Energy

Nota:

• The enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states under standard conditions e.g. K(s) + 1/2Cl2(g) -> KCl(s)
• Usually exothermic for an ionic compound.

Nota:

• The enthalpy change that takes place when one mole of gaseous atoms forms from the element in its standard state E.g. K(s) -> K(g) or 1/2Cl2(g) -> Cl(g)
• Always exothermic as a bond is being broken

Nota:

• The enthalpy change the accompanies the removal of one electron from each atom to form one mole of gaseous 1+ ions  E.g. Ca(g) -> Ca+(g) + e^-
• Always endothermic as the electron being lost has to overcome attraction from the nucleus

Nota:

• The enthalpy change that accompanies the addition of an electron to one mole of gaseous atoms to form one mole of gaseous 1- ions E.g. Cl(g) + e^- -> Cl^-(g)
• Always exothermic as the electron is attracted into the outer shell of an atom by the nucleus

Nota:

• If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route,

Nota:

• The energy change that accompanies the formation of one mole of an ionic compound from its gaseous elements under standard conditions e.g. 2K^+(g) + S^2-(g) -> K2S(s)
• This is an exothermic change Indicates the strength of an ionic lattice, higher the lattice enthalpy the stronger the bond Covalent structures do not have a lattice enthalpy as their are no ions in the structure. Impossible to measure directly as you cant form one mole of a ionic lattice in gaseous form

Nota:

• The energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one mole of H2O(l) under standard conditions Ionic equation = HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)

Nota:

• The enthalpy change that takes place  when one mole of a compound is completely dissolved in water under standard conditions. E.g. KCl(s) +aq -> K^+(aq) + Cl^-(aq)

Nota:

• The enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions e.g. K+(g) + aq -> K+(aq)

1. Lattice enthalpy
   1. Ionic radius
      1. As ionic radius increases, the attraction between the ions is weaker and the lattice enthalpy becomes less exothermic
         1. The bonds are weaker and therefore the boiling and melting points are lower
      2. For a smaller radius, there is greater attraction from a larger charge density
   2. Ionic charge
      1. The stronger the charge, the more exothermic the lattice enthalpy
2. Enthalpy change of hydration
   1. Ionic radius
      1. As smaller molecules exert more attraction on water molecules releasing more energy, as the radius increases, the reaction becomes less exothermic
   2. Ionic charge
      1. As the charge increases, the radius decreases so the attraction increases, making the enthalpy change more exothermic

Nota:

• A quantitiative measure of disorder in a system
• As molecules are always moving, S is always positive

Nota:

• ∑S(products) - ∑S(reactants)

1. Gas expanding in a room
   1. How things dissolve
      1. Heat dispursing

Nota:

• Entropy always increases when particles become more disordered
• The entropy of a pure substance increases with increasing temperature

1. Changing state
   1. Solid/liquid

      Nota:

      • Perfect crystals have 0K entropy. as they heat up and change to a liquid, entropy increases as the particles become more randomly placed.
   2. Liquid/gas

      Nota:

      • When water evaporates, its entropy changes from 70 to 189
2. More moles of gas

   Nota:

   • If a reaction  has more moles of gas on one side than the other, the more moles of gas will have the highest entropy, shown by giving the reaction. e.g. 3O2-> 2O3 is a decrease in entropy
3. Dissolving

   Nota:

   • When something dissolves the entropy increases.

Nota:

• A reaction is feasible if ΔG has a negative value.

Nota:

• The balance between enthalpy, entropy and temperature for a process ΔG=ΔH-TΔS ΔH is the enthalpy change to the surroundings ΔS is the entropy change in the system T is the temperature in Kelvin (degrees C + 273)