a 2kg block of ice is heated from -20c to 30c. how much heat is needed to conver

Question

a 2kg block of ice is heated from -20c to 30c. how much heat is needed to convert the block of ice to steam at 130c? (hint, the latent heat of fusion and the latent heat vaporization for water are 334,000 j/kg and 2260,000 j/kg respectively.) in addition to showing all the equations used and work please discuss the following: a) why is the latent heat of vaporization much greater than the latent of fusion. b) why does the temperature remain constant during phase changes?

Explanation / Answer

the given problem goes like this -20 'C (ice) ------> 0 'C (ice) --------> 0 'C (water) -------> 100 'C (water) -----> 100 'C (vap.) 100 'C (vap.) ----> 130 'C (vap.) step 1: -20 'C (ice) ------> 0 'C (ice) heat required = m * sp. heat of ice * (temp. diff.) = 2k * 2.11 * 20 = 84.4 kJ step 2: 0 'C (ice) --------> 0 'C (water) heat required = m * L(latent heat of fusion of ice) = 2 * 2260k = 4520 kJ step 3: 0 'C (water) -------> 100 'C (water) heat required = m * sp. heat of water * (temp. diff) = 2k * 4.181 * 100 = 836.2 kJ step 4: 100 'C (water) -----> 100 'C (vap.) heat required = m * L(latent heat of vaporization of water) = 2 * 334k = 668 kJ step 5: 100 'C (vap.) -----> 130 'C (vap.) heat required = m * sp. heat of vapour * (temp. diff) = 2k * 2.080 * 30 = 124.8 kJ total heat required = 84.4 + 4520 + 836.2 + 668 + 124.8 = 6233.4 kJ a) The latent heat of vaporization can be viewed as the energy required to overcome the intermolecular interactions in the liquid (or solid, in the case of sublimation). Hence helium has a particularly low enthalpy of vaporization, 0.0845 kJ/mol, as the van der Waals forces between helium atoms are particularly weak. On the other hand, the molecules in liquid water are held together by relatively strong hydrogen bonds, and its enthalpy of vaporization, 40.8 kJ/mol, is more than five times the energy required to heat the same quantity of water from 0 °C to 100 °C (cp = 75.3 J K-1 mol-1). Care must be taken, however, when using latent heat of vaporization to measure the strength of intermolecular forces, as these forces may persist to an extent in the gas phase (as is the case with hydrogen fluoride), and so the calculated value of the bond strength will be too low. The heat of fusion can be observed if you measure the temperature of water as it freezes. If you plunge a closed container of room temperature water into a very cold environment (say -20 °C), you will see the temperature fall steadily until it drops just below the freezing point (0 °C). The temperature then rebounds and holds steady while the water crystallizes. Once completely frozen, the temperature will fall steadily again. The temperature stops falling at (or just below) the freezing point due to the heat of fusion. The energy of the heat of fusion must be withdrawn (the liquid must turn to solid) before the temperature can continue to fall. Therefore, as additional energy is required to overcome the intermolecular attractions in case of latent heat of vaporization, hence, its value is more as compared to latent heat of fusion. b) Heat is being added to ice as it melts, but that heat is going into turning the water from a solid into a liquid (breaking intermolecular bonds), rather than increasing the temperature. This is also true when it boils, and that is why a boiling pot of water won't reach a higher temperature than 100 °C. When something changes phase from solid to liquid, or from liquid to gas, it takes energy to break the intermolecular interactions. These interactions between the water molecules are what make it solid. When you have ice, these interactions are strongest, which is why ice is hard. Then when you have water, the interactions are not as strong, and although the water still "stays together" it is now a liquid and moves and flows freely. Then when all the interactions are broken, it become a gas, or steam, and now none of the water molecules are attached to any other molecules. Whenever it goes through a phase change like this, the energy goes into breaking up these interactions, and so the temperature stays constant until all the interactions are broken. Once all the ice is melted, or all the water has turned to steam, then any added heat will act to raise them temperature again.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.