页数：58页文件大小：4.01 MB来源： Atkins’PhysChem物理化学ppt课件
本课件进入热力学第一定律。从能量守恒原理出发，界定系统、环境与边界，区分功与热两类能量传递方式，比较自由膨胀、恒外压膨胀与可逆膨胀的功，并给出焓的定义及其与定压热的关系。
The principle of the conservation of energy Energy can be neither created nor destroyed.
The basic concepts System a reaction vessel an engine an electrochemical cell a biological cell and soon
The basic concepts Boundary The type of system depends on the characteristics of the boundary that divides it from the surroundings.
The basic concepts
The basic concepts
The basic concepts
功、热与能量
2.1 Work, heat, and energy Work is done when an object is moved against an opposing force. The energy of a system is its capacity to do work. When work is done on an otherwise isolated system, the capacity of the system to do work is increased, so the energy of the system is increased.
When the system does work, the energy of the system is reduced and it can do less work than before.
功、热与能量
2.1 Work, heat, and energy When the energy of a system changes as a result of a temp. difference between system and its surroundings we say that energy has been transferred a she at. When heat transfer from surroundings into the system, the capacity of the system to do work is increased.
But not all boundaries permit the transfer of energy even though there is a temp. diff. between the system and its surroundings.
2.1 Work, heat, and energy
2.1 Work, heat, and energy
2.1 Work, heat, and energy Exothermic process Endothermic process temperature fall rise adiabatic
2.1 Work, heat, and energy Endothermic process Exothermic process diathermic
Molecular interpretation of work, heat, and energy Heat is the transfer of energy that makes use of disorderly molecular m
热与功的分子解释
2.2 The First Law The internal energy, U, is a state function in the sense that its value depends only on the current state of the system and is independent of how that state has been prepared. changing anyone of the state variables, such as P, results in a change in internal energy.
The internal energy, U, is an extensive property.
2.2 The First Law An extensive property is a property that depends on the amount of substance in the system (n, V). An intensive property is a property that is independent of the amount of substance in the sample (T, P, ρ).
热与功的分子解释
(a) The conservation of energy From experiments we found that: The internal energy of a system maybe changed either by doing work on the system or by heating it. Heat and work are equivalent ways of changing a system’ s internal energy.
If a system is isolated from its surroundings, then no change in internal energy takes place.
(a) The conservation of energy The First Law of thermodynamics is expressed as: The internal energy of an isolated system is constant. Δ U = q + w (2.2) Acquisitive convention
热与功的分子解释
(b) The formal statement of the First Law The expression of the First Lawineqn 2 have some unsatisfactory features, such a show we define and measure ‘heat’. So now we consider adiabatic system. In an adiabatic system of a given composition, the same increase in tem
(b) The formal statement of the First Law The work needed to change an adiabatic system from one specific state to another specific state is the same however the work is done. Δ wad = U f - U i = Δ U (2.3)
(c) The mechanical definition of heat To strip away the thermal insulation around the system and make it diathermic. To keep the internal energy the same as before qwq = wad – w (2.4) Now
热与功的分子解释
Work and Heat We now p a your attention to infinitesimal changes of state and infinitesimal changes in the internal energy dU. The work done on the system isdw and the energy supplied to itasheatisdq. Then we have: dU = d q + d w (2.5) To useeqn 5 we must able to related qanddw to events taking place in the surroundings.
2.3 Expansion work Expansion work is the work arising from a change in volume. Many chemical reaction result in the generation or consumption of gases, and the thermodynamics characteristics of a reaction depend on the work it can do. 体积功（或膨胀功）
状态函数与内能
(a) The general expression for work In physical definition, the work required move an object a distanced z against an opposing force of magnitude F: d w = - Fdz (2.6) The negative sign means that when the system moves an object against an opposing force, the internal energy of the system doing the work will decrease.
