Consider the reaction pathway shown below - Scottish Highers Chemistry - Question 21 - 2022

According to Hess's Law, the enthalpy changes for a chemical reaction are additive. In this case, we can derive the relationship between the enthalpy changes given in the reaction pathway. To find the relationship we need to consider the direction of the reactions and apply Hess's Law:

From W to X, we have:

$$\Delta H = a$$

From X to Z:

$$\Delta H = c$$

From Z to W:

$$\Delta H = b$$

And from W to Y:

$$\Delta H = d$$

Setting up the equation based on the pathway shown, we have:

$$\Delta H = a + b - c - d$$

Rearranging, we get:

$$\Delta H: b = a - c - d$$

Thus, the correct answer according to Hess’s Law is option A.