Lecture 33 - Topics for the day Administrative stuff...

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Topics for the day Administrative stuff Properties of liquids Viscosity Surface tension Capillary action Vaporization Administrative stuff Quest Practice exam (optional, worth no points) is available now Quest HW5 up on Friday ALEKS Objective 5 due now. Objective 6 is on thermodynamics. Due in two weeks. Exam review TODAY, 4 - 5 pm, WEL 3.502 Blackboard gradebook has not been updated yet. I’ll tell you when it has. 1 2 3
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Where were we The molecular description of liquids and solids. .. iClicker Time Which of KBr or CH 3 Br is likely to have the higher normal boiling point? A. They are the same B. CH 3 Br C. KBr KBr is an ionic solid ion/ion forces CH 3 Br has a dipole dipole/dipole forces Boiling = converting liquid to gas Intermolecular forces between liquid molecules must be broken. Higher boiling point implies stronger forces ion/ion >> dipole/dipole Properties of liquids What follows is a range of different properties of liquids, all of which can be understood in terms of intermolecular forces. 4 5 6
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Viscosity is a liquid’s resistance to fow. A high viscosity liquid is hard to pour (e.g., honey, molasses) When liquids fow, the molecules have to move past each other. There are two diFFerent Factors that infuence viscosity. Stronger intermolecular forces lead to higher viscosity Pentane (C 5 H 12 ) viscosity << Dodecane (C 12 H 26 ) viscosity because the increased size leads to increased dispersion Forces. Glycerol is very viscous due to increased hydrogen bonding. Viscosity Glycerol Long molecules can get entangled resulting in higher viscosity. Multigrade automobile oil (e.g., 10W-30) has added polymers so that the oil can be used For all seasons. In the winter, the oil is viscous enough to coat and lubricate engine parts. In the summer, the long chain polymer molecules increase the viscosity oF the oil enough so that even at the highest engine temperatures reached, the oil will still lubricate eFFectively rather than run oFF and pool in the bottom oF the engine. Why does honey get runny when you heat it up? Increased temperature means increased kinetic energy means there is more energy available to overcome the intermolecular Forces. iClicker Time Predict which oF ethanol (CH 3 CH 2 OH) or dimethyl ether (CH 3 OCH 3 ) has the greater viscosity. Assume that they are both at the same temperature and in their liquid Form A. Equal viscosities B. ethanol C. dimethyl ether Hydrogen bonds > dipole-dipole Forces
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Lecture 33 - Topics for the day Administrative stuff...

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