chapter 17.3

Why does temperature stay constant during a phase change?

Added or released thermal energy goes toward changing the state of matter rather than changing temperature.

Molar Heat of Fusion (ΔHfus)

Heat absorbed by 1 mole of a solid as it melts into a liquid at constant temperature (Endothermic, ΔHfus>0).

Molar Heat of Solidification (ΔHsolid)

Heat released by 1 mole of a liquid as it freezes into a solid at constant temperature (Exothermic, ΔHsolid<0).

what is the relationship between molar heat of solidification and molar heat of fusion?

ΔHfus=−ΔHsolid

Molar Heat of Vaporization (ΔHvap)

Energy absorbed by 1 mole of a liquid to become a gas at its boiling point (Endothermic, ΔHvap>0)

Molar Heat of Condensation (ΔHcond)

Energy released by 1 mole of a gas when it condenses into a liquid at its boiling point (Exothermic, ΔHcond<0).

Why does sweating cool you down, while steam causes severe burns?

• Sweat: Absorbs heat of vaporization from your skin as it evaporates.

• Steam: Releases a large amount of heat directly onto skin as it condenses into liquid water.

Molar Heat of Solution (ΔHsoln)

Heat absorbed or released when 1 mole of a solute completely dissolves in water

Exothermic vs. Endothermic Dissolution

• Exothermic (ΔHsoln<0): Solute releases heat into solution; temperature increases (feels hot).

• Endothermic (ΔHsoln>0): Solute absorbs heat from water; temperature decreases (feels cold).