thermodynamics

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spontaneous and non spontaneous changes

 a spontaneous change is one which 

a. proceeds on its own without any external influence under a given set of conditions and, 

b. cannot be brought back to its original state unless external influence is exerted

eg.  diffusion of gas particles, violent reaction between magnesium metal and water

a non-spontaneous change is one with 

a. no natural tendency to occur

eg.  forming back of gas in small volumes after diffusion, combining NaOH and H2 to form Mg and water again

CENTRAL POINT : processes which are spontaneous in one direction are non-spontaneous in the opposite direction. (i.e. non-reversible)

 

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the delta H

delta H is the measure of heat content of a rxn

most energetically favoured (exo) rxns are spontaneous

most energetically not favoured (endo) rxns are non-spontaneous 

therefore, gibbs free energy change : delta G = delta H -T delta S should be used to find spontaneous and non-spontaneous rxns

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the delta S

delta s, entropy is the measure of the number of ways in which particles and energy can be arranged and distributed in a system 

- increasing no. of ways to distributed particles and energy will increase magnitude of entropy (delta S)

- definition :  delta S Sfinal - Sinital ( units : J mol-1 K-1 )

increase in S = change in S is +ve 

decrease in S = change in S is -ve

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factors affecting entropy of system

properties :

a. kinetic energy (K.E.) determines the no. of ways particles can be arranged, where max. K.E. = gaseous state 

b. no of mols. determines no. of ways to distribute particles 

(SEE FLASH CARDS FOR FACTORS!! )

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the delta G

delta G : gibbs free energy when 1 mol of substance formed from constituent elements at r.t.p.

both enthalpy and entropy, plus temperature is important for the determination of a spontaneous reaction, hence the equation : delta G = delta H - T delta S (units : KJ mol-1) 

(http://www.charbase.com/images/glyph/120563)represents standard conditions of 298k and 1 bar

delta G < 0 : rxn is energetically feasible in the forward reaction

delta G > 0 : rxn is not energetically feasible ; rxn can only b carried out w more energy supplied , but is energetically feasible in the reverse direction

delta G = 0 : system at equilibrium, rxn is reversible and there is no net change in amt of pdts and rxtants 

FOR A PROCESS TO B ENERGETICALLY FEASIBLE : deltaH - TdeltaS < 0

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impt assumption to take note :

whilst calculating delta G for ALL cases, delta H and delta S remain constant at all temperatures, as long as there is no phase change.

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