## 6th edition Question 8.67

Carissa Young 1K
Posts: 65
Joined: Fri Sep 28, 2018 12:17 am

### 6th edition Question 8.67

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)?

David S
Posts: 54
Joined: Fri Sep 28, 2018 12:15 am

### Re: 6th edition Question 8.67

The opposite would be the negative value of $\Delta H_{vap}$. 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.

Arianna Ko 2E
Posts: 33
Joined: Fri Sep 28, 2018 12:20 am

### Re: 6th edition Question 8.67

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)