Slide 1 / 48 Slide 2 / 48 New Jersey Center for Teaching and Learning Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written Solutions: permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to Formation and Properties make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org www.njctl.org Slide 3 / 48 Slide 4 / 48 Solution Formation Heats of Solution Solute formation requires the weakening of the Coulombic The heat of solution will vary depending on the affinity of the attractions within the solvent and solute so new Coulombic solute for the solvent. attractions may form between the solvent and solute. Example: Formation of an aqueous glucose solution. Ethanol (CH 3 CH 2 OH) has very different heats of solution when dissolved in water (H 2 O) and hexane (C 6 H 14 ). C 6 H 12 O 6 (s) --> C 6 H 12 O 6 (aq) Ethanol dissolved in water ( H = -10.7 kJ/mol) Enthalpy Step What happens? Change Intermolecular forces between glucose 1 + (solute) molecules must weaken. The hydrogen bonds between the solute and solvent Intermolecular forces between solvent 2 + (water) molecules must weaken. release large amounts of energy when formed. New coulombic attractions will form 3 - Ethanol dissolved in hexane ( H = +23 kJ/mol) between the solute and solvent Since ethanol is polar and hexane is non-polar, very few The net change in enthalpy for the solution formation coulombic attractions form to offset the energy required to process is called the heat of solution and is specific to a particular solute-solvent combination. weaken the solute-solute and solvent-solvent attractions. Slide 5 / 48 Slide 6 / 48 Heats of Solution Heats of Solution Ideal solutions are solutions in which when the solutes are The heat of solution can be calculated by monitoring the mixed the heat of solution would be equal to zero. temperature change when the solute and solvent are mixed. Example: When 5.3 grams of NH 4 Cl are dissolved in 100 grams Solutions behave most ideally when the solute and solvent of water @22.0 C, the temperature of the solution drops to 18.7 are extremely similar in molecular structure and polarity. C. Assuming the specific heat of the solution is 4.2 J/gC, what is Examples of nearly ideal solutions the heat of solution? CH 3 OH and CH 3 CH 2 OH Energy lost by solution = 105.3 g x 3.3 C x 4.2 J = 1460 J C 6 H 14 and C 7 H 14 g C C 6 H 6 and C 7 H 8 Expressed in kJ/mol = 1.460 kJ /0.1 mol NH 4 Cl = 14.6 kJ/mol The heat of solution for these solutions is near zero because the the Coulombic attractions between the solute molecules and This process is endothermic and therefore will likely solvent molecules are almost identical to those that would form become more favorable as the temperature increases. between solute and solvent.
Slide 7 / 48 Slide 8 / 48 Solution Formation Electrolytes Soluble ionic compounds and strong acids make excellent Ionic solutes dissociate into ions in aqueous solvent while electrolytes. Covalent molecular materials make poor covalent molecular solutes do not. electrolytes as they do not dissociate into ions. Dissolution of NaCl in H 2 O The more ions that are produced in solution, the stronger the electrolyte. NaCl(s) --> Na + (aq) + Cl - (aq) Each ion becomes solvated by water molecules Comparing equimolar NaCl(aq) and MgCl 2 (aq) NaCl(s) --> Na + (aq) + Cl - (aq) MgCl 2 (s) --> Mg 2+ (aq) + 2Cl - (aq) Dissolution of glucose (C 6 H 12 O 6 ) in H 2 O C 6 H 12 O 6 (s) --> C 6 H 12 O 6 (aq) Which compound is the stronger electrolyte? The entire glucose molecule becomes solvated by water molecules Ionic solutes are called electrolytes . Why? MgCl 2 produces 1 ion of Mg 2+ and 2 ions of Cl - Move to see answer Since ionic solutes produce ions in solution resulting in increased when it dissociates, so it is the stronger electrolyte. electrical conductivity, there are referred to as electrolytes Slide 9 / 48 Slide 10 / 48 Electrolytes 1 Which of the following is NOT true regarding the formation of an aqueous glucose solution? Soluble ionic compounds and strong acids make excellent electrolytes. Covalent molecular materials make poor electrolytes as they do not dissociate into ions. A Covalent bonds within the glucose molecule must be broken The more ions that are produced in solution, the stronger the electrolyte. B Intermolecular coulombic forces will form between glucose molecules and water molecules Comparing equimolar HF(aq) and HBr(aq) HF(aq) --> H + (aq) + F - (aq) HBr(aq) --> H + (aq) + Br - (aq) C The hydrogen bonding network between water Answer molecules must be disrupted Which compound is a stronger electrolyte? D The glucose molecule remains un-ionized Since HBr is a strong acid, it produces many more ions E All of these are true Move to see answer compared to HF, a weak acid in which very few of the HF molecules have ionized. Slide 11 / 48 Slide 12 / 48 2 What is the heat of solution in (kJ/mol) of KCl if when 3 How much would the temperature of a solution 14.8 grams of KCl was dissolved in 400 grams of prepared by dissolving 10.6 grams of LiNO 2 in 300 water, the temperature dropped 4.3 C? Assume a grams of water increase? Assume a specific heat of specific heat of solution of 4.2 J/gC. solution of 4.2 J/gC and a heat of solution of LiNO 2 of -11.0 kJ/mol. Answer Answer
Slide 13 / 48 Slide 14 / 48 4 Which of the following would be the strongest 5 Which of the following correctly ranks the solutions electrolyte when dissolved in water? from highest to lowest conductivity? A 0.1 M NaF > 0.1 M CH 3 OH > pure water A HCN B 0.2 M AlCl 3 > 0.2 M NaF > pure water B CH 3 OH C pure water > 0.1 M NaF > 0.1 M CH 3 OH C C 6 H 12 O 6 Answer Answer D 0.1 M CH 3 OH > 0.1 M AlCl 3 > 0.1 M NaF D H 2 SO 4 E None of these E HC 2 H 3 O 2 Slide 15 / 48 Slide 16 / 48 Colligative Properties 6 Which of the following pairs of liquids would form the most IDEAL solution? Colligative properties of solutions depend exclusively on the A C 6 H 14 (l) and H 2 O(l) number of solute particles in the solution, not on their kind. Examples of colligative properties B CH 3 OH(l) and C 6 H 14 (l) Vapor pressure lowering C CH 3 OH(l) and CH 3 COCH 3 (l) Boiling point elevation D C 5 H 12 (l) and C 6 H 14 (l) Answer Freezing point depression E None of these Osmotic pressure elevation In essence, the addition of solute to any solvent will decrease the vapor pressure and hence raise the boiling point. The solution will freeze at a lower temperature and require more pressure to prevent osmosis into the solution. Slide 17 / 48 Slide 18 / 48 Vapor Pressure Vapor Pressure The vapor pressure is influenced by the strength of the solvent's When a liquid or solid evaporates, the vapor above the liquid particle interactions. exerts pressure on the surface of the liquid. The stronger the particle interactions, the lower the vapor pressure of a pure liquid at a given temperature. When condensation and evaporation occur at equal rates, the vapor is in equilibrium with its liquid. H 2 O CH 3 COCH 3 Vapor VP = 55.3 mm Hg @ 40 C VP = 400 mm Hg @ 40 C Liquid H-Bonds no H-bonds less evaporation more evaporation
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