Example Question 2 : Phase Changes. Possible Answers:. Correct answer:. Explanation : The enthalpy of vaporization gives the amount of energy required to evaporate a liquid at its boiling point, in units of energy per mole. Example Question 3 : Phase Changes. How much heat must be added to raise a sample of g of water at K to K? Explanation : The following fomula gives the heat needed to generate a given temperature change for a substance of known specific heat capacity: where is the heat input in Joules, is the mass of the sample in grams, and is the specific heat capacity in.
Example Question 4 : Phase Changes. Why does water boil at a lower temperature at high elevation? Possible Answers: Hydrogen bonds are easier to disrupt at high elevation.
The higher the elevation, the denser water is. There is a lower heat of fusion at higher elevation. The atmospheric pressure is lower at high elevations. Water has a higher vapor pressure at high elevation. Correct answer: The atmospheric pressure is lower at high elevations. Explanation : Increasing temperature means that vapor pressure increases as well. Example Question 5 : Phase Changes. Possible Answers: 2, 3, and 4. Correct answer: 1, 3, and 5. Explanation : Remember, temperature is a measure of the average kinetic energy of molecules.
Example Question : Ap Chemistry. Using the heating curve, determine which segment s relate to an increase in potential energy. Correct answer: 2 and 4. Possible Answers: 1. Correct answer: 3. Explanation : When the kinetic energy is increasing the temperature is also increasing the substance is not going through a phase change.
In the given heating curve, which segment s correlate to a mixture of phases? Possible Answers: 1, 3, 5. Correct answer: 2, 4. Explanation : When kinetic energy is increasing molecules are simply moving faster. Possible Answers: 5. Correct answer: 4. Explanation : Boiling is a phase change from liquids to gas. Possible Answers: The beginning of segment 1. Correct answer: The end of segment 5. Explanation : Remember, temperature is a measure of the average kinetic energy.
Copyright Notice. View AP Chemistry Tutors. The liquid and solid phases are continuously giving off vapor because some of the molecules have high enough speeds to enter the gas phase, a process called evaporation; see a.
For the molecules to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces. When only a small proportion of the molecules meet these criteria, the rate of evaporation is low. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. If a lid is placed over the container, as in b , evaporation continues, increasing the pressure, until sufficient vapor has built up for condensation to balance evaporation.
Then equilibrium has been achieved, and the vapor pressure is equal to the partial pressure of water in the container. Vapor pressure increases with temperature because molecular speeds are higher as temperature increases. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid decreases. This phenomenon is also called evaporative cooling.
This is why evaporating sweat cools the human body. Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure. For example, laundry on a clothes line will dry by evaporation more rapidly on a windy day than on a still day. Thus, it can evaporate without limit at this temperature and pressure. But why does it form bubbles when it boils? This is because water ordinarily contains significant amounts of dissolved air and other impurities, which are observed as small bubbles of air in a glass of water.
The pressure inside the bubble is fixed at 1. As the temperature rises, the amount of air in the bubble stays the same, but the water vapor increases; the bubble expands to keep the pressure at 1. It cannot reach this pressure, however, since the bubble also contains air and total pressure is 1. The bubble grows in size and thereby increases the buoyant force. The bubble breaks away and rises rapidly to the surface, resulting in boiling. The bubble expands to keep its pressure at 1.
The bubble grows and rises to the surface. Privacy Policy. Skip to main content. The conservation of energy means that, assuming no energy is transferred to the environment, any energy transferred to a material will be distributed between the chemical store and the thermal store of the internal energy. Whether the energy breaks bonds, increases the speed of the particles to stretch bonds, or just increases the speed of the particles depends on the temperature and state of the material.
Internal energy When a material is heated or cooled, two changes may happen to the particles within the material: Chemical bonds between the particles may form, break or stretch.
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