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# 热力学第二定律 - PowerPoint PPT Presentation

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### 热力学第二定律

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·气体向真空膨胀

· 高温物体传热给相接触的低温物体

·浓溶液的扩散

·锌片投入硫酸铜溶液中的反应

·电解水的反应

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1．理想气体自由膨胀: Q=W=U=H=0, V>0

U=0, H=0, Q = W，

2．热由高温物体传向低温物体: 冷冻机做功后，系统恢复原状，…

3．化学反应: Cd(s)+PbCl2(aq)=CdCl2(aq)+Pb(s)

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• 克劳修斯（Clausius，1850）的表述：

“不可能把热从低温物体传到高温物体，而不引起其它变化。”

• 开尔文（Kelvin，1851）的表述：

• “不可能从单一热源取出热使之完全变为功，而不发生其它的变化。”

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“第二类永动机是不可能造成的”

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1824 年，法国工程师N.L.S.Carnot (1796~1832)设计了一个循环，以理想气体为工作物质，从高温 热源吸收 的热量，一部分通过理想热机用来对外做功W，另一部分 的热量放给低温 热源。这种循环称为卡诺循环。

N.L.S.Carnot

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 (热机)=W/Q2

(冷冻机)= - Q1/W

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1.恒温可逆膨胀

(p1V1T1)――(p2V2T1)

2.绝热可逆膨胀

(p2V2T1)――(p3V3T2)

3. 恒温可逆压缩

(p3V3T2)――(p4V4T2)

4. 绝热可逆压缩

(p4V4T2)――(p1V1T1)

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1mol理想气体的卡诺循环在pV图上可以分为四步：

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• 根据绝热可逆过程方程式

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“=” 号为可逆过程

“>” 号为自发过程

“=” 号为处于平衡状态

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“>” 号为自发过程

“=” 号为可逆过程

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3 为可逆过程，整个循环为不可逆循环。、理想气体状态变化（仅有体积功)

1、等容变温

2、等压变温

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QR：可逆相变热

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• (a)自由膨胀

• (b)反抗恒定外压Pamb=lOOkPa

• (c)可逆膨胀

• 到末态压力lOOkPa。求各过程的ΔS及ΔSiso

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dU = δQ + δW = δQ + δWe + δW′

=δQ - pdV + δW′

TdS－dU－pdV＋δW′ >0

> 可以发生的不可逆过程

＝假想的可逆过程

< 不可能发生的过程

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－d(U＋pV－TS) >－δW′

（“＜”0为自发, “=0”为可逆,

“＞0”在上述条件下是不可能的﹚

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ΔG 的计算与应用

G≡H－TS≡U＋pV－TS

② 对于固体和液体: ΔGT = V (p2－p1)

ΔG=0 （可逆相变）

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Qp=Qv+nRT

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