CHEM UNIT 4: ENTROPY

Entropy tells you how much disorder there is

• it is a measure of ways that the energy can be shared out between the particles and the number of ways the particles can be arranged
• substances are more energetically stable when there's more disorder so they naturally move to try t increase the entropy
• physical state affects entropy - solid < liquid < gas
• amount of energy  substance has also affects entropy - energy can be measured in quanta (fixed 'packages' of energy), more energy quanta a substance has the more ways it can be arranged and therefore the greater entropy it has
• more particles = more netropy: N2O4(g) --> 2NO2, energy increases because the no. of moles increases

entropy of a substance increases with temp.

raising temp, increases the amount of energy particles have - more ways this energy can be distributed = higher entropy

• Standrad entropy - entropy of 1 mole of that substance under standard conditions (100kPa pessure and 298K) Units: JK^-1mol^-1
• simple atoms or molecules tend to have lower standard entropies than more complex molecules

entropy increase may explain spontaneous endothermic reactions

-normally have to supply energy to make an endothermic reaction happen, but if entropy increases enough, the reaction will happen by itself

NaHCO3(s) + H+(aq) --> Na+(aq) + CO2(g) + H2O(l)

=large increase in entropy - product has mre particles and are in higher entropy states overall (gas and liquid rather than a solid)

For a spontaneous reaction, the total entropy change must be positive

∆S_total = ∆S_system + ∆S_surroundings ∆Ssystem = Sproducts - Sreactants ∆Ssurroundings = -∆H/T

• total entropy of a system and its surroundings has to increase for a spontaneous reaction to happen
• If Stotal is positive the reaction is kinetically favourable - can happen spontaneously
• If Stotal is negaive the reaction is kinetically stable - won't react on their own
• Ssurroundings = - H/T - in an exothermic reaction  H is negative, so it is always positive - heat given out by the system increases te entropy of the surroundings  :   in an endothermic reaction  H is positive so it is always negative - heat taken out by the system lowers the entropy of the surroundings
• At higher temps, Ssurroundings gets smaller so as a result it makes a smaller contribution to the total entropy change than it does at lower temps
• if the increases in Ssystem is big enough in an endothermic reaction then it will happen

Just because a reaction can happen, doesn't mean it wll happen fast

• a spontaneous reaction or change happens without you having to input energy - it is thermodynamically favourable
• just because a reaction can happen spontaneously doesn't mean that it happens quickly. reaction kitnetics will tell you this.

Enthalpy change of solution - overall effect on enthalpy when something dissolves

Enthalpy change of solution - enthalpy change when 1 mole of solute is dissolved in sufficient solvent that no further enthalpy change occurs on further dilution, e.g. NaCl(s) ---> NaCl(g)

Standard Lattice energy - the enthalpy change when 1 mole of a solid ionic compound is formed from gaseous ions under standard conditions e.g. Na+(g) + Cl-(g) --> NaCl(s)

Enthalpy change of hydration - enthalpy change wen 1 mole of aqeous ions is formed from gaseous ions, e.g. Na+(g) --> Na+(aq)

[Use enthalpy cycles to find the enthalpy change of solution]

Ionic charge and size affects lattice energy

• larger the charge on the ions, the more energy is released when an ionic lattice forms = lattice energy will be more negative
• smaller the ionic radii of the ions involved the more exthermic (more negative) the lattice enthalpy
• factors that affect the lattice enthalpy also affect the hydration enthalpy. so the smaller the highly charged ion, the more negative the hydration enthalpy

Dissolving also involves entropy changes