If reactants a and b contain a total of 248 kj of chemical energy, and products c and d contain a total of 841 kj of chemical energy, is the reaction endothermic or exothermic?

The answer is endothermic.

Reaction is endothermic

Explanation:

The given reaction is :

A + B → C + D

The enthalpy change of a reaction (ΔH) is given by the difference between the enthalpy of the products and that of the reactants.

ΔH = H(products) - H(reactants) (1)

Now, enthalpy is a form of chemical energy which is essentially a measure of the heat content of the system. It is given that:

Chemical energy of reactants (A+B) = 248 kJ

Chemical energy of products (C+D) = 841 kJ

Based on equation (1) we have:

ΔH = H(C+D) - H(A+B) = 841-248 = +593 kJ

Since, the enthalpy change is positive, this implies that the reaction is endothermic i.e. energy is absorbed or required to carry out the reaction.