[ThatGuy]

I have attached the problem statement.

It says that 1 mole of water (18.02 grams) at 25 C is evaporated. Calculate the required heat.

I know how to do this using q=mcDT but I am not sure how to approach this problem. I thought about using the Hess' Law approach but the 25 C gets in the way.

If someone can just point out how to get started, I would greatly appreciate it.

[Borek]

Assuming both equations give standard enthalpy of formation, they can be used to find the standard enthalpy of evaporation. Standard means at 298 K, so 25°C.

You don't need to heat the water to evaporate it, hint given is rather misleading.

[ThatGuy]

Assuming both equations give standard enthalpy of formation, they can be used to find the standard enthalpy of evaporation. Standard means at 298 K, so 25°C.

You don't need to heat the water to evaporate it, hint given is rather misleading.

It does say that the heating and evaporating of water is required. Wouldn't that mean the 25 C water would first be heated to 100 C then boiled?

[Borek]

1. Looks like question requires you to find the evaporation enthalpy from the given data, but it doesn't say anything about the specific heat of water. Perhaps you are expected to take the specific heat of water from tables (or memory), but if you are expected to use the tables, there is no need to bother yourself with using data given to find the evaporation enthalpy.

2. Have you ever left a wet towel in the bathroom only to find it dry the next day? Water at 25°C will eventually dry out without heating it to 100°C, so there is no need to heat it up first.

3. You are not given data that can be used to calculate enthalpy of evaporation at 100°C, and it is for sure different from the enthalpy of evaporation at 25°C (the one you can calculate from the data given). So using data given you can't calculate how much heat is needed to evaporate water at 100°C.

For me question is confusing and inconsistent and it is hard to say what kind of answer is expected. Perhaps the idea is to assume enthalpy of evaporation is identical at 25°C and 100°C. The difference is not large after all. But if so, I find wording misleading, as data given suggest something else.

[ThatGuy]

1. Looks like question requires you to find the evaporation enthalpy from the given data, but it doesn't say anything about the specific heat of water. Perhaps you are expected to take the specific heat of water from tables (or memory), but if you are expected to use the tables, there is no need to bother yourself with using data given to find the evaporation enthalpy.

2. Have you ever left a wet towel in the bathroom only to find it dry the next day? Water at 25°C will eventually dry out without heating it to 100°C, so there is no need to heat it up first.

3. You are not given data that can be used to calculate enthalpy of evaporation at 100°C, and it is for sure different from the enthalpy of evaporation at 25°C (the one you can calculate from the data given). So using data given you can't calculate how much heat is needed to evaporate water at 100°C.

For me question is confusing and inconsistent and it is hard to say what kind of answer is expected. Perhaps the idea is to assume enthalpy of evaporation is identical at 25°C and 100°C. The difference is not large after all. But if so, I find wording misleading, as data given suggest something else.

Suppose I was asked to find the heat of sublimation or deposition of water, instead.

Would I be able to apply the same logic? Because there is no temperature change in sublimation/deposition, correct? I would simply add the heat of fusion and vaporization?

[Borek]

Sorry, I don't understand your question.

Please elaborate. I guess you left out things you think are obvious, but in the context of earlier discussion it is not clear to me what kind of setup/data given you mean.

[ThatGuy]

Ok.

Let's say the I was asked to calculate the heat of sublimation for, let's say H₂O. I would just add the heat of fusion and the heat of vaporization, correct? This would be because there is no temperature change as the  H₂O would not experience the liquid phase.

[Borek]

Why don't you check by googling all three values and checking if they fit?