Transcript Chapter 4
4 Properties of Matter A burning log undergoes a chemical change resulting in the release of energy in the form of heat and light Foundations of College Chemistry, 14th Ed. Morris Hein and Susan Arena Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. Chapter Outline 4.1 Properties of Substances 4.2 Physical and Chemical Changes 4.3 Learning to Solve Problems 4.4 Energy 4.5 Heat: Quantitative Measurement A. Energy in Chemical Changes B. Conservation of Energy 4.6 Energy in the Real World © 2014 John Wiley & Sons, Inc. All rights reserved. Properties of Substances Each substance has a set of properties that are characteristic and give it a unique identity. Properties are classified as either physical or chemical. Physical properties are inherent characteristics that can be determined without altering the composition. Examples include: color taste odor state of matter density melting point boiling point © 2014 John Wiley & Sons, Inc. All rights reserved. Properties of Substances Chemical properties describe the ability of a substance to either undergo a reaction with another substance or to decompose. Consider chlorine (Cl2). Physical Properties: 4 times heavier than air, a gas at room temperature, greenish-yellow in color, bad odor. Chemical Change: 2 Na (s) + Cl2 (g) © 2014 John Wiley & Sons, Inc. All rights reserved. 2 NaCl (s) Properties of Substances No two substances have identical physical and chemical properties! © 2014 John Wiley & Sons, Inc. All rights reserved. Physical and Chemical Changes Physical changes are changes in physical properties (such as size and density) or changes in states of matter without a change in composition. No new substances are formed during a physical change! Sawing wood is a physical change – the wood changes shape, but the resulting pieces are still wood! © 2014 John Wiley & Sons, Inc. All rights reserved. Physical and Chemical Changes In a chemical change, new substances are formed that have different properties and composition from the original material. When copper metal (Cu) is heated in air, the shiny metal turns black as copper(II) oxide is formed on the surface. 2 Cu (s) + O2 (g) D 2 CuO (s) D represents heat © 2014 John Wiley & Sons, Inc. All rights reserved. Common Physical and Chemical Changes Be able to distinguish between a physical and chemical change. © 2014 John Wiley & Sons, Inc. All rights reserved. Let’s Practice! a. b. c. d. e. Which of the following is a physical change? grinding a rock into powder hydrogen and oxygen reacting to form water a shovel rusting an acid and base reacting to form water burning sugar Which of the following is a chemical change? a. b. c. d. e. melting aspirin breaking a plate boiling antifreeze putting sugar in tea lighting a match © 2014 John Wiley & Sons, Inc. All rights reserved. Chemical Equations A chemical equation is used to represent chemical change. For example, water decomposes into its elements when electrolyzed. 2 H2O (l) 2 H2 (g) + O2 (g) Understanding a chemical equation: reactants the starting substances (in this case 2 H2O) products the substances formed (in this case 2 H2 + O2) means “produces”; points towards products A physical change usually accompanies a chemical change. © 2014 John Wiley & Sons, Inc. All rights reserved. Learning to Solve Problems To succeed in chemistry, you must learn to solve complicated problems. READ Read the problem carefully and determine what is known and desired. Use units! PLAN Determine the unit relationships needed to solve the problem. Set up the problem so that the units cancel correctly. CALCULATE Do the math. Make sure the answer contains the proper units and significant figures. CHECK Check the answer — is it reasonable? © 2014 John Wiley & Sons, Inc. All rights reserved. Energy Energy is the capacity of matter to do work. There are many types of energy including mechanical, chemical, electrical and nuclear energy. Potential energy (PE) is stored energy, the energy an object possesses due to its position. A ball located 20 feet above the ground has more PE than when it is located 10 feet above the ground. A diver poised on a diving board has a large amount of PE. When the diver leaves the board, the energy is converted. © 2014 John Wiley & Sons, Inc. All rights reserved. Energy Kinetic energy (KE) is energy that matter possesses due to its motion. When water held