Latent Heat

Phases of Matter  Solid  Gas  Liquid  The properties of solids, liquids, and gases differ.  How does matter change state?

Watched Pot  You want to cook a pot of potatoes in boiling water. To use energy most efficiently, you should A) Heat it up quickly and leave it at a full boilA) Heat it up quickly and leave it at a full boil B) Heat it up slowly and turn it up to a full boilB) Heat it up slowly and turn it up to a full boil C) Heat it up quickly and turn it down to simmerC) Heat it up quickly and turn it down to simmer D) Heat it up slowly and leave it at a low boilD) Heat it up slowly and leave it at a low boil

Energy of Phase  Internal energy can be stored as bonds between atoms and molecules. Solids have the strongest bondsSolids have the strongest bonds Energy required to break them to a liquidEnergy required to break them to a liquid

Heat of Transformation  As a system reaches the point of a phase change heat no longer changes the temperature.  The heat is used to change the phase. heat of fusionheat of vaporization

Latent Heat  The heat of transformation per unit mass is called the latent heat (L). Measured in J/kgMeasured in J/kg  It takes as much energy to melt 1 g of ice as it does to raise the temperature from 0  C to 80  C. Material (f or v)Latent Heat Mercury (f)11.3 kJ/kg Lead (f)24.7 kJ/kg Uranium (f)82.8 kJ/kg Copper (f)205 kJ/kg Water (f)334 kJ/kg Oxygen (v)213 kJ/kg Water (v)2300 kJ/kg

Phase Diagrams  For each temperature and pressure a material is in a specific state.  The triple point is uniquely defined for each material. P 3 for water is used to define the KelvinP 3 for water is used to define the Kelvin  The critical point is where there is no difference between liquid and gas. Phase diagram for water

Sublimation  At low pressures a solid can go directly to a gas and vice versa.  This process is called sublimation. Dry ice to gaseous carbon dioxideDry ice to gaseous carbon dioxide Water vapor to frostWater vapor to frost  There is a latent heat of sublimation Phase diagram for carbon dioxide

Summer Cooler  200 g of ice at -10  C is added to 1.0 kg of water at 15  C.  Is there enough ice to cool the water to 0  C?  Find the amount of heat needed to bring the ice to 0  C and melt it. Q i = mc  T + mL f Q i = 70.9 kJ  Find the heat needed to cool the water down to 0  C. Q w = mc  T Q w = 62.8 kJ  Yes, there’s enough ice. next