Chemistry PDF - Chapter 9 Energy and Chemistry

Larry Brown Tom Holme Chapter 9 Energy and Chemistry Jacqueline Bennett SUNY Oneonta www.cengage.com/chemistry/brown Chapter Objectives Explain the economic importance of conversions between different forms of energy and the inevitab...

Larry Brown Tom Holme Chapter 9 Energy and Chemistry Jacqueline Bennett SUNY Oneonta www.cengage.com/chemistry/brown Chapter Objectives Explain the economic importance of conversions between different forms of energy and the inevitability of losses in this process. Define work and heat using the standard sign conventions. Define state functions and explain their importance. State the first law of thermodynamics in words and as an equation. 2 Chapter Objectives Use calorimetric data to obtain values of E and H for chemical reactions. Define Hfo and write formation reactions for compounds. Explain Hess’s law in your own words. Calculate Ho for chemical reactions from tabulated data. 3 Energy Use and the World Economy A nation’s energy consumption is an indicator of economic growth. There is a direct relationship between Gross Domestic Product and energy consumption. 4 Energy Use and the World Economy In 2011, the total energy supply for the United States was 107.66 quadrillion Btu. Quadrillion = 1015 Btu = British thermal unit, 1 Btu = 1054.35 J. Energy supply can be broken down into coal, natural gas, crude oil, NGPL, nuclear energy, and renewable energy. Domestic production, 70.47 quadrillion Btu Imports, 31.02 quadrillion Btu. 5 Energy Use and the World Economy Energy production and consumption (in quadrillion Btu) in the United States during the year 2011. 6 Energy Use and the World Economy Energy consumption is broken down into four main components. Residential, 22% Commercial, 19% Industrial, 31% Transportation, 28% Nearly half of all domestic energy use is in the production of electricity. “Conversion losses” account for nearly two-thirds of the energy consumed to generate electricity. 7 Energy Use and the World Economy Summary of the generation and consumption of electricity in the United States during the year 2011. 8 Energy Use and the World Economy U.S. domestic consumption has increased over the last 50 years (actual consumption shown 1980-2011). The consumption of various energy sources fluctuates due to availability of raw material and the price and availability of imported fuels. 9 Forms of Energy Two broad categories of energy: potential energy and kinetic energy. Potential energy - associated with the relative position of an object. Kinetic energy - associated with motion. 1 2 Kinetic energy = mv 2 10 Forms of Energy Internal energy - the combined kinetic and potential energies of atoms and molecules that make up an object or system. Chemical energy - energy released or absorbed during a chemical reaction. Other forms of energy include radiant, mechanical, thermal, electrical, and nuclear. Thermochemistry - the study of the energetic consequences of chemistry 11 Heat and Work Heat is the flow of energy between two objects because of a difference in temperature. Heat flows from the warmer object to the cooler object. Work is the transfer of energy accomplished by a force moving a mass some distance against resistance. Pressure-volume work (PV-work) is the most common work type in chemistry. Releasing an inflated balloon before it is tied off illustrates an example of PV-work. 12 Energy Units The Joule is the SI unit of energy. 1 Joule = 1 kg m2/s2 m W = mass ´ acceleration ´ distance = kg ´ 2 ´ m s Other energy units include the Btu and the calorie. 1 Btu is the energy required to raise 1 lb of water 1°F. 1 Btu = 1055 J 1 calorie is the energy required to raise 1 g water from 14.5 to 15.5°C. 1 calorie = 4.184 J 13 Energy Transformation and Conservation of Energy During energy transformation, the total energy must be conserved. The sum of all energy conversions and energy transfers must equal the total energy present which must remain constant. To account for energy transformations and conversions, the system and surroundings must be specified. System - the part of the universe being considered. Surroundings - the remainder of the universe. System + Surroundings = Universe System and surroundings are separated by a boundary. 14 Energy Transformation and Conservation of Energy For a system or surroundings, the only possible forms of energy flow are heat, q, and work, w. The delta, Δ, means “change in” and is defined as the difference in the final and initial states. DE = q + w DE = Efinal - Einitial 15 Example Problem 9.1 If 515 J of heat is added to a gas that does 218 J of work as a result, what is the change in the energy of the system? 