Chemistry: The Central Science. Chapter 5: Thermochemistry

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1 Chemistry: The Central Science Chapter 5: Thermochemistry Study of energy and its transformations is called thermodynamics Portion of thermodynamics that involves the relationships between chemical and heat is called thermochemistry 5.1: The Nature of energy Energy is ability to do work or transfer heat o Work energy used to cause an object to move against a force o Heat energy used to increase the temperature of an object Kinetic Energy and Potential Energy o Kinetic energy energy of motion o Potential energy energy base on position of the object E p = mgh m is mass of the object in question g is gravitational constant, 9.8 m/s 2 h is the height of the object relative to some reference height o Electrostatic potential energy Arise from the interactions between charged particles E el = (ĸQ 1 Q 2 )/d ĸ = J-m/C 2 C = coulomb, unit of charge of electron Electric charge Q 1 and Q 2 are typically on the order of magnitude of the charge of electron ( C) Q 1 and Q 2 have same sign, they repels and E el is positive Likewise, Q 1 and Q 2 have same sign, the attract and E el is negative Units of Energy o Joule SI unit for energy o Often use kj instead of J o calorie a non-si unit for energy 1 cal = J o 1 Cal = 1000 cal = 1 kcal System and Surroundings o System the portion single out for study o Surroundings everything else

2 o Open system matter and energy can be exchange with surroundings o Close system only energy can be exchange with surroundings o Isolated system no matter or energy can be exchange with surroundings Transferring Energy: Work and Heat o Two general was energy is transferred between system and surround is work and heat o Work is the energy used to move an object against a force o Heat is the transfer of energy from the object with higher energy to the object with lower energy 5.2: The First Law of Thermodynamics First law of thermodynamics energy can be neither created nor destroyed Internal Energy energy that an object contains o Cannot find the internal energy o Can only find the change in energy ΔE = E final - E initial E final > E initial = the system has gained energy Relating ΔE to Heat and Work o Internal energy of a system ΔE = q + w E final < E initial = the system has lost energy q is heat of a system (+ = gain, - = loss) w is work of a system (+ = done on, - = done by) ΔE is the total change in energy of a system (+ = net gain, - = net loss) Endothermic and Exothermic Processes o Exothermic reactions that release energy o Endothermic reactions that absorb energy State Functions o A property of a system that is determined only by the present state not the path it took to become that stage. 5.3: Enthalpy Pressure-volume work involves the work in expansion or compression of gases o

3 o there s negative to show that when volume increase, the work done BY the system Enthalpy thermodynamic function accounts for heat flow in processes at constant pressure o H = E + PV o Change in enthalpy at constant temperature is given by ΔH = Δ(E + PV) = ΔE + PΔV ΔH = ΔE + PΔV = (q p + w) w = q p Subscript p on q shows that the change in enthalpy occurs at constant pressure ΔH is positive = endothermic, ΔH = negative = exothermic 5.4: Enthalpies of Reaction ΔH = H products - H reactants Change in enthalpy that accompanies a reaction is enthalpy of reaction (ΔH rxn ) Balance equations that show the associated enthalpy change is called thermochemical equations Enthalpy is an extensive property o The magnitude of ΔH is directly proportional to the amount of reactant consumed in the reaction ΔH for a reaction is equal in magnitude but opposite sign, to ΔH of a reverse reaction ΔH of a reaction also depends on the state of matter of the reactants and products 5.5: Calorimetry ΔH can be determined by using calorimetry, the measurement of heat flow, and calorimeter, a device to measure heat flow Heat Capacity and Specific Heat o The temperature increase of each substance with the same amount of heat added depend on the object s heat capacity o Heat capacity (C) is the amount of heat to raise the temperature of the substance by 1K Molar heat capacity (C m ) the amount of energy needed to raise 1 mole of the substance by 1K Specific heat capacity (or just specific heat) (C s ) the amount of energy needed to raise 1 gram of the substance by 1K o Liquid water has high specific heat capacity

4 Constant-Pressure Calorimetry o Coffee-cup calorimeter is often use in general chemistry lab Have pore so the pressure is the same as the atmosphere Have thermometer to measure the temperature o The heat loss by the reaction will be gain by the solution o The heat gain by the reaction will be loss by the solution q soln = (specific heat of solution) (grams of solution) ΔT Bomb Calorimetry (Constant-Volume Calorimetry) o Use a bomb calorimeter o Put a sample in a container pressurized with oxygen o Add energy through wire and combust the sample o The heat produce by the reaction is absorb by the water o Measure the change in temperature Need to know the heat capacity of the calorimeter Heat evolved in the reaction q rxn = -C cal ΔT 5.6: Hess s Law ΔH is a state function so it only depends on the initial state and final product Hess s Law if a reaction is carried out in a series of steps, ΔH for the overall reaction will equal the sum of the enthalpy changes for the individual steps 5.7: Enthalpies of Formation ΔH f is the enthalpy of formation in which the subscript f indicates that the substance has been formed Standard enthalpy change (ΔH o ) is the enthalpy change of a reaction when all reactants and products are in their standard state (substance in its pure form at 1 atm and 289 K) Standard enthalpy of formation (ΔH o f) is the reaction the forms one mole of the compound from its elements, with all substances in standard state o ΔH o f of any compound is zero Using Enthalpies of Formation to Calculate Enthalpies of Reaction o Use the table for ΔH o f to calculate the enthalpies of reaction Add and subtract the appropriate ΔH to get the ΔH o rxn o ΔH o rxn = ΣnΔH o f (products) - ΣmΔH o f (reactants) 5.8: Foods and Fuels

5 Fuel value the energy released when one gram of material is combusted Foods o Carbohydrate metabolism produces water, CO 2 and energy C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O Average fuel value of carbohydrate is 17 kj/g or 4 kcal/g o Fats also produce water and CO 2 but gives off 38 kj/g or 9 kcal/g Insoluble in water and produce more energy than protein or carbohydrate o Protein s metabolism produce less energy in body than in calorimeter because it produce different product Fuel value of protein on average is 17 kj/g or 4 kcal/g o The amount of energy needed depend on the mass of the person Fuel o The higher the percentage of carbon and hydrogen in the compound, the higher its fuel value o Fossil fuels decomposition of plants and animals from millions of years ago o Natural gas gaseous hydrocarbons o Petroleum Liquid composed of hundred of compounds, mostly hydrocarbons, with remainder sulfur, nitrogen, or oxygen o Coal solid contain hydrocarbons of high molecular weight and sulfur, oxygen, oxygen Abundant Produce SO 2, a very troublesome air pollutant Turn coal to syngas Heated steam the coal to remove sulfur-containing compounds, water, and CO 2 Easily transported because it s gaseous Other Energy Sources o Nuclear energy and fossil fuels are nonrenewable resources o Researching renewable resources Solar energy, wind energy, geothermal energy, hydroelectric energy, and biomass energy Solar cells are being made