Module Five [[Thermodynamics]] Entropy and Free Energy

Spontaneous Reactions

* Spontaneous reactions occur without having energy supplied to them from the surroudings.

* Most spontaneous reactions which occur are exothermic (gives out heat) as endothermic reactions require heat from t he surroundings so should not occur spontaneously.

* As always, there are exceptions to the rule, and some endothermic reactions do occur spontaneously.

NaHCO3(s) + HCL ---> NaCL(aq) + H2O(l) + CO2(g)

*This indicates that there is another factor (other than enthalpy) which decides whether or not a reaction is spontaneous

Entropy

* The measure of disorder within a system (the randomness of a system)

* As the disorder increases, then the entropy increases

* Solids are highly ordered and so have low entropy values

* Liquid particles have more room to move around, so they are less ordered, and have higher entropy values

* Gaseous particles move with rapid, random motion and so are even less ordered and have even higher entropy values.

Gas>Liquid>Solid

* Simple structures have high entropy

* S is the symble

* Units are Jk-1 mol

* Entropy Values in Joules not KJ

* Ions and molecules in solution have higher entropies than their corresponding solids.

ANY REACTION THAT RESULTS IN THE FORMATION OF GAS, OR AN INCREASE IN THE NUMBER OF GASEOUS MOLES, WILL RESULT IN AN INCREASE OF ENTROPY.

'Delta S' (change in entropy) can be calculated:

Sum of the Entropy of the Products - Sum of the Entropy of the reactants = delta S

Delta S = Heat energy Transfered (delta H)/Temperature (T)

The second law of thermodynamics states that all chemical and physical reactions must result in an increase in entropy, as the total entropy of the universe cannot decrease. Therefore, all spontaneous reactions must result in an increase in enthalpy.

Free Energy

A reation is feasible when delta G is negative.

Delta G = delta H - (T x Delta S)

* Spontaneous Reaction: one which occurs without extra energy being added to it

*Feasbility: One which is thermodynamically possible, but may not occur due to large acctivation energy needed.

DeltaH DeltaS DeltaG Feasibility

-ve +ve -ve always
+ve -ve +ve never
-ve -ve -ve (deltaH>TdeltaS) feasible (deltaH>TdeltaS)
+ve +ve -ve (TdeltaS>deltaH) feasible (TdeltaS>deltaH)

Text Book Summary

* Entropy is a measure of disorder of a system

* Spontaneous reaction occurs without the help of an outside agency

* reation is feasible when the free energy is negative or zero

* a feasible reaction may not occur due to activation energy needed

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Chemistry for AQA (text book)