Reaction Quotient, Q, Spontaneity, and Equilibrium Lecture

TP Sulfide ion, S2‐, is slightly larger than chloride ion, Cl‐, since they have the same number of electrons but sulfur has one less proton. Which has a larger lattice enthalpy? 33% 33% 33% 1. MgS NaCl 2. NaCl MgS 3. Too close to know without additional information. • Complete: Predicting relative values of ΔlattH and ΔaqH • Review: Colligative properties • Reaction quotient, Q , spontaneity, and equilibrium 1 TP Sulfide ion, S2‐, is slightly larger than chloride ion, Cl‐, since they have the same number of electrons but sulfur has one less proton. See https://en.wikipedia.org/wiki/Ionic_radius Which has a larger lattice enthalpy? Predicting relative values of ΔlattH and ΔaqH 33% 33% 33% 6 1. MgS NaCl 2. NaCl MgS 3. Too close to know without additional information. 7 1 Lattice enthalpy, ΔlattH Lattice enthalpy, ΔlattH Key idea: Electrical attraction Coulomb’s law between oppositely charged ions in lattice Key idea: Electrical attraction Coulomb’s law between oppositely charged ions in lattice Energy  Energy  The larger charges the greater lattice enthalpy MgS CaCO3 The smaller ion size, the smaller the separation and so … the greater lattice enthalpy NaCl NaF NaCl LiCl NaCl KNO3 8 TP Which of the following has the largest magnitude lattice enthalpy, |ΔlattH |? 33% 33% 33% 9 Enthalpy of aquation, ΔaqH Make a sketch of liquid water at the scale of individual molecules. Represent the molecules as chevrons /\ in various orientations. 1. MgCO3 2. MgS 3. Further information required 10 11 2 Enthalpy of aquation, ΔaqH Enthalpy of aquation, ΔaqH Make a sketch of what a 1 M aqueous solution of MgS s looks like on the scale of your sketch of liquid water. Key idea: Electrical attraction Coulomb’s law between ions and polar water molecules Energy  Represent water molecules as chevrons /\ in various orientations. The smaller the ion size, … the smaller the distance to water and so … the greater enthalpy of aquation Cl– Br– I– Li K NH4 12 13 Enthalpy of aquation, ΔaqH Enthalpy change of solution, ΔsolH Key idea: Electrical attraction Coulomb’s law between ions and polar water molecules When KF s is dissolved in water in a beaker, the beaker becomes warm. What happens when KCl s is dissolved in water? Energy  KF The larger the ion charge … S2– Aquation Solution +821 ‒837 ‒16 (warmer) KCl the greater enthalpy of aquation PO43– Lattice Cl– 14 15 3 Enthalpy change of solution, ΔsolH Enthalpy change of solution, ΔsolH When KF s is dissolved in water in a beaker, the beaker becomes warm. What happens when KCl s is dissolved in water? When KF s is dissolved in water in a beaker, the beaker becomes warm. What happens when KCl s is dissolved in water? Lattice Aquation Solution KF +821 ‒837 ‒16 (warmer) KCl +703 Why is the lattice enthalpy of KCl smaller? Lattice Aquation Solution KF +821 ‒837 ‒16 (warmer) KCl +703 ‒700 +3 (colder) Why is the enthalpy of aquation of KCl smaller? 16 17 Enthalpy change of solution, ΔsolH TP Based on Coulomb’s law, which of the following has the largest magnitude enthalpy of aquation, |ΔaqH |? When KF s is dissolved in water in a beaker, the beaker becomes warm. What happens when KCl s is dissolved in water? 25% 25% 25% 25% Lattice Aquation Solution KF +821 ‒837 ‒16 (warmer) KCl +703 ‒700 +3 (colder) 1. 2. 3. 4. LiCl NaCl KCl Further information required Could we have predicted that when KCl s dissolves the solution get colder? 18 19 4 TP Based on Coulomb’s law, which of the following has the largest magnitude enthalpy of aquation, |ΔaqH |? Quiz Based on Coulomb’s law, which of the following has the largest magnitude enthalpy of change of solution, |ΔsolH |? 33% 33% 33% 33% 33% 33% 1. MgS 2. MgCO3 3. Further information required 1. MgCO3 2. MgS 3. More information needed 20 21 Colligative properties review Non‐volatile solute negligible vapor pressure … • lowers vapor pressure of solvent • raises boiling point of solvent • lowers freezing point of solvent If solute cannot pass through a membrane ... • the solvent will create an osmotic pressure Dynamic chemical equilibrium Details and practice in lab and in discussion 25 49 5 Spontaneous approach to equilibrium: A  B + C Spontaneity of “reactants”  “products” If products right side increase with time, we say … the reaction is spontaneous. If reactants left side increase with time, we say … the reaction is nonspontaneous. If the amount of reactants and products do not change with time, we say … the reaction is at equilibrium. 50 51 Reaction quotient Q measures progress AB C Q products/reactants B C/A Q at t1 is smaller Q at t2 is larger Q thereafter no longer changes For t t2, Q K, equilibrium constant 52 6