NaCl(s) formation?
Physical Properties of Mixtures At a specified T and P a pure substance has well-defined (unique) values for a range of physical properties. These include density, colour, electrical conductivity, vapor pressure and so on. For example, at -5.0 O C ice (H 2 O(s)) has a vapor pressure of 0.402 kPa and a density of 0.917 g∙cm -3. (As the ice is cooled below this T the vapor pressure drops quickly).
Physical Properties/Mixtures – cont’d : Changing the chemical composition of a mixture will affect physical properties. Many food items and biologically important fluids are mixtures. In St. John’s the city council is planning to make “mixtures” this winter by adding rock salt, NaCl(s), to ice. The objective here will be to melt ice - lower the melting point of ice.
Class Examples: 1. A popular consumer product is 5.21% ethanol (C 2 H 5 OH) by volume. Assuming that the remaining 94.8% by volume of this product is water (and that ethanol has a density of 0.789 g/mL) calculate: (a) the % by mass of ethanol in this solution. (b) the molar concentration of ethanol in this solution. (c) the molality of ethanol in this solution.
Molarity and Molality For dilute aqueous solutions the molality and molality of a solution are usually very similar. Why is this the case?
Class Examples 2. A solution is prepared by dissolving 44.6g of Cu(NO 3 ) 2. 6H 2 O(s) in enough water to make 825 mL of solution. What is the molar concentration of Cu 2+ (aq) ions and NO 3 - (aq) ions in this solution? 3. 2.25 L of 0.400 mol. L -1 Al(NO 3 ) 3 (aq) and 2.00L of 0.350 mol. L -1 Ba(NO 3 ) 2 (aq) are mixed. What is the molar concentration of nitrate ions in the resulting solution?
Physical Properties – Concentrations: : The most useful concentration units for physical properties studies show the relative numbers of molecules (or ions) of each substance. The relative number of molecules (of each substance) is the same as the relative number of moles (of each substance). Often we employ mole fractions – especially for vapor pressure calculations.
Molarity and Molality Molarity (mol∙L -1 ), does not indicate the relative amounts of solute(s) and solvent. The next slide helps demonstrate why. An alternate concentration unit, molality, does give an indication of the relative amounts of solute(s) and solvent. We can convert from molarity to molality given the solution density.
Similar Intermolecular Forces Molecules with similar structures often have intermolecular forces of the same type and of similar strength. The next slide shows the structures of benzene and the slightly more complex toluene molecule. What intermolecular forces are important for these two molecules?