AQA CHEM5 NOTES

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THERMODYNAMICS:

 

·       ENTHALPY CHANGE ∆H-the heat taken in or given off during a chemical or physical change that takes place at constant pressure.

·       STANDARD ENTHALPY OF FORMATION ∆Hf-exothermic, the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. Na + ½ Cl2 à NaCl

·       STANDARD PRESSURE: 100KPa

·       STANDARD TEMPERATURE: 298K.

·       ENTHALPY OF ATOMISATION (∆Hat)-endothermic, for a metallic (or atomic) lattice, this is simply the enthalpy of sublimation ∆Hsub. It is related to the forces of attraction between atoms. Magnesium has a greater enthalpy of atomisation than sodium, because the greater ionic charge attracts the two delocalised electrons in the metallic lattice of magnesium more strongly. It is the standard enthalpy change that accompanies the formation of one mole of gaseous atoms. Na(s) à Na(g).

·       ENTHALPY OF IONISATION ∆Hi-endothermic, the standard enthalpy change for the removal of an electron from a species in the gas phase to form a positive ion and an electron, both also in the gas phase. Na(g) à Na+(g) +e-.

·       BOND DISSOCIATION ENTHALPY/ATOMISATION ∆Hdiss-endothermic, the standard enthalpy change that accompanies the breaking of a covalent bond in a gaseous molecule to form two free radicals also in the gas phase. Cl2(g) à 2Cl.

·       ELECTRON AFFINITY ∆Hea-can be exothermic or endothermic, the standard enthalpy change when an electron is added to an isolated atom in the gas phase. Cl(g) + e- à Cl-.

·       LATTICE ENTHALPY ∆HL-exothermic, related to several features of the ions, the amount of energy released is related to the charges on the ions, their size and the type of lattice formed. Negative ∆H value, ∆Hdiss is positive (NaCl(s) à Na+(g) + Cl-(g). The standard enthalpy change that accompanies the separation of one mole of a solid ionic lattice into its gaseous ions.

·       ENTHALPY OF FORMATION: the energy released during the formation of one mole of sodium chloride from its constituent elements in their standard states under standard conditions.

·       HESS’ LAW: the overall enthalpy change in a complete cycle must be 0 i.e. the sum of ∆H = 0. Using Hess’ law, the overall enthalpy change is independent of the number of steps.

·       The standard enthalpy of formation is the enthalpy change involved in the production of one mole of a compound from its elements under standard conditions, reactants and products being in their standard states.

·       The process of dissolving: the polar ends of the water molecules are attracted to the ions in the ionic lattice of a solid. This attraction distorts the charge cloud of the positive and negative ions in the lattice and reduces the forces holding them together. The ions move from the lattice into solution and become surrounded by water molecules-the ions are then said to be hydrated. Positive cations are attracted to the negative end of the dipole (the oxygen atom in the water

Comments

Kelly Davies

Good set of notes. Has all you need for CHEM5.

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