Liquids and Solids: The Molecular Kinetic Theory II. Unit 5

Transcription

1 Liquids and Solids: The Molecular Kinetic Theory II Unit 5

2 Energy Definition Energy is the ability to do work. The ability to make something happen. Different Kinds of Energy: Heat (Thermal) Energy energy transferred from objects at a high temperature to objects at a lower temperature Kinetic Energy energy of motion related to an object s mass and velocity Potential Energy energy by virtue of position

3 Conservation of Energy Energy is never lost or gained, it is only transferred from one object to another. e.g.... energy lost by metal is gained by water.

4 Temperature Vs. Heat Energy Temperature Units are o C or K. Directly proportional to the average velocity of particles in the object. Measured directly by a thermometer Heat Energy Units are calories or Joules (1.00 cal = 4.18 J). Related to energy flow from differences in temperature. Must be calculated. Heat and temperature are related, but are not the same thing!!

5 Calculating Thermal (Heat) Energy The heat energy added or removed from an object depends on three properties of the object: (1) specific heat of the object (2) mass of the object (3) temperature change of the object Heat Added = Specific Heat x Mass x Temperature or Removed Change Joules Joules/ (gram x o C ) grams o C

6 Specific Heat The Specific Heat of a substance is the amount of energy needed to raise 1 gram of the substance 1 o C. Units for Specific Heat are: Joules grams x Specific Heat is a characteristic property that varies from substance to substance. Substance Sp Ht cal / (gx o C) Sp Ht J / (gx o C) Aluminum Copper o C Gold Water

7 Specific Heat Calculations How much energy (in calories and Joules) does it take to heat 25 g of aluminum from 25 o C to 90 o C? 1) Heat = Sp Ht (See previous slide.) x Mass x Temperature 2) Heat = cal / (g x o C) x 25 g x (90 o C 25 o C) 3) Heat = 349 calories 349 calories 4.18 J 4) Heat in Joules = x 1460 Joules 1 1calorie

8 Potential Energy Potential Energy energy that results from an object s position

9 Kinetic Energy Kinetic Energy is the energy of motion. KE is directly proportional to the mass of the object and the square of its speed.

10 Warming and Cooling Curves Description of Warming Curve: A-solid warming B-solid changing to liquid (melting) C-liquid warming D-liquid changing to gas (boiling) E-gas warming

11 Warming and Cooling Curves As a solid: 1. The particles are touching. 2. The particles are held in place. 3. The particles have little kinetic energy.

12 Warming and Cooling Curves As a liquid: 1. The particles are always touching. 2. The particles have more kinetic energy than solids. 3. The particles have limited freedom of motion.

13 Warming and Cooling Curves As a gas: 1. The particles touch only during collisions. 2. The particles have a great deal of kinetic energy and freedom of motion.

14 Warming and Cooling Curves Phase changes: 1. In parts B and D, the temperature doesn t change... the Kinetic Energy is constant. 2. In parts B and D the particles move farther apart... the Potential Energy is increasing.

15 Warming and Cooling Curves Description of Cooling Curve: A. gas cooling B. Gas changing to liquid (condensing) C. Liquid cooling D. Liquid changing to solid (freezing) E. Solid cooling

16 Heat of Fusion ( H fus ) the amount of energy needed to melt one mole of solid at its melting temperature the amount of energy released as one mole of liquid freezes at its melting temperature or Substance H fus Kcal/mol Substance H fus Kcal/mol Neon Sodium 0.63 Chlorine 1.53 Sodium Chloride 6.8 Water 1.44 Copper 3.11

17 Heat of Fusion Calculations Given the H fus of water is 1.44 kcal/mol, how much energy would it take to melt 10 g of ice at 0 o C? 1. Change grams to moles: 10g H 1 O 1mole H2O x 18 g H O mol H 2 O 2. Change moles to energy: 0.56 mol kcal x 1mol 0.80 Kcal 3. Change to KJ (optional): 0.80 Kcal 1 x 4.18 KJ 1Kcal 3.3 KJ

18 Heat of Vaporization ( H vap ) the amount of energy needed to vaporize one mole of liquid at its boiling temperature the amount of energy released as one mole of liquid condenses at its boiling temperature or Substance H vap Kcal/mol Substance H vap Kcal/mol Neon Sodium 24.1 Chlorine 4.88 Sodium Chloride 40.8 Water 9.7 Copper 72.8

19 Heat of Vaporization Calculations Given the H vap of water is 9.7 kcal/mol, how much energy would it take to boil 15 g of water at 100 o C? 1. Change grams to moles 15g H 1 O 1mole H2O x 18 g H O mol H 2 O 2. Change moles to energy 0.83 mol kcal x 1mol 8.1Kcal 3. Change Kcal to KJ (optional) 8.1Kcal 1 x 4.18 KJ 1Kcal 34 KJ

20 Vapor Pressure Vapor Pressure the pressure exerted by fastmoving particles in a liquid as they escape to form a gas.

21 Vapor Pressure increases with temperature Vapor Pressure and Substances with high intermolecular attractions have lower vapor pressures at a given temperature. Temperature

22 Normal Boiling Point The normal boiling point is defined as the temperature where the vapor pressure is equal to 1 atm or 760 mm Hg or Kpa. B.P. Chloroform = 60 o C B.P. Ethanol = 80 o C B.P. Water = 100 o C B.P. Ethanoic Acid =?

23 Abnormal Boiling Points You can trick a liquid into boiling at a higher temperature by increasing atmospheric pressure. This is the principal behind a pressure cooker.

24 Abnormal Boiling Points Freeze drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to gas. Freeze Drying