free energy - does this show stability aswell as spontanaeity? Watch

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ryan750
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hi guys -
i was doing my chemistry industrial and got stuck because the book had sum free energy values of Al2O3 and of Fe2O3 aswell as their enthalpies of formation. They are all negative which i presume means that the oxides of aluminium and iron are more stable than reactants and that the reactions are spontaneous. The aluminium oxide was more negative for delta H and delta G than the iron oxide was. Thus just putting the aluminium oxide into context of its relative stability.
But the book seemed to be focusing on the free energy values as the key to their stability. It says " the high negative free energy of formation of its oxide - delta Gf, make electrolysis of a melt the only feasible method of extraction." then

"here the relative values of delta Hf provide a satisfactory guide to stability of the oxides, as seen by comparison with thise for delta Gf."
the values are:

Al2O3 delta Gf = -1580 delta Hf = -1680
Fe2O3 delta Gf = -820 delta Hf = -740

does the fact that free energy of Al2O3 is less negative than its enthalpy of formation have anything to do with it? because this is not so with Fe2O3.

Please help me - im so confused - the book says that a full explanation of free energy exceeds the scope of the book - i wish that edexcel had entropy on the syllabus because apparently it explains everything.

the text in bold are the things that are confusing me or i am unsure about - please help or even if you cant just write sumthing anyway because i made a post the other night about hysteresis in springs and no one replied. :confused:
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oxymoron
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Feeling sorry for you as no one has replied ... so - here's my random thoughts on entropy which may or may not help.

Entropy is the amount of disorder in a system. The 2nd law of thermodynamics says that the entropy of the universe always increases.
However, this does not mean that entropy of the chemical system always increases in a spontaneous reaction. For example take the condensation of water vapour into water:
Here the entropy is clearly decreasing as the system is becoming more ordered (molecules in water are more ordered than in a gas), but because heat is given out in the reaction (exothermic reaction), this heat increases the entropy of the surroundings (by increasing the molecules' random motion). This increase in surroundings entropy more than compensates for the drop in entropy of the system to make the entire entropy of the universe increase in this reaction.

It is hard to guess what the entropy change of a reaction is like, but usually if the number of molecules is increasing, entropy will increase also:

eg CaCO3 --> CaO + CO2 will have a positive entropy.

If you call the change in entropy of the chemical system in a chemical reaction *S (where * is delta cos I don't know how to type one) and the change in enthalpy *H, then there is an equation defining the gibbs free energy, *G (at temp T) as:

*G = *H - T*S (*'s are all delta signs)

So as you can see, if the *G term is GREATER than the *H term, then entropy change must be negative.
If the *G term is LESS than (ie more negative than) the *H term , then the entropy change is positive.

If the *G term is negative, then the reaction is spontaneous ... ie it is thermodynamically favourable - note this does not say anything about the rate of reaction, as this will depend also on the activation energy.

(I realise that I have not really answered your questions - but I thought this may be useful as you were asking about entropy).

To attempt to answer your first bold point ... because it's oxide has a high delta Gf, it is very thermodynamically stable with respect to the elements (which is what we want to extract here). This means it is hard to break it down by other methods (such as displacement by another element). This is why electrolysis is the only feasable way of extracting it (in an economicallly viable way).
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phishfood
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free energy change dictates spontanaiety, because essentially a process that changes a system from a higher free energy state to a lower free energy state will happen spontaneously. in the universe, all matter wants to go to lower free energy, which might be lower enthalpy but might also be greater entropy. negative free energy change is the interplay of these factors, and if dG is negative then the process is favourable and leads to a real-world state that is more stable

i think the point your book was making was that 'relative values of delta H provide a satisfactory guide' to stability. free energy change is linked to enthalpy change (one goes up, other goes up), and so in the examples in the book, the enthalpy changes are a good indicator of stability since they are indicative also of free energy change.
it is because of the entropy change also involved, that enthalpy change is not the most precise indicator of stability, and in cases where the TdS term is very large, enthalpy change is no longer sufficient to explain stability. this is why in these cases you also have to consider order/disorder. so if a salt dissolving in water is endothermic, it will still happen even though the enthalpy change says the solution is less stable than the undissolved salt, since the solution is more disorderly (which the universe likes, as 'twere)
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phishfood
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the difference between free energy changes and enthalpies of formatoin illustrates the TdS term in oxymoron's equation, so the order/disorder component can be important, but only in the context of how much difference it makes to the overall free energy change.

there must be books for which a 'discussion' is not beyond its scope tho. i'm sure we need to know free energy for A2
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ryan750
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thanks guys - explained alot
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Ben.S.2
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(Original post by ryan750)
thanks guys - explained alot
Of course it's a measure of stability! Stable molecules tend not to spontaneously react with themselves etc. - reflected in a large positive value of delta G for the reaction in which they are destroyed/altered. A large, negative delta G for the formation implies a large positive delta G for the destruction and, hence, indicates that the products of the reaction are stable (exist in a potential energy well).

Ben
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