Name Chemistry / / Melting/Freezing/Boiling & Condensing
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1 Name Chemistry / / Melting/Freezing/Boiling & Condensing As a substance melts, freezes, boils or condenses, heat is either absorbed or released. But, as this change in state occurs, there is no change in temperature. For example, from the time the first molecule of a substance melts until the last molecule of that substances the temperature is constant. The same is true of the reverse process; from the time the first molecule of a liquid freezes until the last liquid molecule freezes, the temperature is constant. Because the temperature is constant, the formula: H = T x mass x Cp would not be accurate. Without a temperature change, T = 0 and our calculation of the heat absorbed or released would be zero. Since we know heat is being added or released a different formula must be used. At the melting/freezing point the formula to be used is: H = H fus x mass. H fus represents heat of fusion which is defined as the amount of energy needed to melt one gram of a substance. Example: Calculate the amount of energy needed to melt 5.00 grams of ice at 0 C. H fus (H 2 O) = J/g. H = H fus x mass H = x 5.00 H = 1680 joules The reverse process(freezing) is an exothermic process. Thus, instead of H fus, H solid (heat of solidification) is used. It has the same numeric value as a substance s H fus but it is negative (H solid (H 2 O) = J/g) At the boiling/condensing point the formula to be used is: H = H vap x mass. H vap represents heat of vaporization which is defined as the amount of energy needed to vaporize one gram of a substance. Example: Calculate the amount of energy needed to change grams of water at 100 C to steam at 100 C. H vap (H 2 O) = J/g. H = H vap x mass H = x H = joules The reverse process(condensing) is an exothermic process. Thus, instead of H vap, H cond (heat of condensation) is used. It has the same numeric value as a substance s H vap but it is negative (H cond (H 2 O) = J/g) Note: The H fus, H cond, H vap & H solid values given above are in joules/gram. Often, however, you will be given values in joules/mole or kilojoules/mole. If this is the case, use the formulas: H = H fus x moles or H = H vap x moles. Be sure to convert mass moles if necessary. See the example below. Example: Calculate the kilojoules of energy needed to melt 10.0 grams of NH 3. H fus = 5.65 kj/mole H vap = 23.4 kj/mol First: Since the H fus value is given in kj/mole, I will use the formula H = H fus x moles. Second: Convert mass to moles by dividing the given value by the gram formula mass = moles Third: Solve it. H = H fus x moles H = 5.65 x H = 3.32 kj
2 Understanding Phase Changes The melting of ice seems rather simple, but leave it to chemists to make it incredibly detailed and complicated. This worksheet will guide you through the process of phase changes and the heat calculations associated with them. As a piece of ice is exposed to a warmer environment, it begins to absorb heat. The heat causes the solid molecules to vibrate faster. Eventually, the ice molecules break loose of their lattice positions and melting will begin to occur. Ice melts at 0 C. At this temperature all of the heat added is used to crack apart the ice structure by breaking the hydrogen bonds between the ice molecules. The temperature remains at the melting point from the time the first molecule of ice melts until the last molecule of ice melts. Although heat added at this time does not cause a rise in temperature, it does increase the potential energy of the water. Once all of the ice molecules have melted, heat added will cause an increase in the water temperature until the boiling point is reached. Water boils at 100 C. At the boiling point, heat added is used to break the intermolecular attractions between molecules. Once all the molecules have changed from liquid to gas, the temperature of the vapor will begin to increase. The graph below breaks this process into 5 stages. Stage 1: Ice is heated to its melting point. Since there is an increase in temperature with the addition of heat, the formula to use to calculate the heat change in this stage is: H = T x mass x Cp. It is important to remember to use the specific heat capacity of the solid state. At stage one, since the temperature is increasing, kinetic energy is increasing. Stage 2: Since added heat does not cause an increase in temperature ( T=0), you cannot use the same formula as in stage one. Instead, for stage two the formula to use is: H = H fus x mass. H fus represents heat of fusion which is defined as the amount of energy needed to melt one gram of a substance. There is no change in temperature at stage two so the heat added causes an increase in potential energy. Stage 3: As in stage one, added heat leads to an increase in temperature. For this stage the formula to use is: H= T x mass x Cp. The Cp of the liquid state should be used. Since the temperature is increasing, kinetic energy is increasing. Stage 4: Like stage two, heat added to the water does not cause an increase in temperature. The formula to use in stage four is: H = H vap x mass. H vap represent the heat of vaporization, the amount of heat needed to vaporize one gram of a substance. There is no change in temperature at stage two so the heat added causes an increase in potential energy. Stage 5: Added heat leads to an increase in temperature (like in stages one & three) so the formula to use is: H = T x mass x Cp. The Cp of the gaseous state should be used. At stage five, since the temperature is increasing, kinetic energy is increasing.
