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
Heat transfer wikipedia , lookup
Marcus theory wikipedia , lookup
Thermodynamics wikipedia , lookup
Bioorthogonal chemistry wikipedia , lookup
George S. Hammond wikipedia , lookup
Internal energy wikipedia , lookup
Chemical thermodynamics wikipedia , lookup
Chapter 5 Thermochemistry The Nature of Energy Kinetic Energy and Potential Energy • Kinetic energy is the energy of motion: 1 2 Ek 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 bicyclist at the top of a hill. The Nature of Energy Kinetic Energy and Potential Energy • Electrostatic potential energy, Ed, is the attraction between two oppositely charged particles, Q1 and Q2, a distance d apart: Q1Q2 Ed d • The constant = 8.99 109 J-m/C2. • If the two particles are of opposite charge, then Ed is the electrostatic attraction between them. Units of Energy • • • • • SI Unit for energy is the joule, J: We sometimes use the calorie instead of the joule: 1 cal = 4.184 J (exactly) A nutritional Calorie: 1 Cal = 1000 cal = 1 kcal 1 2 1 2 Ek mv 2 kg 1 m/s 2 2 1 kg - m/s 2 1 J Systems and Surroundings Analyzing Energy Changes • System: part of the universe we are interested in. • Surroundings: the rest of the universe. Transferring Energy Work and Heat • Force is a push or pull on an object. • Work is the product of force applied to an object over a distance: w F d • Energy is the work done to move an object against a force. • Heat is the transfer of energy between two objects. • Energy is the capacity to do work or transfer heat. The First Law of Thermodynamics Energy is neither created or destroyed • Total energy lost by a system equals the total energy gained by a system. • Internal Energy: total energy of a system (kinetic + potential). • Cannot measure absolute internal energy. • Change in internal energy, E Efinal Einitial Relating E to Heat and Work • Energy of (system + surroundings) is constant. • Any energy transferred from a system must be transferred to the surroundings (and vice versa). • From the first law of thermodynamics: when a system undergoes a physical or chemical change, the change in internal energy is given by the heat added to or absorbed by the system plus the work done on or by the system: E q w Sign Conventions Exothermic and Endothermic Processes • • • • Endothermic: absorbs heat from the surroundings. Exothermic: transfers heat to the surroundings. An endothermic reaction feels cold. An exothermic reaction feels hot. State Functions • State function: depends only on the initial and final states of system, not on how the internal energy is used. Work • Chemical reactions can absorb or release heat. • However, they also have the ability to do work. • For example, when a gas is produced, then the gas produced can be used to push a piston, thus doing work. Zn(s) + 2H+(aq) Zn2+(aq) + H2(g) • The work performed by the above reaction is called pressure-volume work. • When the pressure is constant, w PV Enthalpy • Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure. H E PV • Enthalpy is a state function. • If the process occurs at constant pressure, H E PV E PV Enthalpy • Since we know that w PV • We can write H E PV qP w • When H, is positive, the system gains heat from the surroundings. • When H, is negative, the surroundings gain heat from the system. Enthalpy Enthalpies of Reaction • For a reaction: H H final H initial H products H reactants • Enthalpy is an extensive property (magnitude H is directly proportional to amount): CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802 kJ 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) H = 1604 kJ Enthalpies of Reaction • When we reverse a reaction, we change the sign of H: CO2(g) + 2H2O(g) CH4(g) + 2O2(g) H = +802 kJ • Change in enthalpy depends on state: H2O(g) H2O(l) H = -88 kJ 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. q specific heat grams of substance T Constant Pressure Calorimetry • Atmospheric pressure is constant! qrxn qsoln specific heat of solution grams of solution T Constant Pressure Calorimetry Bomb Calorimetry (Constant Volume Calorimetry) • Reaction carried out under constant volume. • Use a bomb calorimeter. • Usually study combustion. qrxn Ccal T 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 Hess’s Law 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. Enthalpies of Formation • If there is more than one state for a substance under standard conditions, the more stable one is used. • Standard enthalpy of formation of the most stable form of an element is zero. Standard Enthalpies of Formation at 298 K Using Enthalpies of Formation of Calculate Enthalpies of Reaction • We use Hess’ Law to calculate enthalpies of a reaction from enthalpies of formation. Using Enthalpies of Formation of Calculate Enthalpies of Reaction • For a reaction H rxn nH f products mH f reactants Foods and Fuels • • • • 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, H = -2816 kJ • Fats break down as follows: 2C57H110O6 + 163O2 114CO2 + 110H2O, H = -75,520 kJ Fuels • In 2000 the United States consumed 1.03 1017 kJ of fuel. • Most from petroleum and natural gas. • Remainder from coal, nuclear, and hydroelectric. • Fossil fuels are not renewable. Fuels