AQA Chemistry A-level 3.1.8 Thermodynamics Detailed Notes PDF

AQA Chemistry A-level 3.1.8: Thermodynamics Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

AQA Chemistry A-level 3.1.8: Thermodynamics Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.1.8.1 - Born-Haber Cycles Born-Haber cycles are similar to the cycles used with Hess’s Law. They allow ​enthalpy changes​ to be determined which cannot be measured directly. Lattice Enthalpies (∆​le​H) Lattice enthalpies are used for ​ionic substances​. Lattice ​dissociation​ enthalpy is defined as: The energy required to break apart an ionic lattice into its constituent ions in a gaseous state under standard conditions. Lattice ​formation​ enthalpy is defined as: The energy required to form an ionic lattice from its constituent ions in a gaseous state under standard conditions. Atomisation Enthalpy (∆​at​H) This is defined as: The energy required for the formation of a mole of gaseous atoms under standard conditions. Enthalpy of Electron Affinity (∆​ea​H) This is defined as: The enthalpy change when one mole of electrons is added to a mole of gaseous atoms under standard conditions. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Born-Haber Cycles These different enthalpy changes can be ​combined​ to produce a ​Born-Haber cycle​. In a similar way to Hess’s Law, the arrows represent a reaction and are therefore treated like vectors. If the cycle ​counters the direction​ of the arrow, the sign of the enthalpy change represented by that arrow is ​reversed​. Example: In this example, ∆​leH ​ of formation is calculated by following around the cycle, starting at the beginning of the arrow working round to the head of the arrow: ∆H = --355 - 121 - 502 - 107 - 411 = - 786 kJmol-1​ Enthalpies of Solution (∆​sol​H) and Hydration (∆​hyd​H) These can be used as another way of measuring lattice enthalpies ​indirectly​. They are combined using a similar idea to Hess’s Law. Enthalpy of ​solution​ is defined as: The enthalpy change when one mole of ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Enthalpy of ​hydration​ is defined as: The enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions under standard conditions. It is nearly always negative as water has a ∂+ region that naturally attracts -ve ions. They are combined in the following way to form an energy cycle​. Example: Perfect Ionic Model Hydration enthalpy is heavily influenced by the ​size and charge​ of the molecules being dissolved. Therefore it is often assumed that the ions are ​‘perfectly ionic’​. The perfect ionic model assumes that: All the ions are ​perfectly spherical The ions display ​no covalent character​. Covalent character occurs in ions when two joined ions have ​varying sizes or charges ​meaning the distribution of charge is ​not even​. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.1.8.2 - Gibbs Free-Energy and Entropy Entropy (∆S) In chemistry, things tend towards a state of ​disorder​. Entropy is a ​measure of this disorder​, ∆S​. The more disordered a substance, the greater its entropy value. ​Entropy increases as temperature increase​ because the particles ​gain energy ​and move ​further apart​, ie. they become less ordered. Gases​ have the ​greatest entropy​ compared to solids and liquids. When a substance melts or evaporates, there is a ​sudden increase​ in entropy. The entropy change of vaporisation is much greater than that of fusion as a ​gas is much more disordered than a liquid or a solid. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc The overall entropy change for a reaction can be calculated, it is measured in ​JK​-1​mol​-1​: Since all things tend towards a state of disorder, all ​spontaneous​ reactions have a ​positive entropy value​, ie. disorder increases. Gibbs Free-Energy (∆G) This quantity allows ∆S to be found without needing to measure the effects on the surroundings. It uses the​ reaction temperature, enthalpy (∆H) and entropy (∆S)​ changes to determine a value for ∆G: Free-energy is measured in​ kJmol​-1​ so it is important that ∆H and ∆S are used in the same, standard units​. This equation also takes the form ​‘y = mx + c’​ meaning it can be represented graphically as a straight line graph of ∆G against ∆S. ∆G is ​negative for all spontaneous reactions​. This becomes true at a certain, specific temperature which can be found by putting ​∆G equal to zero​. Changing the temperature or the type of reaction occurring will ​change the feasibility​ of the reaction happening. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc