页数：38页文件大小：1.92 MB来源： Atkins’PhysChem物理化学ppt课件
本课件延续第一定律的应用：先介绍弹式量热计与火焰量热计测定焓变的方法，再给出标准生成焓、标准燃烧焓的定义与盖斯定律的计算技巧，最后用基尔霍夫定律处理反应焓随温度的变化。
Brief review System, (open, closed, isolated), surrounding Work, w, expansion work (free, p ex, p in) Heat, q, exothermic, endothermic diathermic, adiabatic process w, q, path - dependence Internal energy, U = q V Enthalpy, H = qpH = U + p V
Heat capacity, Cp - C V = nRfor a perfect gas Δ H = Δ U + Δ ngRT for the reaction with
The variation of enthalpy with temperature
Adiabatic expand of a perfect gaspiViTipfVfTf
(1) ViTiVfTi Δ U = 0 (2) piVfTipfVfTf Δ U = C V Δ T = wad (a) The work of adiabatic change
The reversible adiabatic expansion dU = CVdT = - pdVVfTfc = ViTicc = C V, m / R (a) The work of adiabatic change
The adiabatic expansion at constant pressure (a) The work of adiabatic change
p V γ = constant (b) Heat capacity ratio and adiabats
(b) Heat capacity ratio and adiabatspV γ = constant TV γ － 1 = constant
Application of the First Law to a perfect gas Δ U 0 - p ex Δ VnRTlnw 0
热力学第一定律：基本概念
The study of heat produced or required by chemical reaction is called thermochemistry. A branch of thermodynamics System: reaction vessel and its contents Energy or mass exchange with surroundings Reaction heat, q = Δ U (at const.
V) or q = Δ H (at constp) For an exothermic process, Δ U < 0 or Δ H < 0 For an endothermic process, Δ U > 0 or Δ H > 0
热力学第一定律：基本概念
Standard enthalpy changes: Δ H The standard state of a substance at a specified temperature is its pure form at 1 bar. Δ H for a reaction or a physical process is the difference between the products in their standard states and the reactants in their standard states, all at the same specified temperature, usually, 298.15 K (25.00 ℃)
Δ H in a
Δ H in a change of physical state is called the standard enthalpy of transition, Δ trsH (table 2.4) The standard enthalpy of fusion: Δ fusH H 2 O (s) H 2 O (l) The standard enthalpy of vaporization: Δ vapH H 2 O (l) H 2 O (g) The standard enthalpy of sublimation: Δ subH H 2 O (s) H 2 O (g) (a) Enthalpies of physical changes
Δ subH = Δ fusH + Δ vapH (a) Enthalpies of physical changes
Δ H (A B) = - Δ H (B A) (a) Enthalpies of physical changes
热力学第一定律：基本概念
Two ways to describe the enthalpy change in a chemical reaction. Thermochemical equation: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) Δ H = - 890 k J 2 CH 4 (g) + 4 O 2 (g) 2 CO 2 (g) + 4 H 2 O (l) Δ H = - 1780 kJPure, unmixed reactants in their standard states pure, unmixed products in their standard states Chemical equation and standard reaction enthalpy CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) Δ r H = - 890 kJmol - 1 (b) Enthalpies of chemical changes
热力学第一定律：基本概念
For reactions: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) Δ r H = - 890 k J mol - 1 C 6 H 12 (s) + 6 O 2 (g) 6 CO 2 (g) + 6 H 2 O (l) Δ r H = - 2808 k J mol - 1 Δ r H can be written as Δ cH , The standard enthalpy of combustion The complete oxidation of one mol organic compound to CO 2 gas and liquid H 2 O if the compound contains C, H, and O, and to N 2 if N is also present.
(b) Enthalpies of chemical changes
热力学第一定律：基本概念
The standard enthalpy of an overall reaction is the sum of the standard enthalpies of the individual reactions into which a reaction may be divided. individual reaction: may be hypothetical reactions path - independence of the value of Δ r H q 3 = q 1 + q 2 = q 4 + q 5 + q 6 The path should all be atconstVor constp.
(c) Hess’ s law (Г. И. Г е с с 盖斯）
盖斯定律
The importance of Hess’ s law: For those reactions which is difficult to determine or control directly, difficult to realize, complex to determine can be assembled from information on other reactions through add or subtract them.
(c) Hess’ s law (Г. И. Г е с с 盖斯）
热力学第一定律：基本概念
For the reaction, Δ r H = - 124 k J mol - 1 CH 2 = CHCH 3 (g) + H 2 (g) CH 3 CH 2 CH 3 (g) For propane, Δ cH = - 222 - k J mol - 1 CH 3 CH 2 CH 3 (g) + 5 O 2 (g) 3 CO 2 + 4 H 2 O (l) Calculate the standard enthalpy of combustion of propene.
