Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Eigenstate thermalization hypothesis wikipedia , lookup
Transition state theory wikipedia , lookup
Thermodynamics wikipedia , lookup
Chemical thermodynamics wikipedia , lookup
Heat transfer wikipedia , lookup
Thermal conduction wikipedia , lookup
Heat transfer physics wikipedia , lookup
Thermo chemistry Dealing with ENERGY & HEAT The Nature of Energy Kinetic and Potential Energy From Physics: Force is a push or pull on an object. • Work is the product of force applied to an object over a • distance: w=Fd Energy is the work done to move an object against a force. • Kinetic energy is the energy of motion: • Ek 12 mv 2 Potential energy is the energy an object possesses by virtue of its position. Potential energy can be converted into kinetic energy. • Example: a ball of clay dropping off a building. Energy Units SI Unit for energy is the joule, J: E k 12 mv 2 1 2 2 kg 1 m/s 2 1 kg m 2 / s 2 1J We sometimes use the calorie instead of the joule: 1 cal = 4.184 J (exactly) A nutritional Calorie: 1 Cal = 1000 cal = 1 kcal • Temperature & Energy & Heat Temperature is a measure of hot and cold – an intensive property. Thermometers in various scales are used to compare temperature. Energy is the driving force for changes. A change often is associated with a certain amount of energy and amounts of energy can be measured according to the quantities changed. Like other quantities, energy is an extensive property, unlike temperature. Heat is energy in transfer or energy transferred and heat is often used as another term for energy. In this aspect, it is the quantity of energy transferred or involved. The Nature of Energy Systems and Surroundings System: part of the universe we are interested in. Surroundings: the rest of the universe. A system for Thermochemistry For the study of energy, there must be a reference for its amounts transferred in or out. Such a reference is called a system. The boundary isolates the system from its surrounding, so that the amount of energy or heat transferred in or out can be identified. Picture a system with boundary, surrounding, and heat transfer in your mind Surrounding System Environment Measuring Heat (Energy) Heat = Energy = Work Heat is energy – the ability to do work. Units for work, heat or energy is J 1 cal = 4.184 J (defined. The amount of 1J=1Nm heat to raise 1 g water for 1 K) Gaining heat increase T, as the kinetic energy of molecules increase. Remember these from discussion of gases? Kinetic energy = ½ m u 2 Heat capacity: the amount of heat required to raise T by 1 K Heat capacity = C = ms Specific heat (s): heat capacity of 1gram of substance (no unit) compare to water (1.000 cal K-1 g-1) Calorimetry Heat Capacity and Specific Heat Calorimetry = measurement of heat flow. Calorimeter = apparatus that measures heat flow. Heat capacity = the amount of energy required to raise the temperature of an object by one degree. Molar heat capacity = heat capacity of 1 mol of a substance.Specific heat = specific heat capacity = heat capacity of 1 g of a substance. Amount of heat = q = sm( T) joul q = (specific heat) (mass of substance) T. Calorimeter Experiment q = C * T Heat gained Temperature difference Heat capacity Heat capacity of water = 1 cal deg-1 g-1 molar heat capacity of water = 18 * 4.184 J deg-1 mol-1 = 75.3 J deg-1 mol-1 Heat capacity of water = 1 cal deg-1 g-1 molar heat capacity of water = 18 * 4.184 J deg-1 mol-1 = 75.3 J deg-1 mol-1 Bomb and Coffee-cup Calorimeter q = C * T Use a known amount of q and measure T to determine C. By measuring T , calculate (corection) C = q / T Once C is known, and a known amount of reactants, heat of reaction, Hreaction, can be evaluated Bomb Calorimetry (Constant-Volume Calorimetry) Reaction carried out under constant volume. Use a bomb calorimeter. Usually study combustion. qrxn = -CcalorimeterT First Law of Thermodynamics Endothermic and Exothermic Processes Endothermic: absorbs heat from the surroundings. An endothermic reaction feels cold. Exothermic: transfers heat to the surroundings. An exothermic reaction feels hot. Endothermic