by a dam is released, its PE converts to KE which can be used to produce electricity. Energy can be converted from one form to another. In chemistry, energy is most frequently released as heat. © 2014 John Wiley & Sons, Inc. All rights reserved. Heat: Quantitative Measurement The SI unit for energy is the joule (J). The energy required to change the temperature of 1 g of water by 1 °C is 4.184 J. 4.184 J = 1 calorie Nutritional calories are actually kilocalories (kcal) and are represented with a capital C. Heat and temperature are different concepts. Imagine two beakers containing water. In each, the temperature is raised by 30 °C. The beaker containing twice the water requires twice the heat to reach the same temperature. © 2014 John Wiley & Sons, Inc. All rights reserved. Let’s Practice! Methane (CH4) is the major compound in natural gas. Using the reaction below, determine the amount of energy produced during combustion (in calories). CH4 (g) + 2 O2 (g) Knowns CO2 (g) + 2 H2O (g) + 802.5 kJ 4.184 J = 1 cal 1000 J = 1 kJ Solving for calories Calculate 1000 J x 1 cal x = 1.918 x 105 cal 802.5 kJ 4.184 J 1 kJ © 2014 John Wiley & Sons, Inc. All rights reserved. Heat: Quantitative Measurement Every substance has a characteristic heat capacity. The specific heat of a substance is the amount of heat (gained or lost) required to change the temperature of 1 g of the material by 1 °C. The specific heat of water is much higher than most substances. © 2014 John Wiley & Sons, Inc. All rights reserved. Specific Heat Capacity When an object is heated or cooled, the amount of energy transferred depends on three things: 1. The amount of material 2. The magnitude of the temperature change 3. The identity of the material gaining or losing energy The following equation can be used to calculate heat: q = mCDT q is heat m is mass C is specific heat capacity DT is change in temperature © 2014 John Wiley & Sons, Inc. All rights reserved. Specific Heat Capacity How much heat is needed to raise the temperature of 200. g of water by 10.0 °C? q = mCDT Knowns m = 200. g C = 4.184 J/g°C DT = 10.0 °C Solving For q q = mCDT Calculate q = (200. g)(4.184 J/g°C)(10.0 °C) = 8.37 x 103 J © 2014 John Wiley & Sons, Inc. All rights reserved. Specific Heat Capacity Calculate the specific heat (J/g°C) of an unknown if 1638 J raises the temperature of 125 g from 25.0 °C to 52.6 °C. q = mCDT Knowns q = 1638 J m = 125 g DT = 52.6 °C – 25.0 °C = 27.6 °C q Solving For C C = mDT Calculate C = 1638 J = 0.475 J/g°C (125 g)(27.6 °C) © 2014 John Wiley & Sons, Inc. All rights reserved. Energy in Chemical Changes All chemical reactions either absorb or release energy. Chemical changes can produce different kinds of energy, like electrical energy in a lead storage battery or heat and light when fuel undergoes combustion. Chemical changes can also use energy, such as the electricity used to decompose water or the solar energy used by plants during photosynthesis. © 2014 John Wiley & Sons, Inc. All rights reserved. Conservation of Energy Energy can be changed from one form to another or from one substance to another. The Law of Conservation of Energy states that energy can be neither created nor destroyed. When water decomposes, energy is absorbed by the system so H2 and O2 have higher potential energy. When hydrogen (H2) is used as a fuel, energy is released and water (the product) has lower potential energy. © 2014 John Wiley & Sons, Inc. All rights reserved. Energy in the Real World Energy comes from many sources, including petroleum, coal and woody plants, all derived from the sun. We use petroleum deposits in the forms of gasoline and natural gas. Petroleum is composed of hydrocarbons, compounds containing only carbon and hydrogen in differing ratios. © 2014 John Wiley & Sons, Inc. All rights reserved. Energy in the Real World Natural gas consists mainly of methane (CH4) with small amounts of ethane, propane and butane mixed in. Coal is formed from plant remains stored under high pressure for many years. The higher the carbon content, the more energy available in the coal. © 2014 John Wiley & Sons, Inc. All rights reserved. The Energy Crisis To keep up with increasing energy demands, new renewable energy sources are necessary. Potential resources include solar, nuclear, biomass, wind and synthetic fuels. solar wind biomass © 2014 John Wiley & Sons, Inc. All rights reserved.