IB›IB Chemistry HL›Mind mapsR1.2 Energy cycles in reactionsIB Chemistry HL: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsBond enthalpiesBond breaking absorbs energy, forming releases itΔH=Σ(broken)−Σ(formed)\Delta H = \Sigma(\text{broken}) - \Sigma(\text{formed})ΔH=Σ(broken)−Σ(formed)Values are averages, valid for gases onlyNX2+3 HX2→2 NHX3\ce{N2 + 3H2 -> 2NH3}NX2+3HX22NHX3 gives ΔH=−93 kJ\Delta H = -93\,\text{kJ}ΔH=−93kJHess's lawΔH is independent of the route takenReverse an equation → change the sign of ΔHMultiply an equation → multiply ΔHFormation and combustionΔHf\Delta H_fΔHf: one mole of compound from its elementsΔHf\Delta H_fΔHf of an element in its standard state is zeroΔHc\Delta H_cΔHc: one mole burned completely in oxygenFormation: products − reactantsCombustion: reactants − productsEnergy cyclesHess, bonds, Born–HaberΔH\Delta HΔHkJIEEABorn–HaberHess's law applied to an ionic compoundAtomisation of metal and non-metalIonisation energies of the metalElectron affinity of the non-metalLattice enthalpy: ionic solid → gaseous ionsLattice trendsFirst electron affinity is exothermicSecond electron affinity is endothermic (O⁻ repels e⁻)Higher charge, smaller ion → larger lattice enthalpyMgClX2\ce{MgCl2}MgClX2 forms, not MgCl, due to lattice enthalpyExam tipsFormation equation must make one mole, correct statesNever start a formation equation from gaseous atomsDouble Cl atomisation and EA for MgClX2\ce{MgCl2}MgClX2Combustion arrows point away, so order flips