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Chapter 6 Thermochemistry Law of Conservation of Energy Law

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Chapter 6 Thermochemistry Law of Conservation of Energy Law Chapter 6 I Understanding Heats of Reaction - thermodynamics is the science of the relationships between heat and other forms of energy. Thermochemistry refers to the heat absorbed or evolved during a Thermochemistry chemical reaction. LawLaw ofof ConservationConservation ofof EnergyEnergy Energy • Energy is defined as the capacity to move Energy can be converted from one form to matter. another but can neither be created nor Energy can be in many forms: destroyed. >Radiant Energy -Electromagnetic radiation. (E is constant) >Thermal Energy - Associated with random universe motion of a molecule or atom. >Chemical Energy - Energy stored within the structural limits of a molecule or atom. 1 Energy and Its Units - energy is the potential or capacity to move matter 2 1 Kinetic energy. Ek = ½ mv SI unit of energy is the joule (J) which has units kg @ m2/s2. 1 calorie (cal) is equal to the amt of heat necessary to raise 1 g of H2O by 1 oC. 1 cal = 4.184 J Potential energy and kinetic energy. Photo Courtesy of Tennessee Valley Authority. A Problem to Consider • Consider the kinetic energy of a person whose mass is 130 lb (59.0 kg) traveling in a car at 60 mph (26.8 m/s). 1 2 Ek = 2 (59.0 kg)×(26.8 m / s) 4 Ek = 2.12×10 J • The SI unit of energy, kg.m2/s2, is given the name Joule. 2 Energy A Problem to Consider • Potential Energy: This energy depends on the • Consider the potential energy of 1000 lb of water “position” (such as height) in a “field of force” (453.6 kg) at the top of a 300 foot dam (91.44 m). (such as gravity). • For example, water of a given mass m at the 2 Ep = (453.6 kg)×(9.80 m / s )×(91.44 m) top of a dam is at a relatively high “position” 5 2 2 h in the “gravitational field” g of the earth. Ep = 4.06×10 kg⋅m / s 5 Ep = 4.06×10 J Ep = mgh FirstFirst LawLaw Energy • Internal Energy is the energy of the First Law of Thermodynamics: particles making up a substance. •The total energy of a system is the sum of its kinetic energy, potential energy, and internal The energy of the universe is energy, U. constant. Etot = Ek + Ep + U 3 FirstFirst LawLaw TemperatureTemperature v.v. HeatHeat ∆E = q + w Temperature reflects random motions of particles, therefore related to kinetic energy ∆E = change in system’s internal energy of the system. q = heat Heat involves a transfer of energy between w = work 2 objects due to a temperature difference Figure 6.5: A kinetic-theory WorkWork explanation of heat. work = force × distance since pressure = force / area, work = pressure × volume wsystem = −P∆V Where ∆V = Vfinal -Vinitial 4 StateState FunctionFunction First Law of Thermodynamics ·Both work & heat are ways in which energy can be Depends only on the present state of the transferred. system - not how it arrived there. The way that energy transfer is divided between work and heat depends on the conditions of transfer - and is It is independent of pathway. called the “pathway”. However, the total amount of energy transferred (w + q) will always remain constant. Internal Energy is a State Function Campsite to illustrate altitude. Heat of Reaction Internal Energy • Heat is defined as the energy that flows is a State into or out of a system because of a Function difference in temperature between the system and its surroundings. But work and heat • Heat flows from a region of higher are not! temperature to one of lower temperature; once the temperatures become equal, heat flow stops. 5 SystemSystem andand SurroundingsSurroundings Heat of Reaction System: That on which we focus attention • In chemical reactions, heat is often transferred from the “system” to its Surroundings: Everything else in the universe “surroundings,” or vice versa. • The substance or mixture of substances under Universe = System + Surroundings study in which a change occurs is called the thermodynamic system (or simply system.) •Thesurroundings are everything in the vicinity of the thermodynamic system. Illustration of a thermodynamic 1. An exothermic process is a system. chemical change in which heat Photo courtesy of American Color. is evolved (q is negative) 2. An endothermic process is a System = reaction between HCl and NaOH chemical reaction or physical HCl, NaOH, System change in which heat is NaCl, H2O The surroundings = flask, absorbed (q is positive) air, solvent (if not involved in the reaction) - “everything else”! 