Slide 1 / 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 permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to 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 Slide 2 / 48 Solutions: Formation and Properties www.njctl.org Slide 3 / 48 Solution Formation Solute formation requires the weakening of the Coulombic attractions within the solvent and solute so new Coulombic attractions may form between the solvent and solute. Example: Formation of an aqueous glucose solution. C 6 H 12 O 6 (s) --> C 6 H 12 O 6 (aq) Enthalpy Step What happens? Change Intermolecular forces between glucose 1 + (solute) molecules must weaken. Intermolecular forces between solvent 2 + (water) molecules must weaken. New coulombic attractions will form 3 - between the solute and solvent The net change in enthalpy for the solution formation process is called the heat of solution and is specific to a particular solute-solvent combination.
Slide 4 / 48 Heats of Solution The heat of solution will vary depending on the affinity of the solute for the solvent. 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 ). Ethanol dissolved in water ( H = -10.7 kJ/mol) The hydrogen bonds between the solute and solvent release large amounts of energy when formed. Ethanol dissolved in hexane ( H = +23 kJ/mol) Since ethanol is polar and hexane is non-polar, very few coulombic attractions form to offset the energy required to weaken the solute-solute and solvent-solvent attractions. Slide 5 / 48 Heats of Solution Ideal solutions are solutions in which when the solutes are mixed the heat of solution would be equal to zero. Solutions behave most ideally when the solute and solvent are extremely similar in molecular structure and polarity. Examples of nearly ideal solutions CH 3 OH and CH 3 CH 2 OH C 6 H 14 and C 7 H 14 C 6 H 6 and C 7 H 8 The heat of solution for these solutions is near zero because the the Coulombic attractions between the solute molecules and solvent molecules are almost identical to those that would form between solute and solvent. Slide 6 / 48 Heats of Solution The heat of solution can be calculated by monitoring the temperature change when the solute and solvent are mixed. Example: When 5.3 grams of NH 4 Cl are dissolved in 100 grams of water @22.0 C, the temperature of the solution drops to 18.7 C. Assuming the specific heat of the solution is 4.2 J/gC, what is the heat of solution? Energy lost by solution = 105.3 g x 3.3 C x 4.2 J = 1460 J g C Expressed in kJ/mol = 1.460 kJ /0.1 mol NH 4 Cl = 14.6 kJ/mol This process is endothermic and therefore will likely become more favorable as the temperature increases.
Slide 7 / 48 Solution Formation Ionic solutes dissociate into ions in aqueous solvent while covalent molecular solutes do not. Dissolution of NaCl in H 2 O NaCl(s) --> Na + (aq) + Cl - (aq) Each ion becomes solvated by water molecules 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) The entire glucose molecule becomes solvated by water molecules Ionic solutes are called electrolytes . Why? Since ionic solutes produce ions in solution resulting in increased electrical conductivity, there are referred to as electrolytes Slide 8 / 48 Electrolytes Soluble ionic compounds and strong acids make excellent electrolytes. Covalent molecular materials make poor electrolytes as they do not dissociate into ions. The more ions that are produced in solution, the stronger the electrolyte. Comparing equimolar NaCl(aq) and MgCl 2 (aq) NaCl(s) --> Na + (aq) + Cl - (aq) MgCl 2 (s) --> Mg 2+ (aq) + 2Cl - (aq) Which compound is the stronger electrolyte? MgCl 2 produces 1 ion of Mg 2+ and 2 ions of Cl - Move to see answer when it dissociates, so it is the stronger electrolyte. Slide 9 / 48 Electrolytes Soluble ionic compounds and strong acids make excellent electrolytes. Covalent molecular materials make poor electrolytes as they do not dissociate into ions. The more ions that are produced in solution, the stronger the electrolyte. Comparing equimolar HF(aq) and HBr(aq) HF(aq) --> H + (aq) + F - (aq) HBr(aq) --> H + (aq) + Br - (aq) Which compound is a stronger electrolyte? Since HBr is a strong acid, it produces many more ions Move to see answer compared to HF, a weak acid in which very few of the HF molecules have ionized.
