Latent Heat

1. Latent Heat Changes in State

2. Energy and States of Matter • The energy and organization of the particles in a sample of matter determine the physical state, or phase, of the matter: solid, liquid, or gas.

3. Energy and States of Matter • When the temperature of a substance is at the substance’s melting or boiling point, a change of state, or phase transition, begins.

4. Energy and States of Matter • When a solid is heated, the kinetic energy of the vibrating particles increases and the temperature of the substance rises. • When the melting point is reached, continued heating causes the bonds between particles to begin to break, • The potential energy of the particles increases.

5. Energy and States of Matter

6. Change of State • At the boiling point, the water molecules in the steam and liquid water have the same kinetic energy. • But the molecules in the steam have a lot more potential energy than those in the liquid water, • So the steam has more thermal energy than the liquid water.

7. Latent Heat • Latent heat is the thermal energy that is needed for a phase change. • Latent means “hidden.” • The increase in thermal energy of the object or system is hidden, because no corresponding change in temperature is observed. • The quantity of thermal energy transferred in a phase change from solid to liquid is called the latent heat of fusion, QF.

8. Specific Latent Heat of Fusion • Fusion is another term for melting • The specific latent heat of fusion, LF, of a substance is the quantity of thermal energy needed to melt 1 kg of the substance at its melting point. • It is also the quantity of thermal energy that must be removed from 1 kg of the substance to make it freeze or solidify without temperature change.

9. Specific Latent Heat of Fusion • The specific latent heat of fusion, LF, of a substance is the quantity of thermal energy needed to melt 1 kg of the substance at its melting point. • A phase change requires a huge amount of energy. • For example, the specific latent heat of fusion of water is 333 kJ/kg. It takes 333 kJ of energy to melt 1 kg of ice at 0°C.

10. Specific Latent Heat of Vaporization • The specific latent heat of fusion, LV, of a substance is the quantity of thermal energy needed to vaporize 1 kg of the substance at its vaporization point.

11. Latent Heat Gold melts at 1063°C. It takes 3250 J of energy to melt 50.0 g of gold at this temperature. Determine the specific latent heat of fusion of gold.

12. Latent Heat The boiling point of ethanol is 78.4°C. Calculate the quantity of energy needed to completely vaporize 75.0 g of ethanol if its initial temperature is (a) 78.4°C and (b) 22.4°C. The specific latent heat of vaporization of ethanol is 854 kJ/kg and its specific heat capacity is 2.72 kJ/(kgK).