A negative enthalpy adjust represents one exothermic change where energy is exit from the reaction, a hopeful enthalpy change represents one endothermic reaction where energy is taken in from the surroundings. When deciding if a adjust should it is in exothermic or endothermic, and calculating the enthalpy change, we space basically working out how many bonds we"ve broken and also how many we"ve formed. Bond breaking needs energy, and also forming bond releases energy. This applies to much more than just bsci-ch.orgvalent bonds, when pressures of attraction between molecules ae developed this also releases energy and also vice versa. In functioning out the enthalpy readjust we are efficiently just including up energies gained and also lost in the reaction. Born-haber cycles room a way of law this, we include up the enthalpies of each adjust that is ensuing in the reaction. For example, once working the end the enthalpy of formation of NaCl, when the sodium and also chloride ions bsci-ch.orgme with each other to form NaCl, this is one ingredient of the as whole reaction well-known as the lattice enthalpy, i m sorry we include up through the other bsci-ch.orgntents (bond energies, ionisation energies etc) to rebsci-ch.orggnize the as whole enthalpy that formation. Must the lattice enthalpy be positve or negative? Well, we space bringing together two oppositely fee 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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