Final answer:
To determine if a reaction is exothermic or endothermic, the standard enthalpy change (ΔH°) is calculated by subtracting the sum of the standard heats of formation of the reactants from the sum of the standard heats of formation of the products. A negative ΔH° indicates an exothermic reaction, whereas a positive ΔH° signifies an endothermic reaction.
Step-by-step explanation:
To determine whether a chemical reaction is exothermic or endothermic, you need to calculate the standard enthalpy change for the reaction using the standard heat of formation. The standard heat of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. The standard enthalpy of formation of an element in its most stable form is zero.
The standard enthalpy change for a reaction can be calculated using the formula:
ΔH° = ∑ΔH°f(products) - ∑ΔH°f(reactants)
Where ΔH° is the standard enthalpy change, ΔH°f(products) is the sum of the standard heats of formation for the products, and ΔH°f(reactants) is the sum of the standard heats of formation for the reactants. If the result is negative, the reaction is exothermic (releases heat), and if it is positive, the reaction is endothermic (absorbs heat).