Hess's law ____.

A) makes it possible to calculate ΔH for complicated chemical reactions
B) states that when you reverse a chemical equation, the sign of ΔH does not change.
C) determines the way a calorimeter works
D) describes the vaporization of solids

Highly confused for this question, I originally thought B but, doesn't the sign of ΔH change?

true, that is why A is the correct choice

Hess's law is a principle in thermodynamics that allows us to calculate the enthalpy change (ΔH) for complicated chemical reactions. This means that option A) "makes it possible to calculate ΔH for complicated chemical reactions" is the correct answer.

Let's go through the provided options to understand why the answer is A) and not B):

A) "Makes it possible to calculate ΔH for complicated chemical reactions": This statement aligns with the purpose of Hess's law, which is to determine the overall enthalpy change for a chemical reaction by manipulating other known enthalpy changes of related reactions. By using known enthalpy changes, we can add or subtract them to obtain the desired overall enthalpy change.

B) "States that when you reverse a chemical equation, the sign of ΔH does not change": This statement is actually a misunderstanding. The correct principle is that when an equation is reversed, the sign of ΔH changes. If a reaction occurs in one direction with an associated enthalpy change, reversing the reaction will have the opposite sign for ΔH. This is because the enthalpy change is a measure of energy absorbed or released during a reaction, and reversing the reaction changes the flow of energy.

To better understand why B) is incorrect, we can consider an example reaction: 2H2(g) + O2(g) → 2H2O(g) with ΔH = -572 kJ/mol. Reversing this reaction would yield: 2H2O(g) → 2H2(g) + O2(g), but the sign of ΔH changes to +572 kJ/mol.

Therefore, A) is the correct answer, as Hess's law allows us to calculate ΔH for complicated chemical reactions by manipulating known enthalpy changes.