1.What is the molar heat of vaporization of water, given the following thermochemical equations?
H2 + 1/2 O2 --> H2O + 241.8 kJ
H2 + 1/2 O2 --> H2O + 285.5 kJ
a) 44.0 kJ/mol
b)-44.0 kJ/mol
c)241.8 kJ/mol
2. Which substance has a standard enthalpy of formation delta Hf, equal to zero?
a)Gold
b)water ( liquid state)
c)carbon monoxide
d)zinc
e)water (gas)
I can't seem to get the correct answer...
Isn't the answer to #1 just the difference between the liquid and the gas (which you didn't label)? And since we are talking vaporization, that must be +.
For #2, elements are defined as having Hf = zero; therefore, both Au and Zn qualify.
To determine the molar heat of vaporization of water, we can use the fact that the vaporization of water is the reverse process of its formation. In the given thermochemical equations:
1. H2 + 1/2 O2 → H2O + 241.8 kJ
2. H2 + 1/2 O2 → H2O + 285.5 kJ
In equation 1, the enthalpy change is +241.8 kJ, which means energy is absorbed during the formation of 1 mole of water. On the other hand, in equation 2, the enthalpy change is +285.5 kJ, indicating that more energy is absorbed during the formation of water compared to equation 1.
Since the formation of water in these equations involves vaporization, we can conclude that equation 2 represents the vaporization of water. Therefore, the molar heat of vaporization of water is 285.5 kJ/mol.
So the correct answer for question 1 is:
c) 241.8 kJ/mol
Regarding question 2, the substance with a standard enthalpy of formation (ΔHf) equal to zero means that no energy is released or absorbed during its formation from its elements in their standard states. The standard enthalpy of formation for an element in its most stable form is defined as zero.
Among the substances listed, gold, carbon monoxide, zinc, and water in both liquid and gas states have formation enthalpies different from zero since they involve energy changes during their formation.
However, water (liquid state) has a standard enthalpy of formation (ΔHf) equal to zero, as it is essentially the reference point for measuring ΔHf values for other substances.
So the correct answer for question 2 is:
b) water (liquid state)
To find the molar heat of vaporization of water (H2O), we need to use the given thermochemical equations and the concept of Hess's Law.
1. Start with the first thermochemical equation: H2 + 1/2 O2 --> H2O + 241.8 kJ
This equation tells us that when 1 mole of water (H2O) is formed, 241.8 kJ of energy is released.
2. Next, consider the second thermochemical equation: H2 + 1/2 O2 --> H2O + 285.5 kJ
This equation tells us that when 1 mole of water (H2O) is formed, 285.5 kJ of energy is released.
3. By comparing the two equations, we can see that the only difference is the amount of energy released when 1 mole of water is formed. The difference is 285.5 kJ - 241.8 kJ = 43.7 kJ.
4. The molar heat of vaporization of water is defined as the amount of energy required to convert 1 mole of liquid water into 1 mole of water vapor. Since in this case, we are dealing with the energy released when water is formed, the correct answer is the negative of the difference: -43.7 kJ/mol.
Therefore, the correct answer to question 1 is b) -44.0 kJ/mol.
Moving on to question 2:
To determine which substance has a standard enthalpy of formation (ΔHf) equal to zero, we need to understand that the standard enthalpy of formation represents the energy change when 1 mole of a compound is formed from its constituent elements in their standard states.
1. Gold (Au) is an element and does not have a standard enthalpy of formation since it is not formed from any other elements.
2. Water in the liquid state (H2O, l) does not have ΔHf equal to zero since it requires energy to form from its constituent elements (H2 and O2).
3. Carbon monoxide (CO) is formed from its constituent elements (C and O2) and has a standard enthalpy of formation, but it is not equal to zero.
4. Zinc (Zn) is an element and does not have a standard enthalpy of formation since it is not formed from any other elements.
5. Water in the gas state (H2O, g) also does not have ΔHf equal to zero since it requires energy to form from its constituent elements (H2 and O2).
Therefore, the correct answer to question 2 is none of the given options.