Calculating Change in Entropy

$\Delta S = \frac{q_{rev}}{T}$

regina_ho_3A
Posts: 13
Joined: Fri Jul 15, 2016 3:00 am

Calculating Change in Entropy

When we calculate change in entropy, how do we know when to use the enthalpy of formation vs the enthalpy of vaporization/fusion?
For example, on question 9.15 why do we use the enthalpy of formation to calculate the freezing of H2O instead of using the enthalpy of fusion?

Ranica Hortelano 2D
Posts: 9
Joined: Wed Nov 18, 2015 3:00 am

Re: Calculating Change in Entropy

You would use standard enthalpy of formation when the elements in your equation are in their most stable form. For example, aluminum's most stable form would be its solid form and carbon's most stable form would be graphite. In solving a standard enthalpy of formation problem you would be given a chart with the enthalpies of formation (or you would have to look up the enthalpies of formation) and from there you would subtract the sum of the enthalpies of the reactants from the sum of the enthalpies of the products (this is dependent on your given reaction).

Enthalpy of fusion/vaporization would be used when dealing with heating curves or phase changes. Problems like these usually give you an initial temperature and final temperature as well as an initial phase and an end phase. For example, a problem might ask how much energy is needed to heat 8 grams of ice at -4.4 degrees Celsius into steam at 120 degrees Celsius.