1. Heat: Phase Change

2. 'change of phase' 'change of state'. The term 'change of phase' means the same thing as the term 'change of state'. o These changes of phase always occur with a change of heat. o Heat (which is energy) either comes into the material during a change of phase or heat comes out of the material during this change of phase. o Although the heat content of the material changes, the temperature does not.

3. How could there be a change in heat during a phase change without a change in temperature? the energy is used to break the bonds between the molecules If heat is coming into a substance during a phase change, the energy is used to break the bonds between the molecules of the substance.

4. What that means is… Heat enters a liquid to break apart the bonds when the liquid boils (evaporates) into a gas, and heat enters a solid to break apart the bonds when it melts into a liquid. Heat leaves a gas to form the molecular bonds when the gas condenses into a liquid, and heat leaves a liquid to form the molecular bonds when it freezes into a solid. Therefore, the temperature does not change during a change in phase! The heat content does change. Therefore, the temperature does not change during a change in phase! The heat content does change.

5. 2.02 2.06

6. That means… When calculating the heat absorbed or released during a phase change, use this formula: Q = m  H f  H f (for fusion: ice  water) = 333J/g v  H v (for vaporization: water  steam) = 2260J/g

7. Let’s try a problem like this… 50.0g of ice at 0 0 C melts. a. How much energy does this involve? b. Is the energy given off or absorbed? c. Is this endothermic or exothermic? Q = m  H f Q = ? Q = ? m = 50.0g  H f = 333 J/g Q = (50)(333) Q = 16650 J Q = 16700J Heat must be absorbed to make the ice melt. The reaction is therefore, endothermic.

8. 245g of water at 100.0 o C boils and changes to steam. A. How much energy does this involve? B. Is the energy released or absorbed? C. Is this endothermic or exothermic? Q = m  H v Q = ? Q = ? m = 245 g  H v = 2260 J/g Q = (245)(2260) Q = 553700 J Q = 554000J Heat must be absorbed to make the water evaporate. The reaction is therefore, endothermic.

9. How much energy does it take to convert 22g of ice at 0 o C to water at 10. o C? Ice to water Q = m  H f Q = (22)(333) Q = 7300J It takes 7300J of energy to make the ice into water. Water at 0 o C to water at 10 o C Q = mc  t Q = (22.0)(4.18)(10.) Q = 920J It takes 920J of energy to get the water to 10 o C. It takes 7300 + 920 = 8200J of energy to make the ice at 0 o C into water at 10 o C. Two things have to happen. The ice has to change to water and then the water has to warm up to 10. o C.

10. How much energy does it take to make 125g of water at 50.0 o C into steam at 140.0 o C? Water at 50.0 o C to water at 100. o C Q = mc  t Q = (125)(4.18)(50.) Q = 26100J It takes 26100J of energy to get the water to 100 o C. Water to steam Q = m  H v Q = (125)(2260) Q = 283000J It takes 283000J of energy to change the water into steam Steam at 100 o C to steam at 140 o C Q = mc  t Q = (125)(2.02)(40) Q = 10100J It takes 10100J of energy to change the steam at 100 o C to steam at 140 o C. The total: 26100 + 283000 + 10100 = 319000J It takes 319000J of energy to change the water at 50 o C to steam at 140 o C.

11. How much heat does it take to make 175g of ice at -30.0 o C into water at 45.0 o C? ice at -30.0 o C to ice at 0 o C Q = mc  t Q = (175)(2.06)(30.0) Q = 10800J It takes 10800J of energy to get the ice to 0 o C. ENDOTHERMIC ice to water Q = m  H f Q = (175)(333) Q = 58300J It takes 58300J of energy to change the ice into water ENDOTHERMIC water at 0 o C to water at 45 o C Q = mc  t Q = (175)(4.18)(45) Q = 32900J It takes 32900J of energy to change the water to 45.0 o C. ENDOTHERMIC The total: 10800 + 58300 + 32900 = 102000J It takes 102000J of energy to change the ice at –30.0 o C to water at 45.0 o C.

12. How much heat energy is released when 5.00g of steam at 100. o C changes to water at 80.0 o C? steam to water Q = m  H v Q = (5.00)(-2260) Q = -11300J 11300J of energy is released when the steam changes to water. EXOTHERMIC Water at 100. o C to water at 80 o C Q = mc  t Q = (5.00)(4.18)(-20.) Q = -418J 418J of energy is released. EXOTHERMIC -11300 + - 418 = -11700J of energy is developed when the steam changes to water.