IB›IB Chemistry SL›Mind mapsR1.2 Energy cycles in reactionsIB Chemistry SL: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsBondsBreaking bonds absorbs energy (endothermic)Forming bonds releases energy (exothermic)Exothermic overall: more energy released than absorbedBond enthalpy: energy to break one mole of a bond, gaseousBond enthalpy ΔHΔH=Σ(broken)−Σ(formed)\Delta H = \Sigma(\text{broken}) - \Sigma(\text{formed})ΔH=Σ(broken)−Σ(formed)Data booklet gives average values for C–H, O–H, C–CDiatomic values (H₂, HCl) are exactCX2HX4+HX2→CX2HX6\ce{C2H4 + H2 -> C2H6}CX2HX4+HX2CX2HX6: broken 1050, formed 1174ΔH = −124 kJ mol⁻¹LimitationsAverages are not exact for a particular moleculeOnly valid for gasesLiquids and solids: energy of state change missingExperimental value is more reliableEnergy cyclesbonds and Hess's lawΔHkJmolHess's lawΔH is independent of pathwayFollows from conservation of energyUse an energy cycle when ΔH cannot be measured directlyUsing cyclesReverse an equation → change sign of ΔHMultiply an equation → multiply ΔH by same factorAdd equations, cancel species on both sidesS+1½OX2→SOX3\ce{S + 1½O2 -> SO3}S+1½OX2SOX3: −297 + ½(−198) = −396 kJExam tipsFormed − broken gives the wrong signScale ΔH whenever you scale an equationCheck physical states when calculated and experimental differSay which value is more reliable and why