What is the change in enthalpy of the following reaction?

2nabr(aq) + pbcl2(aq) 2nacl(aq) + pbbr2(aq)

given:
nacl: ∆h = -411 kj
nabr: ∆h = -360 kj
pbcl2: ∆h = -359 kj
pbbr2: ∆h = -277 kj

We are given with the balanced chemical reaction and the individual enthalpy of formation of the species involved in the reaction. To get the enthalpy of the reaction, the difference between the sum of the enthalpy of formations of the products and the reactants is calculated taking in mind the stoichiometry of the reaction.
∆Hrxn = [2(-411) + (-277)] - [2(-360) + (-359)]
 ∆Hrxn = -20 kJ

Ifound this on google “laws of chemical combination. it states that matter can neither be created nor destroyed. it stated that a given compound always contains exactly the same proportion of elements by weight.” i hope this you for what your looking for.