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Unformatted text preview: 11/7/11 Thermodynamic Deﬁni�ons Thermodynamic Deﬁni�ons Thermodynamics is the science of heat and work. It is a precise quan�ta�ve subject whose concepts must be deﬁned carefully. One fundamental mo�on is the state of a system. In thermodynamic analyses, we o�en describe the state of a system or changes in the state of a system, but what do we mean by system and state? A thermodynamic system is any part of the universe that we want to describe and study by itself. For example, an automobile, a thundercloud or the earth. A�er we have selected a par�cular thing to be a system, the rest of the universe is deﬁned to be the surroundings. System + Surroundings = Universe Thermodynamic Deﬁni�ons Thermodynamic Deﬁni�ons The condi�ons that describe a system are collec�vely called its state. When these condi�ons change, we speak of a change of state. Condi�ons that must be speciﬁed to establish the state of a system are called state variables. For chemical systems, variables are o�en familiar quan��es: Pressure (P), Volume (V), Temperature (T), amounts of substances (n). Frequently, state variables are related in some way. A mathema�cal equa�on that describes such a rela�onship is an equa�on of state (e.g. PV = nRT). Changes of state occur during most interes�ng processes in thermodynamics. Energy Energy is the founda�on of thermodynamics. It is neither created nor destroyed in any process although it may be transferred from one body to another or converted from one form into another. Energy is ALWAYS conserved. Energy is the capacity to do work or move ma�er. Energy-‐releasing reac�ons are said to be exothermic. Energy-‐absorbing reac�ons are said to be endothermic. 1 11/7/11 Energy Kine�c and Poten�al Energy The understanding of energy ﬂow and conserva�on is complicated because energy takes on several diﬀerent forms. Kine�c, poten�al, thermal and radiant energy all play important roles in chemistry. Any moving object possesses energy of mo�on which is called kine�c energy (KE). KE varies with the mass of the body and the velocity at which it is moving. KE = mv2/2 The eq. states that the units for E be (mass)(distance)2
(�me)-‐2, so the SI unit for KE is (kg)(m)2(s)-‐2. It always has these same dimensions and units. It is called the Joule (J). Energy = (mass)(distance)2/(�me)2 = (kg)(m)2/(s)2 = Joules (J). Kine�c and Poten�al Energy Kine�c and Poten�al Energy Poten�al energy (PE) is stored energy. For example, a rock teetering high on a ledge is about to release stored gravita�onal energy, a cloud on the verge of “hurling” a thunderbolt earthward is about to release stored electrical energy and gasoline in the cylinder of your car engine is about to release stored chemical energy. In each case, the stored energy is called PE. Energy and Heat In chemistry, one of the most important forms of E is thermal mo�on. A change in thermal mo�on of a system requires a ﬂow of E and a transfer of thermal energy is known as heat (q). Heat transfers are changes in E so they are measured in Joules (J). A ﬂow of heat frequently causes a temperature (T) change. T is not E but it is related to E. A change in an object’s T (ΔT) depends on three factors: 2 11/7/11 Energy and Heat Energy and Heat 1. ΔT depends on q, the amount of heat that has been transferred. That is, the transfer of 50 J of heat to an object causes an increase in T that is twice as large as the increase caused by 25 J of heat. 2. ΔT depends on the amount of material. That is, the transfer of 50 J of heat to 1 mol of a substance causes a T increase that is twice as large as the increase caused by the transfer of 50 J of heat to 2 mol of the same substance. 