16 Energy Transformation and Conservation of Energy The sign resulting from the difference in the final and initial states indicates the direction of the energy flow. Negative values indicate energy is being released. Positive values indicate energy is being absorbed. 17 Energy Transformation and Conservation of Energy First law of thermodynamics states that energy can be transformed from one form to another but cannot be created or destroyed. DEuniverse = DEsurroundings + DEsystem = 0 18 Waste Energy A common way to obtain work from a system is to heat the system. Heat flows in and is converted to work. It is impossible to completely convert all heat to work. Heat not converted to work is considered waste energy, which may contribute to thermal pollution. Thermal pollution is the temperature change in a body of water from hot or cold waste streams resulting in temperatures different from normal seasonal ranges. The efficiency of conversion from heat to work can be expressed as a percentage. Increases in energy consumption can be offset by increasing energy efficiencies. 19 Waste Energy Typical efficiencies of some common energy conversion devices. 20 Waste Energy Predicted efficiency gains by the year 2030 for various technologies. 21 Heat Capacity and Calorimetry Calorimetry is a laboratory method for observing and measuring the flow of heat into and out of a system. Different systems will absorb different amounts of energy based on three main factors. The amount of material, m or n. m is mass and n is number of moles The type of material, as measured by c or Cp. c is the specific heat capacity, or specific heat, and Cp is the molar heat capacity. The temperature change, ΔT. 22 Heat Capacity and Specific Heat The specific heat capacity, or specific heat, is a physical property of a substance that describes the amount of heat required to raise the temperature of one gram of a substance by 1ºC. Represented by c. Specific heat is compound and phase specific. The molar heat capacity is a physical property of a substance that describes the amount of heat required to raise the temperature of one mole of a substance by 1 ºC. Represented by Cp (the subscript “p” indicates constant pressure). Molar heat capacity is compound and phase specific. 23 Heat Capacity and Specific Heat The amount of heat energy absorbed can be quantified. q = mcDT q = nC p DT 24 Heat Capacity and Specific Heat Specific heat and molar heat capacities for some common substances. 25 Example Problem 9.2 Heating a 24.0 g aluminum can raises its temperature by 15.0°C. Find the value of q for the can. 26 Example Problem 9.3 The molar heat capacity of liquid water is 75.3 J mol -1 K-1. If 37.5 g of water is cooled from 42.0 to 7.0°C, what is q for the water? 27 Calorimetry Heat flow is measured using a calorimeter. A calorimeter measures the heat evolved or absorbed by the system of interest by measuring the temperature change in the surroundings. qsystem = - qsurroundings qgained = - qlost 28 Example Problem 9.4 A glass contains 250.0 g of warm water at 78.0°C. A piece of gold at 2.30°C is placed in the water. The final temperature reached by this system is 76.9°C. What was the mass of gold? The specific heat of water is 4.184 J g-1 °C-1, and that of gold is 0.129 J g-1 °C-1. 29 Calorimetry There are two steps in a calorimetric measurement. Calibration - the calorimeter constant, Ccalorimeter, is determined by dividing the known amount of heat released in the calorimeter by the temperature change of the calorimeter. Actual Measurement - heat released or absorbed in a reaction of known quantity of material is measured. q = Ccalorimeter ´ DT 30 Calorimetry Actual Measurement - temperature change for the calorimeter and the calorimeter constant are used to determine the amount of heat released by a reaction. qcalorimeter = Ccalorimeter ´ DTcalorimeter qreaction = - qcalorimeter 31 Calorimetry Diagram of a bomb calorimeter and standard choice for system and surroundings in a bomb calorimetry experiment. 32 Example Problem 9.5 In the calibration of a calorimeter, an electrical resistance heater supplies 100.0 J of heat and a temperature increase of 0.850°C is observed. Then, 0.245 g of a particular fuel is burned in this same calorimeter and the temperature increases by 5.23°C. Calculate the energy density of this fuel, which is the amount of energy liberated per gram of fuel burned. 33