CHEM 481 Lecture Notes _Engle 2nd - Chapter 5_

CHEM 481 Lecture Notes _Engle 2nd - Chapter 5_ - Chapter 5-...

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Last printed 2/23/2010 1:07 PM 5-1 Chapter 5- The Second Law G ETTING S TARTED The 2 nd Law of thermodynamics deals with predicting the spontaneity of a process. For the purposes of thermodynamics, we define. .. Spontaneous process A direction of change that does not require work to bring it about. o Confine gas to smaller volume o Cool an object (change its temperature) o Make rxn go in reverse All require work and are NOT spontaneous Direction of Spontaneous Change: What determines the direction of spontaneous change? Is it the energy of the system tending towards a minimum? NO Expansion of a gas into a vacuum (i.e. irreversible expansion with P ext =0) It is spontaneous, but because no work is done (w=0), the internal energy remains constant. (Doesn’t matter if it is adiabatic or isothermal) Spontaneous change is always accompanied by a conversion of energy into a more dispersed form. On each bounce. .. ... Ball induces random thermal motion of atoms in surface, loses some energy and doesn’t bounce as high. ... Energy which is initially all in the ball, is dispersed into the surface. You will never see ball at rest start bouncing. In order for that to happen, the random thermal motion of atoms in the surface would have to change into directed motion upwards. .. Direction of Spontaneous
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Last printed 2/23/2010 1:07 PM 5-2 This is considered so unlikely (improbable) that it is considered impossible . E NTROPY (S) Second Law of Thermodynamics The entropy (S) of an isolated system increases in the course of spontaneous change. Tot S0 Δ > Together the system and surroundings (i.e. the universe) constitute one big isolated system. To predict whether a process is spontaneous must determine how S changes for the whole univers. .. N N sys sur Entropy Entropy Change in Change in Surroundings System dS dS dS =+ We will consider both of these, but to go further we will need to be more specific about the mathematical form of dS Mathematical Form of dS: We might think that. .. N N Reflects Amt of heat change in supplied "order" reflects change in random motion of atoms in surr. dS dq = This would be a bad choice, because q is not a state function. To get correct form consider the heat differential, dq. Random Motion All atoms in surface move upwards at the same time Surroundings System
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Last printed 2/23/2010 1:07 PM 5-3 Heat differential : To motivate this discussion, we will consider a specific case, but the result is general Specific Case q rev (V,T) supplied to an ideal gas for small changes in V & T along a reversible path . V rev rev rev VT Constant V, No additional work C qq dq dT dV TV ∂∂ ⎛⎞ =+ ⎜⎟ ⎝⎠ ±²³ ²´ Derivative in 1 st term rev V V q C Constant V, No additional work T = Derivative in 2 nd term N ext int rev bcz rev rev T dU 0 (isothemal) nRT dq dw P dV P dV dq dV V q nRT VV = =− = = = = Is dq rev exact? Test for exactness… v rev rev
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CHEM 481 Lecture Notes _Engle 2nd - Chapter 5_ - Chapter 5-...

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