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BIOENERGY. The sugars in sugarcane, produced from CO 2, H 2 O, and sunshine via photosynthesis, can be converted into ethanol, which is used as an alternative to gasoline. In certain climate, such as that in Brazil, the sugarcane crop replenishes itself rapidly, making cane- based ethanol a sustainable fuel source.

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I NTRODUCTION E NERGY AND C HEMISTRY Energy is necessary for all life. Plants, sunlight, and photosynthesis Human beings derive energy from plants and animals Energy changes accompany chemical reactions All of the energy we use are derived from chemical reactions Thermodynamics is the study of energy and its transformations

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5.1 T HE N ATURE OF E NERGY D EFINITION OF E NERGY Energy is the capacity to do work or to transfer heat Work is the energy used to cause an object with mass to move against a force Heat is the energy used to cause the temperature of an object to increase

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5.1 T HE N ATURE OF E NERGY K INETIC E NERGY AND P OTENTIAL E NERGY Objects that have mass and motion possess kinetic energy Potential energy is the stored energy that arise from the attractions and repulsions an object experiences in relation to other objects

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5.1 T HE N ATURE OF E NERGY U NITS OF E NERGY The SI unit of energy is the joule (J) An older, non-SI unit is still in widespread use: the calorie (cal), the amount of energy required to raise the temperature of 1 g of water by 1 C 1 cal = 4.184 J

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5.1 T HE N ATURE OF E NERGY S YSTEM AND S URROUNDINGS The system is the portion we single out for study and typically includes the molecules we want to study (here, the hydrogen and oxygen molecules). The surroundings are everything else (here, the cylinder and piston). Open systems: matter & heat exchangeable Closed systems: only heat exchangeable Isolated systems: no exchange

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5.1 T HE N ATURE OF E NERGY T RANSFERRING E NERGY : W ORK AND H EAT Energy used to move an object over some distance is work. w = F d where w is work, F is the force, and d is the distance over which the force is exerted.

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5.1 T HE N ATURE OF E NERGY T RANSFERRING E NERGY : W ORK AND H EAT Energy can also be transferred as heat. Heat flows from warmer objects to cooler objects.

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5.1 T HE N ATURE OF E NERGY T RANSFERRING E NERGY : W ORK AND H EAT

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5.2 T HE F IRST L AW OF T HERMODYNAMIC E NERGY IS C ONSERVED Energy is neither created nor destroyed. In other words, the total energy of the universe is a constant; if the system loses energy, it must be gained by the surroundings, and vice versa.

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5.2 T HE F IRST L AW OF T HERMODYNAMIC I NTERNAL E NERGY The internal energy (E) of a system is the sum of all kinetic and potential energies of all components of the system E = E k + E p

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5.2 T HE F IRST L AW OF T HERMODYNAMIC I NTERNAL E NERGY By definition, the change in internal energy, E, is the final energy of the system minus the initial energy of the system: E = E final E initial

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If E > 0, E final > E initial Therefore, the system absorbed energy from the surroundings. 5.2 T HE F IRST L AW OF T HERMODYNAMIC I NTERNAL E NERGY

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If E < 0, E final < E initial Therefore, the system released energy to the surroundings. 5.2 T HE F IRST L AW OF T HERMODYNAMIC I NTERNAL E NERGY

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5.2 T HE F IRST L AW OF T HERMODYNAMIC R ELATING E TO W ORK AND H EAT When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w). That is, E = q + w.

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5.2 T HE F IRST L AW OF T HERMODYNAMIC R ELATING E TO W ORK AND H EAT

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5.2 T HE F IRST L AW OF T HERMODYNAMIC E NDOTHERMIC AND E XOTHERMIC P ROCESSES

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5.2 T HE F IRST L AW OF T HERMODYNAMIC S TATE F UNCTION Consider chemical changes of H 2 O: The value of a state function (internal energy, E, in this case) depends only on the present state of the system, not on the path the system took to reach that state

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5.2 T HE F IRST L AW OF T HERMODYNAMIC S TATE F UNCTION However, q and w are not state functions. Whether the battery is shorted out or is discharged by running the fan, its E is the same. But q and w are different in the two cases.

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5.3 E NTHALPY E NTHALPY Consider a chemical reaction: w = -P V

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5.3 E NTHALPY E NTHALPY Enthalpy is a measure of the total energy of a system Enthalpy accounts for heat flow in processes occurring at constant pressure when no forms of work are performed other than P-V work Enthalpy (H) is expressed by: H = E + PV (state function) When the system changes at constant pressure, the change in enthalpy, H, is H = (E + PV) = E + P V = (q p + w) w = q p (state function?) H equals the heat gained or lost at constant P

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When H is positive: endothermic process When H is negative: exothermic process 5.3 E NTHALPY E NTHALPY

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At constant V, the change in E is equal to the heat gained or lost At constant P, the change in H is equal to the heat gained or lost The difference between E and H is the amount of P-V work done by the system when the process occurs at constant P, P V In many reactions, V is close to zero, which makes P V (the difference between E and H) small Generally H can be used to measure energy changes during most chemical processes

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5.3 E NTHALPY (a) H > 0, endothermic (b) H < 0, exothermic

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5.4 E NTHALPIES OF R EACTION E NTHALPY OF R EACTION thermochemical equation enthalpy diagram

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5.4 E NTHALPIES OF R EACTION This tragedy, in Lakehurst, New Jersey, on May 6, 1937, led to the discontinuation of hydrogen as a buoyant gas in such craft

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5.4 E NTHALPIES OF R EACTION E NTHALPY OF R EACTION Enthalpy is an extensive property. H for a reaction in the forward direction is equal in size, but opposite in sign, to H for the reverse reaction. H for a reaction depends on the state of the products and the state of the reactants. for 1 mol of CH 4

