How Does the State of Matter Influence Enthalpy of Formation Values?

[yuuri14]

Homework Statement

Predict the value for change in Hf for:
Br2(g):change in Hf = Br(g) greater than Br(l) ... (-)

Br2(l):change in Hf= Br(l) less than Br(g)...(+)

I2(g): change in Hf= I2(g) greater than I (s)...(-)

I2(s) : change in Hf= I2(s) less than I(l)...(+)

Homework Equations

The Attempt at a Solution

Would I be right on my prediction by using negative or negative signs since it said to predict the value of change on Hf so solid to liquid to gas process.

 

[nate808]

Yes, you are correct in predicting the change in Hf using negative or positive signs. In general, the change in enthalpy of formation (Hf) for a substance is negative when it goes from a more complex or higher energy state to a simpler or lower energy state (such as from a gas to a liquid or solid). Conversely, the change in Hf is positive when a substance goes from a simpler or lower energy state to a more complex or higher energy state (such as from a solid or liquid to a gas). So, for the examples given, your predictions are correct.

 

[Blueshift5]

I cannot provide a specific numerical value for the change in Hf without knowing the specific values for the enthalpies of formation of each species. However, I can provide some insights on the predicted trends based on the given information.

Firstly, it is important to note that the enthalpy of formation (Hf) is defined as the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states. Therefore, the enthalpy of formation is dependent on the state of each species involved in the reaction.

Based on the given information, it can be predicted that the change in Hf for Br2(g) would be negative, as the formation of Br2(g) from its constituent elements Br(g) and Br(l) would release energy. This is because Br2(g) is in a lower energy state compared to Br(g) and Br(l).

Similarly, the change in Hf for I2(g) would also be negative, as the formation of I2(g) from its constituent elements I(s) would release energy. However, the change in Hf for I2(s) would be positive, as the formation of I2(s) from its constituent elements I(l) would require energy.

In summary, the change in Hf for a substance will depend on the specific states of its constituent elements and the direction of the reaction. Predictions can be made based on the relative energies of the different states involved.