2. The diagram below represents the bright-line spectra of four elements and a bright-line spectrum
produced by a mixture of two of these elements.
Which two elements are in this mixture?
(1) barium and hydrogen
(2) barium and lithium
(3) helium and hydrogen
(4) helium and lithium

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.