A double replacement reaction can be best described as a reaction in which a substitution takes place.
two atoms of a compound are lost.
ions are exchanged between two compounds.
electrons are exchanged between two atoms.

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.