entropy

• internal energy is the total amount of energy within a system, the sum of the kinetic energies and potential energies of all the particles
• entropy is defined as the measure of dispersal of energy in a system which is greater the more disordered the system
• Gibbs free energy is the amount of energy given off by a reaction that is free and available to do work
• spontaneous reactions are processes which can take place by themselves once they have been initiated, also called feasible reactions
• the 1st law of thermodynamics is that energy cannot be created or destroyed it can only change from one type to another
• the 2nd law of thermodynamics is the fact that the total entropy at the end of the reaction is always more than at the start of the reaction
• exothermic reactions release energy into the surroundings
• endothermic reactions absorb energy from the surroundings
• the driving force for all chemical reactions is the tendency for greater disorder or greater entropy
• the total entropy change of a reaction is always positive as every physical and chemical change leads to an overall increase in entropy
• total entropy is only equal to zero when the system is in equilibrium
• if an entropy change is negative then the reaction must have caused a much bigger positive entropy change to the surroundings
• total entropy change formula:

  $\Delta S\left(total\right)=$$\Delta S\left(system\right)+$$\Delta S\left(surroundings\right)$
• factors affecting entropy:

  • volume of the system
  • number of particles
  • energy
  • complexity of the molecules
• when particles are confined to a small volume the particles are relatively ordered and the entropy is lower, in a larger volume the entropy is higher as the movement of the particles in the system is more unpredictable and chaotic
• one particle has a higher entropy than 2 particles as energy can only be absorbed by the one particle, in 2 particles there is less certainty about which particle will absorb the energy
• triatomic molecules have more vibrational states than diatomic molecules and single atom molecules so triatomic molecules have the greatest entropy, if all the molecules have the same molecular mass and are at the same temperature
• a gas has a higher entropy than a liquid as it has a greater volume than liquids so it has more positions in which they can be arranged and a gas has higher kinetic energy than a liquid
• a diatomic gas has a higher entropy than a monoatomic gas as there are more ways in which quanta energy can be distributed as well as translation energy states diatomic molecules have more vibrational and rotational energy states
• if all of the molecules have the same number of atoms (diatomic) then the molecule with the highest number of protons, neutrons and electrons will have the highest entropy as the same energy in all 3 molecules can be dispersed in more ways in the molecule with the highest mass number
• only part of the energy released in a spontaneous reaction can be used to do work ($\Delta G$) the rest causes an entropy change
• Gibbs free energy equation:

  $\Delta G=$$\Delta H-T\Delta S$
• if gibbs free energy is negative then the system loses energy to do useful work
• if gibbs free energy is positive then the system gains energy and work is done on the system to make the reaction happen
• if enthalpy change is exothermic $\left(\Delta H<0\right)$ and the entropy change increases$\left(\Delta S>0\right)$the spontaneous reaction always happens
• if enthalpy change is exothermic $\left(\Delta H<0\right)$ and the entropy change decreases $\left(\Delta S<0\right)$the spontaneous reaction sometimes happens
• if enthalpy change is endothermic $\left(\Delta H>0\right)$and the entropy change increases$\left(\Delta S>0\right)$the spontaneous reaction sometimes happens
• if enthalpy change is endothermic $\left(\Delta H>0\right)$and the entropy change decreases $\left(\Delta S<0\right)$ the spontaneous reaction never happens
• in equilibrium conditions if the reaction goes to completion the value of Kc would be large and the value of gibbs free energy would be negative
• in equilibrium conditions if the equilibrium mixture mainly contains reactants the value of Kc would be small and the value of gibbs free energy would be positive
• in equilibrium conditions when the magnitude of gibbs free energy is more than 60kJ/mol (positive or negative) the reaction is considered to either go to completion or not to occur at all
• when gibbs free energy is equal to xero this is the turning point of reaction and is the minimum temperature required for a reaction to become feasible
• when the value of gibbs free energy is negative the reaction is feasible
• substances can dissolve when the enthalpy change of solution is positive as there is an increase in the entropy of the ions as they go from being ordered in the crystal to being disordered in solution, this increase in entropy acts as the driving force of reaction
• substances can be unable dissolve when the enthalpy change of solution is negative as the highly charged ions attract water molecules and immobilise them causing a big negative shift in entropy and causes dissolving to become unfavourable
• dissolving becomes unfavourable when enthalpy change of solution is negative as the increase in entropy of the surroundings and the ions cannot offset the decrease in entropy of the solvent molecules meaning the substances will not dissolve