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KU CHEM 170 - Measuring the Molecular Weight of Air

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Chem175 1st Edition Lecture 8 Outline of Last Lecture I. Intermolecular Forces and PhasesII. Types of Intermolecular ForcesIII. Intermolecular Forces Affect the Behavior of SubstancesOutline of Current Lecture IV. Upcoming Lab: Measuring the Molecular Weight of AirV. Last Lecture: VocabularyVI. Vapor Pressure of LiquidVII. Clausius Clapeyron EquationVIII. Boiling PointCurrent LectureUpcoming Lab: Measuring the Molecular Weight of AirThere’ll be two balloons filled with gas. One will be filled with helium and the other will be filled with an unknown gas. The goal of the lab will be to identify the unknown gas, bymeasuring the pressure, volume and temperature to calculate the number of moles. Then the mass of the balloons will be measured to find the molecular weight of the unknown gas. Ch. 12 – Liquids and SolidsLast Lecture: Intermolecular forces are the forces between molecules and atoms. The magnitude of intermolecular forces determine whether a substance is a gas, liquid or a solid at a certain temperature. Intermolecular forces come from columbic interactions, the attractive force between positive and negative charges.-Cohesion refers to the forces that hold like molecules together.-Adhesion refers to the forces attract unlike molecules to each other.-Surface tension is the measure of tightness of surface film.-The caterpillar effect occurs when water travels up a tube, because the water molecules are attracted to the molecules of the tube (adhesion). -Meniscus is the curved surface of a liquid.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Vapor Pressure of LiquidWater can evaporate. This is a phase change from a liquid to a gas.H2O(l )→ H2O(g)This rate of this reaction is directly related to temperature. As the temperature increases, the kinetic energy of the molecules increases. As the kinetic energy of the molecules increases the molecules can break their bonds to each faster and escape into the atmosphere.Water can also condensate. This is a phase change from a gas to a liquid. People most commonly see this happen when they walk outside on a hot and humid daywith a cold drink in their hand. On the glass you will see droplets of water appearing, because when the gaseous water molecules hit the cold glass they slow down and become liquid water molecules.H2O(g)→ H2O(l)Dynamic equilibrium occurs a reversible reaction becomes balanced. Becoming balanced, means that there is no net change and the two reactions occur at the same rate. In this case the reversible reaction is H2O(l )↔ H2O(g). If we place alid on a cup of water there will be a time when the rate at which water evaporates is equal to the rate at which water condenses. The pressure of the gasat this moment is the vapor pressure of water.Clausius Clapeyron Equationln(P)=(∆ HvapR)1T+CWhere P = pressure, ∆ Hvap = the enthalpy of vaporization, R = gas constant, T=temperature, and C is the intercept.∆ Hvap= qpThis means that the enthalpy is equal to the heat at constant pressure.∆ Hvap=−∆ HcondensationNote that vapor pressure is directly related to temperature.ln(P1)=(∆ HvapR)1T1+Cln(P2)=(∆ HvapR)1T2+Cln(P1)−ln(P2)=(∆ HvapR)1T1+C−((∆ HvapR)1T2+C)ln(P1P2)=(∆ HvapR)(1T2−1T1)Only use this equation when ∆ Hvap is constant at certain temperature ranges.ex. Water under 17.5 mmHg undergoes a temperature change of 293.2 K to 291.2 K. The ∆ Hvapis 44KJmol. What was the initial pressure?ln(P1P2)=(∆ HvapR)(1T2−1T1)ln(P117.5 mmHg)=(44KJmol8.314Jmol × K)(1293.2−1291.2)P1=15.5 mmHgBoiling Point is the temperature at which the vapor pressure is equal to the pressure of the surrounding atmosphere. The boiling point is directly related to the strength of the intermolecular forces in a liquid. Boiling occurs when the vapor pressure ofthe liquid equals the atmospheric


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