Given the heats of formation calculate the heat of reaction (simple application)

Given the heats of formation calculate the heat of reaction (simple application)

Given:

C(diamond) + O2(g) → CO2(g) ΔH = −395.4 kJ

C(graphite) + O2(g) → CO2(g) ΔH = −393.5 kJ

a) Find ΔH for the manufacture of diamond from graphite:

C(graphite) → C(diamond)

H = -393.5 + 395.4 = + 1.9 kJ

b) Is heat absorbed or evolved as graphite is converted to diamond?

Absorbed

When given the heats of formation, we can calculate the heat of reaction for a specific process by applying the concept of Hess's law. Hess's law states that the overall heat of a reaction is independent of the pathway taken. This allows us to use known heats of formation to determine the heat of reaction for a desired process.

Let's consider the given example of the reaction between graphite (C(graphite)) and oxygen gas (O2) to form carbon dioxide (CO2): C(graphite) + O2(g) → CO2(g) ΔH = -393.5 kJ

To find the heat of reaction for the conversion of graphite to diamond, we need to compare the heats of formation of diamond (C(diamond)) and graphite (C(graphite)). By using the concept of Hess's law, we can subtract the heat of formation of graphite from the heat of formation of diamond.

a) Find ΔH for the manufacture of diamond from graphite: C(graphite) → C(diamond) ΔH = -393.5 kJ + 395.4 kJ = +1.9 kJ

By adding the heats of formation of the reactants and products, we find that the heat of reaction for the manufacture of diamond from graphite is +1.9 kJ. This positive value indicates that heat is absorbed during the conversion process, as the energy required to form the diamond is greater than the energy released during the formation of graphite.

b) Is heat absorbed or evolved as graphite is converted to diamond? Heat is absorbed.

Based on the positive value of ΔH (+1.9 kJ) for the conversion of graphite to diamond, we can conclude that heat is absorbed during this process. This means that energy is taken in from the surroundings to convert graphite into diamond.

To summarize, when given the heats of formation, we can calculate the heat of reaction by applying Hess's law. In the example provided, we determined the heat of reaction for the conversion of graphite to diamond by subtracting the heats of formation of the reactants and products. The positive value of ΔH indicates that heat is absorbed during this process, indicating an energy input required for the conversion. Understanding this concept helps us analyze the energy changes and transformations that occur during chemical reactions.