Chapter 3: Energy and Its Conservation 3.1 Types of Energy 3.2 Thermodynamics 3.3 Energy Changes in Chemical Reactions 3.4 Measuring Energy Changes: Calorimetry 3.5 Enthalpy 3.6 Energy Sources Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.1 Types of Energy Learning objective: Recognize the types of energy of interest to chemists. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.1 Types of Energy Energy – the ability to do work Kinetic Energy Potential Energy: 1. 2. a. b. c. 3. 4. Electrical Chemical Mass Thermal Energy Radiant Energy Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Kinetic Energy Every moving object has kinetic energy which is dependent on the velocity (v) and mass (m) of an object: 1 E kinetic mv 2 2 Joule (J): the SI unit of energy kg m 2 1 joule 1 J 1 2 s Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Potential Energy Electrical Energy - energy from positive and negative ions held a small distance apart. Eelectrical q1q2 =k r q1 and q2 are the charges of two ions r is the distance between the ions in pm k = 2.31 x 10-16 J pm Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Potential Energy Chemical Energy: energy resulting from attraction of the electrons and nuclei in molecules (bond energy) Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Potential Energy Mass: transformation of mass into energy (E = mc2) Thermal Energy: the total energy of random movements of molecules Radiant Energy: energy as a result of electromagnetic radiation. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Energy Transfer and Transformations Energy can be transferred from one type to another. Energy transformations accompany chemical reactions. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.2 Thermodynamics Learning objective: Understand the first law of thermodynamics and the concepts of heat and work. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.2 Thermodynamics The study of energy transfers and transformations “how much energy goes where” Terms: System: whatever we want to describe and study by itself. Surroundings: everything else but the system Boundary: - what separates the system from its surroundings Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Conservation of Energy Energy is neither created nor destroyed in any process, although it may be transferred from one body to another or transformed from one form into another or, restated Energy may be transferred as work or heat, but no energy can be lost, nor can heat or work be obtained from nothing Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Heat Thermal energy that is exchanged with its surroundings is referred to as heat (q) and is measured in joules (J). Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Heat Flows and Temperature 1. 2. • • 3. 4. T depends on q, the amount of heat transferred. T depends on the direction of heat flow: If a substance absorbs heat, T > 0 If a substance releases heat, T < 0 T depends inversely on the amount of material. T depends on the identity of the material. Molar heat capacity (C) – amount of heat needed to raise the temperature of 1 mol of substance by 1° C. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Temperature Change q ΔT = nC Where q is the amount of heat transferred, n is the number of moles of material and C is the molar heat capacity of the substance in J mol-1 °C-1. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. So you can also say that: H eat transferred q m C T Energy transfer is directional, so we must keep track of the signs associated with heat flows qsurroundings = – qsystem Note: T = final temperature - initial temperature Remember to use the correct algebraic sign! If Tf < Ti, then the T should be negative. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Example 3 - 1 Calculate the temperature change that results from adding 250 J of thermal energy to each of the following (a) 0.75 mol of Hg; (b) 0.35 mol of Hg; (c) 0.35 mol of H2O. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Example 3 - 2 An aluminum frying pan that weighs 745 g is heated on a stove from 25°C to 205°C. What is q for the frying pan? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Work Work (w): energy used to move an object against an opposing force The amount of work depends on the magnitude of the force wsurroundings = – wsystem Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. First Law of Thermodynamics Simply a restatement of the law of conservation of energy E = q + w Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. State and Path Functions State Functions: properties that depend only on the conditions that describe the system. Energy is a state function. Path Functions: properties that depend on how the change occurs. Distance travelled is a path function. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Distance Travelled is a Path Function… …but distance between two cities is a state function Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Thermodynamic Path Functions Energy is a state function, but heat and work are path functions Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.3 Energy Changes in Chemical Reactions Learning objective: Understand the origins of energy changes in chemical reactions Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.3 Energy Changes in Chemical Reactions Bond breakage requires energy Bond formation releases energy. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Example 3 - 3 The Haber reaction for the formation of ammonia releases energy: N2 + 3 H2 2 NH3 E = 40.9 kJ How much energy is released in the production of 1.00 kg of ammonia? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Path Independence A change in any state function is independent of path. Thus, the energy change in a chemical reaction is independent of the manner in which the reaction takes place. