Entropy: How far

Notes on chapter two, edexcel a2 chemistry 2nd edition george facer

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Entropy ­ How far? Revision
The second law of thermodynamics states that spontaneous changes result in disorder or entropy.
This dictates whether or not the reaction will take place (spontaneity) at a particular temperatures
and the position of equilibrium. It is split into entropy of system and entropy of surroundings, so
Stotal = Ssurroundings + S system
If s-total is positive, the change is thermodynamically spontaneous. The value of s-total can be
altered if you change the temperature. S-total also determines the extent of the reaction. The more
positive the value, the more the position of equilibrium will lie to the right.
Entropy change of the system
This is affected by the reaction equation. Entropy of a solid has the lowest entropy, and gases have
the highest.
P4(s) + 5O2(g) P4O10(s)
The right hand side has more solids, which have lower entropy compared to the 5 moles of gas on
the left hand side, so the entropy of the system decreases, the S system is negative. Each component
in the reaction has a standard entropy value, so a numerical value for the S system can be calculated
by the following equation:
S system = nS(products) - nS(reactants)
n = stoichiometric numbers in the chemical equation
Entropy change of the surroundings
This is affected by the environment of the surroundings. Heating the surroundings increases its
Ssurroundings is negative for all endothermic reactions (as endothermic reactions take in heat from the
environment) and positive for all exothermic reactions(as exothermic reactions release heat into the
Ssurroundings = - H
If the temperature increases for an exothermic reaction, Ssurroundings becomes less positive, this in
turn will make Stotal less positive. If temperature increases for endothermic reactions, Ssurroundings
becomes less negative; this in turn will make Stotal less negative.

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If the surroundings are hot, the entropy increase is small because the molecules all have high entropy
and are already in chaotic motion.
Ionic Compounds
As it dissolves, the lattice structure of an ionic solid breaks down and the ions are separated. This is
very endothermic, so you might expect this not to be feasilble. However, due to the highly
exothermic nature of hydration (when strong ion-dipole forces form between the cations and the -
oxygens and between the anions and the + hydrogens).…read more


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