6 A test tube that contains anhydrous copper (II) sulfate and a thermometer that registers 26.1º C. Endothermic Photo courtesy of and American Color. exothermic. Water has been added. The thermometer now registers 90.2º C. Barium hydroxide octahydrate and Photo courtesy of ammonium salt are mixed. American Color. Photo courtesy of This dissolving American Color. process is exothermic! H2O, -heat CuSO4 (s) º 2+ 2- Cu (aq) + SO4 (aq) 7 The flask and board are frozen ExoExo andand EndothermicEndothermic solidly together. Photo courtesy of American Heat exchange accompanies chemical reactions. Color. Ba(OH)2 8 H2O (s) + Exothermic: Heat flows out of the system º 2 NH4Cl (s) + heat (to the surroundings). BaCl2 (s) + 10 H2O (s) + 2 NH3 (s) Endothermic: Heat flows into the system An endothermic reaction! (from the surroundings). Enthalpy and Enthalpy Change Enthalpy and Enthalpy Change • The heat absorbed or evolved by a reaction • Enthalpy and Internal Energy depends on the conditions under which it – The internal energy of a system, U, is precisely defined as the heat at constant pressure plus the occurs - the “pathway”. work done by the system: Usually, a reaction takes place in an open vessel, and therefore at the constant pressure of the U = q p + w atmosphere. – In chemical systems, work is defined as a change The heat of this type of reaction is denoted qp, the in volume at a given pressure, that is: heat at constant pressure. w = −P∆V (if volume of system expands – it does work on surroundings) 8 C Enthalpy and Enthalpy Change Internal energy (U). The sum of kinetic & potential energies of the atoms making up Enthalpy is a state function. A state function a substance as well as the subatomic is a property of a system that depends only on particles of each atom. its present state, which is determined by variables such as temperature and pressure, and is independent of any previous history of ETotal = Ek + EP + U the system. Thus, changes in enthalpy, ∆H, depend only on the initial and final states of the system not on the path (or way) in which the final state is reached. EnthalpyEnthalpy Enthalpy and Enthalpy Change Enthalpy = H = E + PV (all state functions) • Enthalpy, denoted H, is an extensive property ∆E = ∆H −∆(PV) of a substance that can be used to obtain the ∆H = ∆E + ∆ (PV) heat absorbed or evolved in a chemical reaction. At constant pressure, –Anextensive property is one that depends on the quantity of substance. (other examples - volume, qP = ∆E + P∆V, or mass, energy). An intensive property does not )E = qp -P∆V depend on amount of substance - e.g. temperature, vapor pressure. where qP = ∆H at constant pressure, )E = Uf -Ui – Enthalpy is also a state function, a property of a system that depends only on its present state and is ∆H = energy flow as heat (at constant pressure) independent of any previous history of the system. 9 An enthalpy Enthalpy of Reaction (∆H) is the change diagram. in enthalpy for a reaction at a given temperature and pressure. 2 Na (s) + 2 H2O (l) —> 2 NaOH (aq) + H (g) ∆H = H(products) - H(reactants) 2 = qp (at constant pressure) So ∆H is essentially the heat evolved or absorbed by the system (reaction) in an open vessel where the work portion of ∆U is unmeasured. Figure 6.9: Pressure-volume Measuring Heats of Reaction work. • To see how heats of reactions are measured, we must look at the heat required to raise the temperature of a substance, because a thermochemical measurement is based on the relationship between heat and temperature change. 2 Na (s) + 2 H2O (l) —> 2 NaOH (aq) + H2 (g) • The heat required to raise the temperature of a ) E = (-367.5 kJ - P )V) substance is its heat capacity. 10 HeatHeat CapacityCapacity Measuring Heats of Reaction • Heat Capacity and Specific Heat heat absorbed –The heat capacity, C, of a sample of substance C = is the quantity of heat required to raise the mole substance increase in temperature temperature of one mole of the sample of substance one degree Celsius. J J – Changing the temperature of the sample = or requires heat equal to: mole° C mole K q = nC∆T SomeSome HeatHeat ExchangeExchange TermsTerms specific heat capacity, s Measurement of Heat of Reaction may be heat capacity per gram = J/°C g or J/K g done using a calorimeter. A calorimeter is a q = s m )T , where m is in g, )T = T -T f i device used to measure the heat absorbed or evolved during a physical or chemical change molar heat capacity, C heat capacity per mole = J/°C mol or J/K mol q = nC )T, where n = no. moles, )T = Tf -Ti 11 Heats of Reaction: Calorimetry Coffee-cup calorimeter. •Acalorimeter is a device used to measure the heat absorbed or evolved during a physical or chemical change. (see Figure Reaction is done at 6.11) constant pressure, so • The heat absorbed by the calorimeter and its we are measuring qp or )H contents is the negative of the heat of reaction.
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