3 Example: Calculate the amount of heat needed to raise the temperature of 250 grams of water from -80 C to 200 C. Important constants for H 2 O: Melting point: 0 C, Boiling point: 100 C, Cp(solid): 2.09 J/g C, Cp(liquid): J/g C, Cp(gas): 2.01 J/g C, H fus : J/g, H vap : J/g. Stage 1: H = T x mass x Cp H = (0 C (-80 C)) x (250 g) x (2.09 J/g C) H = J Stage 2: H = H fus x mass H = (335 J/g) x (250 g) H = J Stage 3: H = T x mass x Cp H = (100 C - 0 C) x (250 g) x (4.184 J/g C) H = J Stage 4: H = H vap x mass H = (2259 J/g) x (250 g) H = J Stage 5: H = T x mass x Cp H = (200 C C) x (250 g) x (2.01 J/g C) H = J H total = Stage 1 + Stage 2 + Stage 3 + Stage 4 + Stage 5 H total = J J J J J H total = J = kj = Calories If the reverse of the above reaction were carried out, the magnitude of the H total would be the same but sign would be negative ( kj or Calories). The negative value indicates that it would be an exothermic process. A positive H value indicates that a process is endothermic. In cooling, heat of solidification ( H solid ) is used instead of heat of fusion ( H fus ) and heat of condensation ( H cond ) is used instead of heat of vaporization ( H vap ). Heats of fusion and solidification are the exact same magnitude but of different sign ( H solid = - H fus ) and heats of vaporization and condensation are the exact same magnitude but of different sign ( H cond = - H vap ). Homework: Answer each of the following questions. 1. Calculate the amount of energy required to change 15.0 grams of ice at 0 C to 15.0 grams of water at 0 C. H fus = 80 cal/gram 2. If the heat of fusion of water is 3.4 x 10 2 J/g, calculate the amount of heat energy required to change 40.0 grams of ice at 0 C to 40.0 grams of water at 0 C. 3. Calculate the amount of energy released as grams of steam are condensed into liquid water at 100 C. H fus : J/g, H vap : J/g. 4. If the heat of fusion of FeO is 32.2 kj/mol, calculate the amount of heat energy required to melt 17.7 grams of FeO. 5. Calculate the kilojoules of energy needed to vaporize 70.0 grams of NH 3. H fus = 5.65 kj/mole H vap = 23.4 kj/mol 6. Calculate the amount of energy released as 55.0 grams of bromine is changed from liquid to solid. H fus = kj/mol
4 Use this graph to answer questions Which number represents the freezing point? 8. Which number represents the melting point? 9. Which number represents the vaporization of a liquid? 10. Which number represents the condensation point? 11. Which number represents the warming of a solid? 12. Which number represents the crystallization? 13. Which number represents the solid state? 14. Which number represents the melting of a solid? 15. Which number represents the liquid state? 16. Which number represents the gaseous state? 17. Which number represents the warming of a vapor? 18. Which number represents the warming of a liquid? 19. Which number represents the boiling point? 20. Which numbers show a change in potential energy? 21. Which numbers show a change in kinetic energy? 22. What formula is used when there is a change in kinetic energy? 23. What formula is used at the melting point of a substance? 24. What formula is used at the boiling point of a substance? 25. Heat of condensation is an (endothermic / exothermic) process. Circle one. 26. Heat of fusion is an (endothermic / exothermic) process. Circle one. 27. Heat of vaporization is an (endothermic / exothermic) process. Circle one. 28. Heat of solidification is an (endothermic / exothermic) process. Circle one. 29. Heat of fusion is used at the: (melting point / boiling point / freezing point / condensing point) Circle one. 30. Heat of vaporization is used at the: (melting point / boiling point / freezing point / condensing point) Circle one. 31. Heat of solidification is used at the: (melting point / boiling point / freezing point / condensing point) Circle one. 32. Heat of condensation is used at the: (melting point / boiling point / freezing point / condensing point) Circle one.
5 33. Draw a heating curve for ammonia between -120 C & 20 C. Label: solid, liquid, gas, boiling point, freezing point, melting point, condensing point and the formula that would be used at each stage. Boiling point (NH 3 ): C, Melting point (NH 3 ): C. 34. Calculate the amount of heat (in Calories) needed to raise the temperature of grams of water from -20 C to 120 C. Important constants for H 2 O: Melting point: 0 C, Boiling point: 100 C, Cp(solid): 2.09 J/g C, Cp(liquid): J/g C, Cp(gas): 2.01 J/g C, H fus : J/g, H vap : J/g. d. Stage 4: e. Stage 5: f. Total: 35. Calculate the amount of heat (in Calories) released as 1.0 liter of water at 80 C is frozen to -3 C. Important constants for H 2 O: Melting point: 0 C, Boiling point: 100 C, Cp(solid): 2.09 J/g C, Cp(liquid): J/g C, Cp(gas): 2.01 J/g C, H fus : J/g, H vap : J/g. d. Total:
6 36. Calculate the amount of heat (in Calories) burned if you eat grams of ice at -5 C and it is warmed to body temp (37 C). Important constants for H 2 O: Melting point: 0 C, Boiling point: 100 C, Cp(solid): 2.09 J/g C, Cp(liquid): J/g C, Cp(gas): 2.01 J/g C, H fus : J/g, H vap : J/g. d. Total: 37. Draw and label a cooling curve for water and calculate the amount of heat (in Calories) released as 750 ml of H 2 O from 200 C to -100 C. Important constants for H 2 O: Melting point: 0 C, Boiling point: 100 C, Cp(solid): 2.09 J/g C, Cp(liquid): J/g C, Cp(gas): 2.01 J/g C, H fus : J/g, H vap : J/g. d. Stage 4: e. Stage 5: f. Total:
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