Chap 06 Lecture Outline - Thermochemistry PDF

Chapter 06 Thermochemistry 1 2 Some Forms of Energy Chemical Energy Electrical: kinetic energy associated with the flow of electrical charge Heat or Thermal Energy: kinetic en...

Chapter 06 Thermochemistry 1 2 Some Forms of Energy Chemical Energy Electrical: kinetic energy associated with the flow of electrical charge Heat or Thermal Energy: kinetic energy associated with molecular motion Light or Radiant Energy: kinetic energy associated with energy transitions in an atom Nuclear: potential energy in the nucleus of atoms Chemical: potential energy in the attachment of atoms or because of their position Compounds contain chemical energy that can be released when its chemical bonds are broken Any substance that can be used as a fuel contains chemical energy 3 4 Units of Energy Units of Energy The amount of kinetic energy an A joule (J) is the amount of energy needed to object has is directly proportional to move a 1 kg mass to a distance of one meter. its mass and velocity. A calorie (cal) is the amount of energy needed to When the mass is in kg and raise the temperature of one gram of water 1 °C. velocity is in m/s, the unit – kcal = energy needed to raise 1000 g of water 1 °C for kinetic energy is: – food Calories = kcals 1 joule of energy is the amount of energy needed to move a 1 kg mass at a speed of 1 m/s. 5 6 1  Energy Use System and Surroundings Energy Energy Energy System: the material or process within which we required to Energy required to used by an are studying the energy changes within. raise used to Unit light 100-W run 1 mile average Surrounding: everything else with which the temperature bulb for 1 US citizen of 1 g of hour (approx) in 1 day system can exchange energy. water by 1°C We study the exchange of energy between the joule (J) 4.18 3.60 x 105 4.2 x 105 9.0 x 108 system and the surroundings. calorie (cal) 1.00 8.60 x 104 1.0 x 105 2.2 x 108 Calorie (Cal) 0.00100 86.0 100. 2.2 x 105 kWh 1.16 x 10-6 0.100 0.12 2.5 x 102 7 8 Conservation of Energy Three types of Systems The amount of energy gained or lost by the system must be Open System: Both mass & energy transfer takes equal to the amount of energy place across the boundaries lost or gained by the Closed System: Only energy (no mass) transfer surroundings. takes places across boundaries The law of conservation of Isolated System: No transfer of mass or energy energy states that energy takes place across the boundaries cannot be created or destroyed. When energy is transferred between objects, or converted from one form to another, the total amount of energy present at the beginning must be present at the end. 9 10 Energy Flow and Conservation of Energy The First Law of Thermodynamics Conservation of energy requires that the sum of Thermodynamics is the study of energy and its the energy changes in the system and the interconversions surroundings must be zero. The first law of thermodynamics is the law of conservation of energy You can never design a system that will continue to produce energy without some source of energy. 11 12 2  Internal Energy (E) The internal energy is the sum of the kinetic and potential energies of all the particles that State Function compose the system. The change in the internal energy of a system depends only on the amount of energy in the To reach the top of the mountain you can take a long & system at the beginning and the end. winding trail, or a short but steep trail Internal energy is a state function: However, regardless of the trail, when you reach the top, you will be 10,000 ft above the base. A state function is a The distance from the base to the peak of the mountain is a state function. It depends only on the difference in elevation between the base and the peak, not on how you arrive there! 