The requiring reaction isCH 2 = CHCH 3 (g) + 9 / 2 O 2 (g) 3 CO 2 + 3 H 2 O (l)
盖斯定律算例
Assemble the reactions: Δ r H / (k J mol - 1) CH 2 = CHCH 3 (g) + H 2 (g) CH 3 CH 2 CH 3 (g) - 124 CH 3 CH 2 CH 3 (g) + 5 O 2 (g) 3 CO 2 + 4 H 2 O (l) - 2220 H 2 O (l) H 2 (g) + ½ O 2 (g) + 286 CH 2 = CHCH 3 (g) + 9 / 2 O 2 (g) 3 CO 2 + 3 H 2 O (l) - 2058
标准生成焓
Standard enthalpy of formation Δ f H of a substance is the standard reaction enthalpy for the compound from its elements in their reference states. The reference state of an element is its most stable state at the specific temperature and 1 bar.
2.8 Standard enthalpies of formation
标准生成焓
Δ f H are expressed as enthalpies per mole of the compound. Δ f H of elements in their reference states are zero at all temperatures. For liquid benzene or solid sodium chloride at 298 K, Δ f H refers to: 6 C (s, graphite) + 3 H 2 (g) C 6 H 6 (l) + 49.0 k J mol - 1 N a (s) + ½ Cl 2 (g) NaCl (s) - 411.15 k J mol - 1 2.8 standard enthalpies of formation
2.8 standard enthalpies of formation 稳定态单质自身的标准摩尔生成焓为零。完全燃烧产物的标准摩尔燃烧焓为零
(a) The reaction enthalpy in terms of Δ f H
Δ r H = {Δ f H (H 2 O 2, l) + 4 Δ f H (N 2, g)} - {2 Δ f H (H N 3, l) + 2 Δ f H (NO, g)} = {- 187.78 + 4 (0)} - {2 (264.0) + 2 (90.25)} = - 896.3 k J mol - 1 (a) The reaction enthalpy in terms of Δ f H ⊖
标准生成焓
Can we construct Δ f H from a knowledge of the chemical constitution of the species? A very approximate procedures based on mean bond enthalpies, Δ H (A – B). A – B (g) A (g) + B (g) Δ H (A – B) The average enthalpy change associated with the breaking of a specific A – B bond.
(b) Group contributions
A molecule is regarded as being built up of thermochemical groups, anatomor physical group of atoms bound to atleast two other atoms. for example: (b) Group contributions … Table 2.7
标准生成焓
Using the thermochemical group, approach to estimate the at 298 K of hexane in (a) the gas phase, (b) the liquid phase. 2 C (H) 3 (C) and 4 C (H) 2 (C) 2 Δ f H (C 6 H 14, g) = - 167.1 k J mol - 1 Δ V H (C 6 H 14, l) = 33.1 k J mol - 1 Δ f H (C 6 H 14, l) = - 200.2 k J mol - 1 The experimental value is – 198.7 k J mol - 1 (b) Group contributions
It is difficult to estimate Δ f H of conformational isomers. Commercial software packages have been used to estimate Δ f H . (c) Δ f H and molecular modelling equatorial axial
Δ r H has to be measured at different temperature. In the absence of this information, Δ r H maybe estimate from heat capacities and reaction enthalpies at some other temperatures. 2.9 The temperature dependence of reaction enthalpies
This is called Kirchhoff’ s law. (基希霍夫定律) A good approximation to assume that Δ rCpis independent of the temperature. 2.9 The temperature dependence of reaction enthalpies
反应焓的温度依赖性
At 298 K, Δ f H (H 2 O, g) = - 241.82 k J mol - 1 Estimate its value at 100 ℃. H 2 O (g): 33.58 J K - 1 mol - 1, H 2 (g): 28.84 J K - 1 mol - 1 O 2 (g): 29.37 J K - 1 mol - 1 Δ r H (T 2) = Δ r H (T 1) + (T 2 - T 1) H 2 (g) + ½ O 2 (g) H 2 O (g) = (H 2 O, g) - {(H 2, g) + ½ (O 2, g)} = - 9.94 J K - 1 mol - 1 Δ f H (373 K) = - 241.82 k J mol - 1 + (75 K) × (- 9.94 J K - 1 mol - 1) = - 262.6 k J mol - 1
反应焓的温度依赖性
1.8 (a): A car tyre was inflated to a pressure of 24 lbin - 2 on a winter’ s day when the temperature was - 5 ℃. What pressure will be found, assuming no leaks have occurred and that the volume is constant, on a subsequent summer’ day when the temp.
is 35 ℃? P f / T f = P i / T i Internal pressure = pumpp + atmospheric p
About homework Problem 1.5 At the given pressures, D ME cannot be treat
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