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Understanding Phase Changes: Heat Transfer in Vaporization and Boiling

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amity-riley
11 diapositives · 246 Views · Nov 28, 2014

This lesson explores phase changes, focusing on vaporization and boiling as reversible physical changes of matter. It defines vaporization, highlighting its endothermic nature, and differentiates between evaporation and boiling. We examine heating curves and their significance in illustrating temperature stability during phase changes. Key concepts include the heat of fusion, which describes the energy required to melt a solid, and the heat of vaporization, relating to the energy absorbed or released during the transition from liquid to gas. Understanding these processes deepens comprehension of thermodynamics.

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Understanding Phase Changes: Heat Transfer in Vaporization and Boiling
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  1. Changes in State • A phase change is the reversible physical change that occurs when a substance changes from one state of matter to another. • Classify each of these processes as endothermic or exothermic.

  2. Vaporization • Vaporization is the changing of a liquid to a gas when the substance absorbs heat energy. • Vaporization occurring at the surface of a liquid is called evaporation. • As water in the perspiration evaporates from your skin it absorbs and carries away heat energy from your body.

  3. Boiling • If enough heat energy is applied to a substance particles inside the liquid can change to gas. • These particles travel to the surface of the liquid and then into the air. This process is called boiling.

  4. Heating Curves A heating curve • graph that illustrates the changes of state as a substance is heated. • As heat is added to a solid, its temperature increases. • A point is reached where the temperature levels out as heat is added. • This is the point at which the solid changes to a liquid. (Melting Point) • Energy from the heat is used to weaken the attractive forces between the particles.

  5. Heating Curve for Water

  6. While a substance changes a state, its temperature does not change until the change of state is complete. • Once the phase change is complete, the temperature will begin to increase as heat is added. • This continues until the liquid reaches its boiling point. • As the liquid changes to a gas, the temperature plateaus until the phase change is complete. (Boiling point)

  7. Heat of Fusion The heat of fusion (DHfus) • is the amount of heat needed to melt 1 gram of a solid at its melting point. • is the amount of heat released when 1 gram of liquid freezes at its freezing point. • for water at 0°CDHfusis 334 J/g. • The quantity of heat that is absorbed during melting or released during freezing can be found using the equation: q = m DHfus

  8. Note- The Heat of Fusion equation is used only at the melting/freezing transition, where the temperature remains the same. • Example Problem: How much heat is needed to melt 15.0 g of ice at 0°C?

  9. Heat of Vaporization The heat of vaporization (DHvap) is the amount of heat • absorbed to vaporize 1 g of a liquid to gas at the boiling point. • released when 1 g of a gas condenses to liquid at the boiling point. • for water at 100°CDHvapis 2260 J/g. • The quantity of heat that is absorbed during melting or released during freezing can be found using the equation: q = m DHvap

  10. Example Problem: Calculate the amount of heat needed to convert 190.0 g of liquid water to steam at 100ºC.

  11. After reflecting on today’s lesson, complete three of the sentence starters on your note card and hand it to me as you leave today. Sentence Starters I’ve learned… I was surprised… I’m beginning to wonder… I would conclude… I now realize that… Outcome Sentences