(a) The general expression for work The external pressure: pexOuter force of the piston: F = pexAdw = - pexAdz = - pexdv (2.7) The total work done from VitoVf: (2.8) If the system is compressed, it is still the external pressure that det
(a) The general expression for work Other types of work: non - expansion work, or additional work. 非体积功（其它功） Collected in Table 2.1 (p 39)
(b) Free expansion When a system expands into a vacuum, or free expansion, the external pressure p ex = 0 From eqn 7, d w = 0, and w = 0 (2.9)
(c) Expansion against constant pressure Now suppose that the external p is constant:
(c) Expansion against constant pressure 示功图Indicator diagram
(c) Expansion against constant pressure
(d) Reversible expansion
(d) Reversible expansion A reversible change infinitesimal modification A system is in equilibri
(e) Isothermal reversible expansion Consider the isothermal, reversible expansion of a perfect gas. p = nRT
(e) Isothermal reversible expansion The maximum work is done in the reversible expansion
(f) One time compression
(f) Multi - time compression
(f) Reversible compression The minimum work is done in the reversible compression
2.4 Heat transactions In general, the change in internal energy of a system is: dU = d q + dw
(a) Calorimetry Adiabatic bomb Calorimetry Δ U = qVq = C Δ T Calorimetry constant: C =
(b) Heat capacity The slope of the tangent to the curve of the internal energy plotted ag
(b) Heat capacity The internal energy varies with the temp. and the volume of the sample,
焓随温度变化的计算
Heat capacities are extensive properties. The molar heat capacities are intensive properties. In general, heat capacities depend on the temp. and decrease atlowtemp. But over small range of temp. at and above room temp., the variation is quite small and can be treated as independent of temp.
approximately. (b) Heat capacity
焓随温度变化的计算
For the infinitesimal change of temp. dU = CVdT (at constant volume) (2.21 a) Over small range of temp. Δ U = C V Δ T (at constant volume) (2.21 b) And q V = C V Δ T (at constant volume) (2.22) For a given quantity of heat, large heat capacity leads to a small Δ T, and small CVleads to large Δ T, infinite C V, zero Δ T.
But not the case as it is at a phase transition. (b) Heat capacity
Δ U ≠ q (not at constV) Some of the energy supplied a she at to the system is returned to the surroundings as expansion work. 2.5 Enthalpy 焓
焓
H = U + p V (2.23) The enthalpy, H, is also a state function. Eqn 23 implies that the change in enthalpy is equal to the heat supplied at constant pressure as long as the system does no additional work. d H = d q (at constant p, no additional work) (2.24 a) And Δ H = q p (2.24 b) (a) The definition of enthalpy, H
Δ H = qpqp = C Δ T calorimetry constant: C = q / Δ T (b) The measurement of an enthalpy change Adiabatic flame calorimetry
DSC measures the heat transition atconstp, T = T 0 + α t, Δ T = T - T 0 = α tqp + q p, ex = (Cp + Cp, ex) Δ TCp, ex = q p, ex / Δ T = q p, ex / α t = P ex / α (b) The measurement of an enthalpy change
焓
Δ U is 0.21 k J when 0.1 molCaCO 3 in the form of calcite converts to aragonite. Calculate the difference between Δ Hand Δ U when the pressure is 1.0 bar given that the densities of the solids are 2.71 gcm - 3 and 2.93 gcm - 3, respectively.
Δ H = H (a) - H (c) = {U (a) + p V (a)} - {U (c) + p V (c)} = Δ U + p {V (a) - V (c)} = Δ U + p Δ VVm = M / ρ: 100 / 2.71 = 37 cm 3 (a); 100 / 2.93 = 34 cm 3 (c) Δ H - Δ U = p Δ V = (1.0 × 105 P a) × (34 - 37) × 10 - 6 m 3 = - 0.3 J Ex 2.2: Relating Δ Hand Δ U
For a perfect gas, p V = nRTH = U + p V = U + nRT (2.25) For the reaction with producing or consuming gas: Δ H = Δ U + Δ ngRT (2.26) where Δ ngis the change in the amount of gas molecules in the reaction. Ex: Relating Δ Hand Δ U
The slope of the tangent to a plot of enthalpy against temp. at constant pressure is call the heat capacity of the system at constant pressure, Cp, at a given temp.: (c) The variation of enthalpy with temperature
焓
For the infinitesimal change of temp. d H = CpdT (at constant pressure) (2.28 a) Over small range of temp. Δ H = Cp Δ T (at constant p) (2.28 b) And q p = Cp Δ T (at constant p) (2.22) The variation of heat capacity with temp.
can sometimes be ignored if the temp. range is small. If it is necessary to take the variation into account: (c) The variation of enthalpy with temperature Table 2.2 in p 50
What is the change in molar enthalpy of N 2 when it is heated from 25 º C to 100 º C? Use the heat capacity information in Table 2.2. Ex 2.4 Evaluating an increase in enthalpy with temperature IfCp = 29.14
焓随温度变化的计算
Most systems expand when heated at constant pressure. Such systems do work on the surroundings and therefore some of the energy supplied to them a she at escapes back to the surroundings. There is a simple relation between the two heat capacities of a perfect gas: Cp - C V = n R (2.31) (d) The relation between heat capacities
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