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### homework 8.67

Posted: Wed Jan 24, 2018 3:59 pm
8.67) Use the information in Tables 8.3, 8.6, 8.7 to estimate the enthalpy of formation of each of the following compounds in the liquid state. The standard enthalpy of sublimation of carbon is +717 kj/mol.
a)H20
b)CH3OH
c)C6H6 (without resonance)
d)C6H6 (with resonance)

How do you find the enthalpies of formation for gases? If H20 has two O-H bonds, and the bond enthalpy of an O-H bond is 463, why the enthalpy of formation not 463x2?

### Re: homework 8.67

Posted: Wed Jan 24, 2018 4:27 pm
By definition, an enthalpy of formation can be found by writing the formation reaction for the compound with all the reactants in their naturally occurring states and a total production of one mole of the product. Once you have an accurate formation reaction written you can then use bond enthalpies to estimate the overall enthalpy of formation.

For example, the formation reaction for H2O is:
H2 (g) + 1/2 O2 (g) -> H20 (l) we can see that there is exactly 1 mole of product and that hydrogen and oxygen are in their naturally occurring states (gaseous). In order to find the enthalpy we must:
break 1 H-H bond (436kJ/mol)
break 1/2 (O-O) bond (496kJ/mol) and
make 2 (O-H) bonds (-463kJ/mol)
This will give us -242kJ/mol, but this value is the enthalpy of formation for water in its gaseous state. In order to find the enthalpy of water in its liquid state we must account for the amount of heat given off when the gaseous water condenses to liquid. This is 44kJ/mol under standard conditions, so we just subtract it from our original value.
Thus, the enthalpy = -242 - 44 = -286kJ/mol

The other parts of this problem are solved in a similar manner.

### Re: homework 8.67

Posted: Wed Jan 24, 2018 4:46 pm
Thanks Remi, one more question: Do two gases always form another gas when they react? More specifically, how did you know that H2 gas and O2 gas reacted to form H20 in its gaseous form?

### Re: homework 8.67

Posted: Wed Feb 14, 2018 1:18 pm
How did you know that the amount of heat released when H2O goes from a gas to a liquid was -44 kj/mol?