13 14 Internal Energy (E) Energy Diagrams: “Graphical” way of The internal energy (E) is the sum of the kinetic showing the direction of energy flow and potential energies of all the particles that If the final condition has Internal Energy compose the system. a larger amount of final internal energy than the Since it is a state function, the change in the initial condition, the initial internal energy of a system depends only on the change in the internal amount of energy in the system at the beginning energy will be + and the end. If the final condition has Internal Energy initial a smaller amount of internal energy than the initial condition, the final change in the internal energy will be ─ 15 16 Energy Flow Energy Flow When energy flows out of a system, When energy flows into a system, DEsystem is negative. DEsystem is positive. When energy flows into the surroundings, When energy flows out of the surroundings, DEsurroundings is positive. DEsurroundings is negative Therefore: Therefore: Surroundings Surroundings DE + DE -- System System DE -- DE + 17 18 3  How is Energy Exchanged? How is Energy Exchanged? Energy is exchanged between the system and Energy is exchanged between the system and surroundings through heat and work. surroundings through heat and work. q = heat (thermal) energy; w = work energy q = heat (thermal) energy; w = work energy q and w are not state functions; their value q and w are not state functions; their value depends on the process. depends on the process. 19 20 Heat Exchange Heat Exchange Heat is the exchange of thermal energy between Heat flows from matter at high temperature to the system and surroundings. matter with low temperature until both objects Occurs when system and surroundings have a reach the same temperature, i.e., thermal difference in temperature. equilibrium The molecules in a sample of hot water move more Temperature rapidly than those in a sample of cold water. Heat flows from matter with high temperature to matter with low temperature until both objects reach the same temperature i.e., thermal equilibrium 21 22 Quantity of Heat Energy Absorbed Specific & Molar Heat Capacity When a system absorbs heat, its temperature increases The specific heat capacity is the amount of heat energy required to raise the temperature of one The increase in temperature is directly gram of a substance 1°C [units: J/(g∙°C)] proportional to the amount of heat absorbed The molar heat capacity is the amount of heat The proportionality constant is called the heat energy required to raise the temperature of one capacity mole of a substance 1°C [units: J/(mol∙°C)] The heat capacity of an object depends on its We can calculate the quantity of heat absorbed mass and the type of material by an object if we know the mass, the specific heat, and the temperature change of the object Heat = (mass) x (specific heat capacity) x (temp. change) 23 24 4  Specific Heat Capacity How much heat is absorbed by a copper penny with mass 3.10 g whose temperature rises from -- 8.0°C to 37.0°C? Specific Heat is the amount of energy needed to Specific heat capacity of copper is 0.385 J/g °C. change the temperature of 1 gram of a substance 1°C 25 26 A 208-g sample of a metal requires 1.75 kJ to change its temperature from 28.2°C to 89.5°C. What is the specific heat of this metal? 27 5  Energy Exchange Change in internal energy (DE) = q + w Exchange of heat energy q = m × Cs × DT Exchange of work: w Thermochemistry Part 2 1 2 Pressure–Volume Work If a balloon is inflated from 0.100 L to 1.85 L If the external pressure is kept constant, against an external pressure of 1.00 atm, how many joules of work is done? 3 4 Summary: Energy exchange between the Measuring DE System & Surrounding Determine DE of a reaction by measuring q & w. Change in internal energy (DE) = q + w If reaction is carried out at constant volume Exchange of heat energy Exchange of work Problem: Cannot observe the temperature changes of individual chemicals in a reaction Solution: Measure the temperature change in the surroundings, using insulated, controlled surroundings, (calorimetry) 5 6 1 Measuring DErxn using the Bomb Calorimeter Measuring DErxn using the Bomb Calorimeter 7 8 When 1.010 g of sugar is burned in a bomb calorimeter, the temperature rises from 24.92°C to 28.33°C. Enthalpy If Ccal = 4.90 kJ/°C, find DE for burning 1 mole of sugar. The enthalpy, H, of a system is the sum of the internal energy of the system and the product of pressure and volume. It is a state function. The enthalpy change, DH, of a reaction is the heat involved in a reaction at constant pressure. Usually, DH and DE are similar in value; the difference is largest for reactions that produce or use large quantities of gas. 9 10 Endothermic and Exothermic Reactions Endothermic and Exothermic Reactions Chemical cold packs contain NH4NO3 that dissolves in When DH is negative, heat is being released by water in an endothermic process: Your hands get cold the system and the reaction is called an because the pack is absorbing your heat. exothermic reaction. Chemical heat packs contain MgSO4 that dissolves in When DH is positive, heat is being absorbed by water in an exothermic reaction: Your hands get warm the system and the reaction is called an because the released heat of the reaction is transferred endothermic reaction. to your hands. 