# The molar heat of fusion for water is 6.01 kJ/mol. How much energy is released when 36.8 g of water freezes at its freezing point?

##### 1 Answer

#### Explanation:

For a given substance, the **molar heat of fusion** basically tells you one thing from *two perspectives*

*how much heat is***needed**in order to melt**one mole**of that substance at its melting point*how much heat must be***removed**in order to freeze**one mole**of that substance at its freezing point

It is **very important** to realize that the molar enthalpy of fusion will carry a **positive sign** when you're dealing with *melting* and a **negative sign** when you're dealing with *freezing*.

That is the case because **heat released** carries a negative sign, while **heat absorbed** carries a positive sign. So, for water, you can say that

#DeltaH_"fus" = +"6.01 kJ/mol" -># heat needed for melting

#DeltaH_"fus" = -"6.01 kJ/mol" -># heat released when freezing

You're interested in finding out how much heat is **released** when **molar mass** to calculate how many moles you have in that sample

#36.8 color(red)(cancel(color(black)("g"))) * ("1 mole H"_2"O")/(18.015color(red)(cancel(color(black)("g")))) = "2.043 moles H"_2"O"#

The heat released can be calculated using the equation

#color(blue)(q = n * DeltaH_"fus")" "# , where

Since you're dealing with **freezing**, you will have

#q = 2.043 color(red)(cancel(color(black)("moles"))) * (-6.01"kJ"/color(red)(cancel(color(black)("mol")))) = -"12.3 kJ"#

What this means is that when **freeze** at water's freezing point, **released** to the surroundings.

Remember, the *negative sign* symbolizes heat **released**.

Having

#q = -"12.3 kJ"#

is equivalent to saying that **released**.