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bond enthalpies and enthalpy change

i'm just confused overall?

why are some answers negative while others are positive?

in the ocr A textbook it says delta H = H(products) - H(reactants) but then it also says delta r H = sum of bond enthalpies in reactants - sum of bond enthalpies in products?

why is it that they're switched around?

Reply 1

Davies Chemistry

Original post by MinMin25

I prefer to draw a Hess' Law diagram (triangle) as shown in the attached diagram. Remember breaking bonds is endothermic (+ value) and making bonds is exothermic (- value). In the triangle you break the reactants into their constituent atoms (endothermic) and then build them back up again (exothermic) into the products. Please have a look at the diagram in the attachment