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Bond Energies Bond energy (BE): energy required to break a bond, always positive Usually expressed in kJ/mol H2 (g) H (g) + H (g) Ebond breaking = BE = + 435 kJ/mol And the reverse process: H (g) + H (g) H2 (g) Ebond making = BE = 435 kJ/mol Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Table 3 – 2: Average Bond Energies Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Reaction Energy Bond energies can be used to estimate the energy change that occurs in a chemical reaction E rxn BE (bonds broken) Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. - BE (bonds form ed) Example 3 - 4 The world produces tens of billions of vinyl chloride annually. Most is converted to the polymer poly(vinyl) chloride (PVC), which is used to make piping, siding, gutters, floor tiles, clothing and toys. Vinyl chloride is made in a two-step process. The balanced overall equation is as follows: Based on average bond energies, what energy change accompanies the formation of one mole of vinyl chloride? Does the synthesis require an input of energy? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.4 Measuring Energy Changes: Calorimetry Learning objective: Apply the principles of calorimetry to determine energy changes in a chemical reaction Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.4 Measuring Energy Changes: Calorimetry Calorimeter: a device used to measure heat flows that accompany chemical reactions. A Styrofoam cup is a simple calorimeter. Exothermic: if the reaction releases heat. Endothermic: if the reaction absorbs heat. qreaction = - qcalorimeter Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. qcalorimeter = CcalT Example 3 - 5 A calorimeter is calibrated with an electrical heater. Before the heater is turned on, the calorimeter temperature is 23.6 °C. The addition of 2.02 x 103 J of electrical energy from the heater raises the temperature to 27.6 °C. Determine the total heat capacity of this calorimeter. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Example 3 - 6 Ammonium nitrate (NH4NO3, M = 80.05 g/mol) is used in cold packs to “ice” injuries. When 20.0 g of this compound dissolves in 125 g of water in a coffee-cup calorimeter, the temperature falls from 23.5 °C to 13.4 °C. Determine the q for the dissolving of the compound. Is the process exothermic or endothermic? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Molar Energy Changes Energy change is an extensive quantity – it is dependent on the amount of substance. ΔEmolar Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. ΔE = n Example 3 - 7 A 0.125 g sample of octane (C8H18, M = 114.2 g/mol) is burned in excess O2 in the constant-volume calorimeter described in Example 3 – 5. The temperature of the calorimeter rises from 21.1 to 32.9 °C. What is Emolar for the combustion of octane? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.5 Enthalpy Learning objective: Understand and calculate enthalpy and internal energy Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.5 Enthalpy Enthalpy (H) is a thermodynamic state function that describes heat flow at constant pressure. H = E + pV Enthalpy change, H: heat transferred into or out of a system at constant pressure. For a reaction, it can be calculated according to: Hreaction ≈ Ereaction + RTngas Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Example 3 - 8 Find the difference between molar H and E for the combustion of octane at 298 K. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Energy and Enthalpy of Vapourization Changes of state always take place at a constant temperature Heat of vapourization Hvap: heat required to convert liquid to gas Hvap ≈ Evap + RTvap Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Enthalpies of Formation A formation reaction produces 1 mol of a chemical substance from the elements in their most stable forms There is a single product with a stoichiometric coefficient of 1. All the starting materials are elements, and each is in its most stable form. Enthalpies of reactions involving gases vary with pressure, so pressures must be specified. Enthalpies of reactions occurring in solution vary with concentration, so concentrations must be specified. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Remember State Functions State function - a quantity whose value is determined only by the state of the system. It does not depend on the path taken to get there. Standard state - is the most stable form of a substance at T = 25 °C and p = 1 bar, and 1 M if it is in solution The superscript ° indicates standard conditions. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Therefore... The standard enthalpy change of a reaction for the formation of 1 mole of a compound directly from its elements is called the standard molar enthalpy of formation, Hf° Mn (s) + O2 (g) MnO2 (g) Hfo = -520.0 kJ/mol Br2 (l) Br2 (g) Hfo = 30.9 kJ/mol Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Enthalpy Changes for Chemical Reactions Hess’s Law: the enthalpy change for any overall process is equal to the sum of enthalpy changes for any set of steps that leads from the starting materials to the products. To calculate the total enthalpy change for a reaction, H°rxn: H°reaction = Sp H°f,p- S r H°f,r Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Enthalpy Changes Under Nonstandard Conditions Energies and enthalpy change as temperature, concentration, and pressure change. Therefore, H depends on these variables, too. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.6 Energy Sources Learning objective: Be familiar with our sources of energy Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 3.6 Energy Sources Energy and Civilization: advances of civilization can be viewed as the results of people figuring how to increase the availability of energy Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Ultimate Energy Sources The vast majority of our energy sources originate in solar energy. Photosynthesis converts some solar energy into more concentrated forms: 6 CO2 (g) + 6 H2O (l) + 2880 kJ C6H12O6 (s) + 6 O2 (g) One nonsolar source is nuclear energy Another is the Earth’s hot interior (geothermal) energy Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Future Resources Economically, we want energy sources to be high intensity, and readily extracted and transported. Environmentally, they would be renewable and environmentally benign. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 3 Visual Summary Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 3 Visual Summary Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 3 Visual Summary Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 3 Visual Summary Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 3 Visual Summary Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 3 Visual Summary Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.