Slide 10 / 48 1 Which of the following is NOT true regarding the formation of an aqueous glucose solution? A Covalent bonds within the glucose molecule must be broken B Intermolecular coulombic forces will form between glucose molecules and water molecules C The hydrogen bonding network between water Answer molecules must be disrupted D The glucose molecule remains un-ionized E All of these are true Slide 11 / 48 2 What is the heat of solution in (kJ/mol) of KCl if when 14.8 grams of KCl was dissolved in 400 grams of water, the temperature dropped 4.3 C? Assume a specific heat of solution of 4.2 J/gC. Answer Slide 12 / 48 3 How much would the temperature of a solution prepared by dissolving 10.6 grams of LiNO 2 in 300 grams of water increase? Assume a specific heat of solution of 4.2 J/gC and a heat of solution of LiNO 2 of -11.0 kJ/mol. Answer
Slide 13 / 48 4 Which of the following would be the strongest electrolyte when dissolved in water? A HCN B CH 3 OH C C 6 H 12 O 6 Answer D H 2 SO 4 E HC 2 H 3 O 2 Slide 14 / 48 5 Which of the following correctly ranks the solutions from highest to lowest conductivity? A 0.1 M NaF > 0.1 M CH 3 OH > pure water B 0.2 M AlCl 3 > 0.2 M NaF > pure water C pure water > 0.1 M NaF > 0.1 M CH 3 OH Answer D 0.1 M CH 3 OH > 0.1 M AlCl 3 > 0.1 M NaF E None of these Slide 15 / 48 6 Which of the following pairs of liquids would form the most IDEAL solution? A C 6 H 14 (l) and H 2 O(l) B CH 3 OH(l) and C 6 H 14 (l) C CH 3 OH(l) and CH 3 COCH 3 (l) D C 5 H 12 (l) and C 6 H 14 (l) Answer E None of these
Slide 16 / 48 Colligative Properties Colligative properties of solutions depend exclusively on the number of solute particles in the solution, not on their kind. Examples of colligative properties Vapor pressure lowering Boiling point elevation Freezing point depression 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 Vapor Pressure When a liquid or solid evaporates, the vapor above the liquid exerts pressure on the surface of the liquid. When condensation and evaporation occur at equal rates, the vapor is in equilibrium with its liquid. Vapor Liquid Slide 18 / 48 Vapor Pressure The vapor pressure is influenced by the strength of the solvent's particle interactions. The stronger the particle interactions, the lower the vapor pressure of a pure liquid at a given temperature. H 2 O CH 3 COCH 3 VP = 55.3 mm Hg @ 40 C VP = 400 mm Hg @ 40 C H-Bonds no H-bonds less evaporation more evaporation
Slide 19 / 48 Vapor Pressure The vapor pressure is directly proportional to the temperature. As heat is added, more evaporation results, leading to a higher vapor pressure. VP H 2 O vs Temp Note the relationship is not linear. Slide 20 / 48 Vapor Pressure VP of Various Substances vs. Temp. Note the larger hydrocarbons have a lower vapor pressure at a given temperature due to higher LDF's. When a liquid's vapor pressure equals the atmospheric/ external pressure it will boil . The stronger the particle interactions, the more energy must be added to raise the vapor pressure hence the higher boiling points. Slide 21 / 48 Vapor Pressure To reach the boiling point, either the vapor pressure must be increased, the atmospheric pressure must be lowered, or both. The BP can be reached by raising the vapor pressure by heating Vapor Pressure Status @ 1 atm external pressure H 2 O @25 C 23.88 mm Hg not boiling! H 2 O @100 760 mm Hg boiling! C The BP can also be reached by lowering the atmospheric pressure The atmospheric pressure at the peak of Mt. Everest is just 253 mm Hg so water must be heated only to roughly 72 C to obtain a vapor pressure of 253 mm Hg and thus boil.
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