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5.4 E NTHALPIES OF R EACTION

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5.5 C ALORIMETRY H EAT CAPACITY AND SPECIFIC HEAT Heat capacity, C The amount of heat required to raise temperature by 1 K ( C) Molar heat capacity, C m The heat capacity of one mole of a substance Specific heat, C s The heat capacity of one gram of a substance

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5.5 C ALORIMETRY H EAT CAPACITY AND SPECIFIC HEAT

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Sample Exercise 5.5 Relating Heat, Temperature Change, and Heat Capacity (a) How much heat is needed to warm 250 g of water (about 1 cup) from 22 C (about room temperature) to near its boiling point, 98 C? The specific heat of water is 4.18 J/g-K. (b) What is the molar heat capacity of water? 5.5 C ALORIMETRY

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Are you running a fever? Difficult to maintain T Maintaining T in our body High heat capacity of water Optimal for muscle function and biochemical rxns in our body at 35.8-37.2 C Hypothalamus controls body T Radiation, convection, and evaporation (humidity and water replenishment) Shivering and reddish skin

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5.5 C ALORIMETRY C ONSTANT - PRESSURE C ALORIMETRY Calorimetry is the measurement of heat flow Consider a system: A reaction in a coffee-cup System: reactants and products Surrounding: water and calorimeter

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5.5 C ALORIMETRY Sample Exercise 5.6 Measuring H Using a Coffee-Cup Calorimeter When a student mixes 50 mL of 1.0 M HCl and 50 mL of 1.0 M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0 C to 27.5 C. Calculate the enthalpy change for the reaction in kJ/mol HCl, assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100 mL, that its density is 1.0 g/mL, and that its specific heat is 4.18 J/g-K. H mol = -2.7 kJ/0.050 mol = -54 kJ/mol

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5.5 C ALORIMETRY C ONSTANT - VOLUME C ALORIMETRY Bomb calorimeter Designed for measuring T by combustion reactions Inlet valve: O 2 supply Electrical contact: initiation Surrounding: water and calorimeter The heat capacity of the calorimeter is measured by combusting a standard sample

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5.5 C ALORIMETRY Sample Exercise 5.7 Measuring q rxn Using a Bomb Calorimeter Methylhydrazine (CH 6 N 2, MW 46.1 g/mol) is used as a liquid rocket fuel. The combustion of methylhydrazine with oxygen produces N 2 (g), CO 2 (g), and H 2 O(l): 2 CH 6 N 2 (l) + 5 O 2 (g) 2 N 2 (g) + 2 CO 2 (g) + 6 H 2 O(l) When 4.00 g of methylhydrazine is combusted in a bomb calorimeter, the temperature of the calorimeter increases from 25.00 C to 39.50 C. In a separate experiment the heat capacity of the calorimeter is measured to be 7.794 kJ/ C. Calculate the heat of reaction for the combustion of a mole of CH 6 N 2.

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5.6 H ESS S L AW C OMBUSTION OF CH 4 (g) Consider the combustion reaction of CH 4 (g) We can think of the reaction as a two-step process

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5.6 H ESS S L AW H ESS S L AW If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps Based on the fact that enthalpy is a state function Useful for calculating H that are difficult to measure directly (Ex. Carbon to CO)

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5.6 H ESS S L AW

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5.7 E NTHALPIES OF F ORMATION An enthalpy of formation, H f, is defined as the enthalpy change for the formation of a compound from its constituent elements The conditions of T, P, and state (aq, s, l,) should be defined. Standard enthalpy change, H H when all reactants and products are in their standard state (1 atm, 273 K or 25 C)

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5.7 E NTHALPIES OF F ORMATION S TANDARD E NTHALPY OF F ORMATION The standard enthalpy of formation of a compound, H f, is the enthalpy change for the reaction that forms one mole of the compound from its elements in their standard states: By definition, H f of the most stable form of any element is zero The enthalpy change of the reaction

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5.7 E NTHALPIES OF F ORMATION S TANDARD E NTHALPY OF F ORMATION

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5.7 E NTHALPIES OF F ORMATION C ALCULATION OF REACTION ENTHALPIES

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5.7 E NTHALPIES OF F ORMATION C ALCULATION OF REACTION ENTHALPIES

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5.7 E NTHALPIES OF F ORMATION 1 mol

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5.7 E NTHALPIES OF F ORMATION Calculate H f of CaCO 3 (s). Calculate H f of CuO(s).

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5.8 F OODS AND F UELS F OODS Fuel value: the energy released when 1 g of a material is combusted The fuel value of carbohydrates: 17 kJ/g (4 kcal/g) The fuel value of fats: 38 kJ/g (9 kcal/g) The fuel value of proteins: 17 kJ/g (4 kcal/g)

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5.8 F OODS AND F UELS F OODS

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5.8 F OODS AND F UELS F UELS

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E X. 5.106 The hydrocarbons acetylene (C 2 H 2 ) and benzene (C 6 H 6 ) have the same empirical formula. Benzene is an aromatic hydrocarbon, one that is unusually stable because of its structure. (a)By using the data in Appendix C, determine the standard enthalpy change for the reaction 3C 2 H 2 (g) C 6 H 6 (l). (b)Which has greater enthalpy, 3 mol of acetylene gas or 1 mol of liquid benzene? (c)Determine the fuel value in kJ/g for acetylene and benzene. (a) H = -631.3 kJ (b) 3 mol acetylene (c) 50 kJ/g (acetylene) and 42 kJ/c (benzene)

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E X. 5.100 How many grams of methane CH 4 (g) must be combusted to heat 1.00 kg of water from 25 C to 90 C, assuming H 2 O(l) as a product and 100% efficiency in heat transfer? 4.90 g of CH 4 (g)