and Exothermic Reactions Use an energy level diagram to help visualize endothermic and exothermic reactions. Explain the sign changes in the following: H = 6.01 kJ at 298 K H2O (s) H2O (l) H = – 6.01 kJ at 298 K H2O (l) H2O (s) H = 44.0 kJ at 273 K H2O (l) H2O (g) H = – 44.0 kJ at 273 K H2O (g) H2O (l) H = 181 kJ 2 NO (g) N2 (g) + O2 (g) H = – 181 kJ N2 (g) + O2 (g) 2 NO (g) Explain why the sign changes when the process is reversed Enthalpy (H) The quantity of heat transferred between the system and surroundings carried out under constant pressure. Can only measure the change in enthalpy: DH = Hfinal - Hinitial = q / no. of moles for example... CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(g) DH = - 890.3 kJ/mol reactants ® products + heat i.e. less Heat content in products than reactants i.e. heat is released!!! for example.. HgO(s) ® Hg(l) + ½ O2(g) DH = + 90.83 kJ/mol i.e. more Heat content in products than reactants i.e. heat is absorbed!!. Hess’s Law Hess’s Law is based on the principle of conservation of energy. The enthalpy change for the overall process is the sum of the enthalpy changes for the individual steps. The heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. Hess’s Law Illustrated A + 2B AB2 H1 A + B AB H2 AB + B = AB2 __add___________________ H1 + H2 A + 2 B = AB2 = H1 2 First Law of Thermodynamics Internal Energy Internal Energy: total energy of a system. • Cannot measure absolute internal energy. • Change in internal • energy, DE = Efinal - Einitial Enthalpies of ReactionFor a reaction DHrxn = H(products) - H (reactants) Enthalpy is an extensive property (magnitude DH is directly proportional to amount): DH = -802 kJ CH4(g) + 2O2(g) CO2(g) + 2H2O(g) 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) DH = -1604 kJ When we reverse a reaction, we change the sign of DH: DH = +802 kJ CO2(g) + 2H2O(g) CH4(g) + 2O2(g) Change in enthalpy depends on state: DH = -88 kJ H2O(g) H2O(l) Hess’s Law Hess’s law: if a reaction is carried out in a number of steps, • H for the overall reaction is the sum of H for each individual step. For example: • CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802 kJ 2H2O(g) 2H2O(l) H = -88 kJ CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890 kJ In the above enthalpy diagram note that H1 = H2 + H3 Enthalpies of Formation If 1 mol of compound is formed from its constituent elements, • then the enthalpy change for the reaction is called the enthalpy of formation, Hof . Standard conditions (standard state): 1 atm and 25 oC (298 K). • Standard enthalpy, Ho, is the enthalpy measured when • everything is in its standard state. Standard enthalpy of formation: 1 mol of compound is formed • from substances in their standard states. Foods and FuelFoods Foods Fuel value = energy released when 1 g of substance is burned. • 1 nutritional Calorie, 1 Cal = 1000 cal = 1 kcal. • Energy in our bodies comes from carbohydrates and fats • (mostly). Intestines: carbohydrates converted into glucose: • C6H12O6 + 6O2 6CO2 + 6H2O, DH = -2816 kJ Fats break down as follows: • 2C57H110O6 + 163O2 114CO2 + 110H2O, DH = -75,520 kJ Fats: contain more energy; are not water soluble, so are good • for energy storage. Fuels U.S.: 1.0 x 106 kJ of fuel per day. Most from petroleum and natural gas. Remainder from coal, nuclear, and hydroelectric. Fossil fuels are not renewable Foods and Fuels Fuel value = energy released when 1 g of substance is burned.Hydrogen has great potential as a fuel with a fuel value of 142 kJ/g. Heat of solution The heat generated or absorbed when a certain amount of solute dissolved in a certain amount of solvent Hsolution = Hsolution - Hcomponent EXAMPLE: Dissolution of NaCl • Na+Clsolid Na+ + Cl- • 1- Lattice energy Gaseous state U = 788 KJ/ mol 2- Heat of hydration Hhyd = - 784 KJ/mol HEAT OF SOLUTION Hsoln = 4 KJ/ mol Na+(aq) + Cl-(aq) Hydrated ions Hsoln = U + Hhyd = 788 + (- 784) = 4 KJ/ mol Copyright 1999, PRENTICE HALL Chapter 5 34 Heat of Dilution Heat change associated with dilution process • EXAMPLE: Dilution of H2SO4, by adding the acid to water slowly with stirring as it is Exothermic process