11 12 2  Enthalpy of Reaction How much heat is evolved in the complete combustion of 13.2 kg of C3H8(g)? The enthalpy change (DH) in a chemical reaction is an extensive property, (i.e., the more reactants you use, the larger the enthalpy change). By convention, we calculate the enthalpy change for the number of moles of reactants in the balanced chemical reaction as written. C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) DH = −2044 kJ 13 14 Measuring DH: 1.00L of 1.00M Ba(NO3)2 solution and 1.00L of 1.00M Na2SO4 solution, both at 25.0°C, are mixed in a Calorimetry at Constant Pressure calorimeter. A white precipitate of BaSO4 forms. The temperature of the mixture increases to 28.1°C. Calculate the enthalpy change for this reaction 15 16 17 18 3  Hess’s Law Hess’s Law Enthalpy is a state function – Change in enthalpy (DH) from initial to final state is independent of the pathway. – From reactants to products, change in enthalpy (DH) is the same whether the reaction is done in one step or in a series of steps. – Oxidation of N2 to NO2 in one step: N2(g) + 2 O2(g) → 2 NO2(g) ΔrH1 = 68 kJmol–1 – Oxidation of N2 to NO2 in two steps: 19 20 Using Hess’s Law Calculate ∆H for the synthesis of diborane from its elements, according to the following equation and data: If a reaction is reversed, the sign of ΔrH is also 2 B(s) + 3 H2 (g) → B2H6 (g) reversed Magnitude of ΔrH is directly proportional to the quantities of reactants and products in a reaction If the stoichiometric coefficients are multiplied by a factor, the value of H is also multiplied by the same factor 21 22 Standard Conditions The standard state is the state of a material at a defined set of conditions. Pure gas at exactly 1 atm pressure Pure solid or liquid in its most stable form at exactly 1 atm pressure and temperature of interest (usually 25 °C or 298 K) Substance in a solution with concentration 1 M 23 24 4  Standard Conditions Formation Reactions The standard enthalpy change, DH°, is the Are reactions of elements in their standard enthalpy change when all reactants and states to form 1 mole of a pure compound products are in their standard states. If you are not sure what the standard state of an The standard enthalpy of formation, DHf°, is element is, find the form in Appendix IIB of the the enthalpy change for the reaction forming textbook that has DHf° = 0. 1 mole of a pure compound from its Because the definition requires 1 mole of constituent elements. compound be made, the coefficients of the The elements must be in their standard states. reactants may be fractions. The DHf° for a pure element in its standard state = 0 kJ/mol. 25 26 Writing Formation Reaction for CO(g) Calculating the Standard Enthalpy The formation reaction is the reaction between the Change for a Reaction elements in the compound, which are C and O. Any reaction can be written as the sum of C + O → CO(g) formation reactions (or the reverse of formation The elements must be in their standard state. reactions) for the reactants and products. – There are several forms of solid C, but the one with The DH° for the reaction is then the sum of the DHf° = 0 is graphite. – Oxygen’s standard state is the diatomic gas. DHf° for the component reactions. The equation must be balanced, but the coefficient of the product compound must be 1. – Use whatever coefficient in front of the reactants is necessary to make the atoms on both sides equal without changing the product coefficient. 27 28 Compute the DHorxn for the following reaction from the given standard enthalpies of formation of components. 4NH3(g) + 7O2(g) → 4NO2(g) + 6H2O(l) Compound DHf° (kJ/mol) NH3(g) -46 O2(g) 0 H2O(l) -286 NO2(g) 34 29 5