3. ΔT depends on the iden�fy of the material. For instance, 50 J of heat increases the T of 1 mol of Au more than it increases the T of 1 mol of water. This sensi�vity to heat is expressed by a quan�ty called the: speciﬁc heat capacity (C) – the ra�o of the heat supplied to some mass of the substance (say, 1.00 g) to the consequent rise in the substance’s T. C = q / mΔT molar heat capacity (Cp) – the heat needed to reaise the T of 1 mol of substance by one kelvin (1 K). Cp = q / nΔT Energy and Heat Problems In both these equa�ons, q is the heat transferred, m is the mass of material involved and n is the number of moles of material involved. These eqns. tell us that the larger C and, hence, Cp are, the more thermal E the substance can store. Also, substances with a high C and Cp cool down more slowly than those with a smaller C and Cp. An aluminum frying pan that weighs 745 g is heated on a stove from 25oC to 205oC. What is q for the frying pan? A thirsty marathon runner pours 200 mL of Gatorade from a can at 25oC. What is the minimum mass of ice that must be added to cool the drink to 0oC? Problems First Law of Thermodynamics Phoebe’s insulated foam cup is ﬁlled with 150ml of coﬀee at (c = 0.907 cal/g °C) 70 °C. It is too hot to drink, so she adds 10ml of milk (c = 0.907 cal/g °C ) at 5.0°C. What is the ﬁnal temperature of the coﬀee and milk mixture? (hint: treat it like a water mixing problem) Emily is tes�ng her baby’s bath water and ﬁnds that it is too cold so she adds some hot water from a ke�le on the stove. If Emily adds 2.00kg of hot water at 80°C to 20kg of bath water at 27°C, what is the ﬁnal temperature of the bath water? E can be transferred as heat or work. Heat and work can be transferred into or out of a system. During a chemical rxn, changes in chemical E may cause heat transfer and/or work transfer. Because E must be conserved, the E change of the system is linked to the ﬂow of heat and work: ΔEsys = qsys + wsys 3 11/7/11 First Law of Thermodynamics First Law of Thermodynamics Scien�sts have found that heat and work transfers are suﬃcient to account for the E change that accompanies any process. ΔEsys = qsurr + wsurr = (-‐qsys) + (-‐wsys) = -‐(qsys + wsys) = ΔEsys This expression is a restatement of the law of conserva�on of E: Any change in the E of a system must be counterbalanced by an opposite change in the E of the surroundings. The 1st law of thermodynamics states that ΔE, the E change of any system, is equal to the heat absorbed by the system plus the work done on the system. Any heat absorbed by the system increases the system’s E and any work done on the system likewise increases the system’s E. The surroundings must provide this E if the total E of the universe is to be conserved. Thus, as the system absorbs heat, the surroundings lose E, and as work is done on the system, the surroundings lose the E needed to do that work. A system o�en First Law of Thermodynamics Calorimeter undergoes a change in E during a process but the surroundings undergo an equal and opposite change in E so the total E of the universe remains unchanged. A device that measures heat ﬂow is called a calorimeter. In a calorimetry experiment, a set of chemicals undergoing a change in E is enclosed in the water bath. The calorimeter and the chemical that it contains act as an isolated system because the insula�on blocks the ﬂow of heat between system and surroundings: Calorimeter Calorimeter qsurr = qsys = 0 So, qsys = qcalorimeter + qchemicals = 0 Thus, qchemicals = -‐qcalorimeter and qcalorimeter = CcalΔT Total heat capacity of the calorimeter is usually determined by measuring ΔT that accompanies the transfer of a known amount of heat. 4 11/7/11 Problems Problem A calorimeter is calibrated using an electrical heater. Before turning on the heat, the calorimeter is 23.55oC. The addi�on of 10.00 kJ of electrical E raises the T to 24.67oC. Determine the total heat capacity of this calorimeter. Ammonium nitrate (NH4NO3) is a salt used in cold packs for icing injuries. When 20.0 g of this salt dissolves in 125 g of water in a coﬀee-‐cup calorimeter, the T falls from 23.5oC to 13.4oC. Is the chemical process exothermic or endothermic? Determine q for the calorimeter. When a 1.000 g sample of the rocket fuel hydrazine, N2H4, is burned in a bomb calorimeter which contains 1200 g of water, the temperature rises from 24.62°C to 28.16°C. If the C for the bomb is 840 J/°C, calculate qreac�on for combus�on of a one-‐gram sample qreac�on for combus�on of one mole of hydrazine in the bomb calorimeter Problem The following acid-‐base reac�on is performed in a coﬀee cup calorimeter: H+(aq) + OH-‐(aq) → H2O(l) The temperature of 110 g of water rises from 25.0°C to 26.2°C when 0.10 mol of H+ is reacted with 0.10 mol of OH-‐. Calculate qwater Calculate ΔH for the reac�on Calculate ΔH if 1.00 mol OH-‐ reacts with 1.00 mol H+ 5 ...
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- Fall '11