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6th edition Question 8.67

Posted: Thu Feb 07, 2019 11:43 pm
by Carissa Young 1K
The question asks to estimate the enthalpy of formation of compounds in the liquid state. After finding the deltaH of the gas, what equation are we supposed to use to find the deltaH of the liquid? In class we learned that the enthlapy of vaporization is the H(vapor)-H(liquid) to go from liquid to a vapor, but what is the opposite (going from vapor to a liquid)?

Re: 6th edition Question 8.67

Posted: Fri Feb 08, 2019 9:08 am
by David S
The opposite would be the negative value of . This is because if you put in X amount of heat energy to get a certain liquid to become a gas, that gas must lose the same amount of energy to return to being a liquid. Losing energy => exothermic process => negative enthalpy for the process of condensation.

Re: 6th edition Question 8.67

Posted: Fri Feb 08, 2019 11:16 am
by Arianna Ko 2E
To find the enthalpy of formation in the liquid state you need to take the the deltaH you calculated for the gas and then add the negative value of the enthalpy of vaporization from it. The value is negative because instead of the positive value you have when finding how much energy must be added to the liquid to transform it into a gas, you get the negative value to find how much heat you are giving off when the gas condenses to the liquid phase.
To find the enthalpy of formation in the liquid state then, you use the equation:
enthalpy of formation of water (gas) + ( -enthalpy of vaporization)