A negative enthalpy change represents an exothermic change where power is released from the reaction, a positive enthalpy change represents one endothermic reaction where power is bring away in indigenous the surroundings. When deciding if a readjust should it is in exothermic or endothermic, and also calculating the enthalpy change, we space basically functioning out how numerous bonds we"ve broken and how plenty of we"ve formed. Link breaking requires energy, and also forming bonds releases energy. This uses to much more than simply rewildtv.comvalent bonds, when forces of attraction between molecules ae created this likewise releases energy and vice versa. In working out the enthalpy change we are efficiently just adding up energies gained and also lost in the reaction. Born-haber cycles are a method of doing this, we add up the enthalpies of each readjust that is occurring in the reaction. Because that example, as soon as working out the enthalpy of formation of NaCl, once the sodium and chloride ion rewildtv.comme with each other to kind NaCl, this is one rewildtv.commponent of the as whole reaction known as the lattice enthalpy, which we add up v the other rewildtv.commponents (bond energies, ionisation energies etc) to determine the in its entirety enthalpy of formation. Must the lattice enthalpy it is in positve or negative? Well, we space bringing with each other two oppositely charged ions, forming an ionic bond, this will certainly release energy, it"s one exothermic change, the lattice enthalpy must be negative. This very same logic deserve to be applied to all other thermodynamic changes. 


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