1. Understanding the Concept:
* Freezing is a phase change where a liquid transforms into a solid. This process releases heat, known as the enthalpy of fusion.
* Enthalpy of fusion (ΔHfus) is the amount of heat energy absorbed or released when one mole of a substance undergoes a phase change from solid to liquid (melting) or liquid to solid (freezing).
* Molar mass is the mass of one mole of a substance.
2. Stoichiometry's Role:
* Provides Molar Mass: Stoichiometry helps determine the molar mass of the substance undergoing freezing. This is crucial for converting the mass of the substance to moles.
* Relates Moles to Energy: Stoichiometry allows us to use the enthalpy of fusion (ΔHfus), which is expressed per mole, to calculate the energy change for a specific mass of the substance.
3. Calculation:
1. Calculate the number of moles (n):
* n = mass (g) / molar mass (g/mol)
2. Use the enthalpy of fusion (ΔHfus) to calculate the energy change (Q):
* Q = n * ΔHfus
Important Notes:
* Sign Convention: ΔHfus is usually reported as a positive value for melting (endothermic process). For freezing (exothermic process), the energy released is negative (Q = -n * ΔHfus).
* Units:
* Mass is typically in grams (g).
* Molar mass is in grams per mole (g/mol).
* Enthalpy of fusion is usually in joules per mole (J/mol) or kilojoules per mole (kJ/mol).
* Energy change (Q) will be in joules (J) or kilojoules (kJ).
Example:
Let's say you want to calculate the energy released when 10 grams of water freezes.
* Molar mass of water (H2O): 18.015 g/mol
* Enthalpy of fusion of water: 6.01 kJ/mol
1. Calculate moles of water: n = 10 g / 18.015 g/mol = 0.555 mol
2. Calculate energy released: Q = - (0.555 mol) * (6.01 kJ/mol) = -3.34 kJ
Therefore, 10 grams of water freezing releases approximately 3.34 kJ of energy.
Stoichiometry provides the framework to connect the mass of a substance with its molar properties, allowing us to calculate the energy changes involved in phase transitions.
