# How to Estimate the Standard Enthalpy of Formation of H2O at 100°C?

• krootox217
In summary, the standard enthalpy of formation of gaseous H2O at 298K is -241.82 kJ mol-1. This value decreases by -241.82 kJ mol-1 when the temperature is raised from 100 to 373K.
krootox217

## Homework Statement

The Standard enthalpy of formation of gaseous H2O at 298K is -241.82 kJ mol-1. Estimate its value at 100 °C given the following values of the molar heat capacities at constant pressure: H2O (g): 33.58 JK-1mol-1, H2 (g): 28.84 JK-1mol-1, O2 (g): 29.37 JK-1mol-1. Assume that the heat capacities are independent of temperature.

## The Attempt at a Solution

The only formula I can think of is this one:

But I don't know how to calculate this exactly. Can some help me getting started?

First, you must devise a pathway to get from reactants at 100C, through the reaction at 25C, and finally to the product at 100C.

Ok, thx

is it correct that the standard enthalpie of H2 and O2 are 0 because of the definition?

Does this mean I that I can use my formula in this way?

krootox217 said:
Ok, thx

is it correct that the standard enthalpie of H2 and O2 are 0 because of the definition?

Does this mean I that I can use my formula in this way?

All correct.

krootox217
Therefore the ΔH(T1) is -241.82 kJ mol-1, the Cp(H2O) is 33.58 JK-1mol-1, the Cp(H2) is 28.84 JK-1mol-1, the Cp(O2) is 29.37 JK-1mol-1 and I just need to insert these values in the equation above and I get the result for ΔH at 373K and therefore my final result?

krootox217 said:
Therefore the ΔH(T1) is -241.82 kJ mol-1, the Cp(H2O) is 33.58 JK-1mol-1, the Cp(H2) is 28.84 JK-1mol-1, the Cp(O2) is 29.37 JK-1mol-1 and I just need to insert these values in the equation above and I get the result for ΔH at 373K and therefore my final result?
Check all your units for consistency.

I think everything is fine, I just need to transform ΔH(T1) from -241.82 kJ mol-1 to -241'820 Jmol-1 and then it should be consistent, I think

Go for it! (Note: If you had left it as kJ, you'd have gotten a much different answer. Experience will tell you that heats of reaction don't change drastically with moderate temperature change, like here, so a "wild" answer is a tipoff that something's wrong.)

krootox217
Thanks a lot!

## 1. What is the definition of standard enthalpy of formation?

The standard enthalpy of formation is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states at a given temperature and pressure.

## 2. How is standard enthalpy of formation calculated?

The standard enthalpy of formation is calculated by subtracting the enthalpies of the products from the enthalpies of the reactants. All values must be in their standard states and the coefficients in the balanced chemical equation are used as the basis for the calculation.

## 3. What is the significance of standard enthalpy of formation?

The standard enthalpy of formation is a measure of the stability of a compound. A negative value indicates that the compound is more stable than its constituent elements, while a positive value indicates that the compound is less stable.

## 4. How does standard enthalpy of formation relate to other thermodynamic quantities?

The standard enthalpy of formation is related to other thermodynamic quantities, such as standard enthalpy change, Gibbs free energy, and entropy, through various thermodynamic equations. It can also be used to calculate the standard enthalpy change of a reaction.

## 5. Can standard enthalpy of formation values vary?

Yes, standard enthalpy of formation values can vary depending on the temperature and pressure at which they are measured. This is why it is important to specify the conditions when reporting a standard enthalpy of formation value.

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