Q= mhvap. H= E+pV Q= mc T H= Hproducts Hreactants

Transcription

1 Student Worksheet for Attempt to work the following practice problems after working through the sample problems in the videos. Answers are given on the last page(s). Relevant Equations C= Q T Q = E + p V Q= mhvap Q = mhf H= E+pV Q= mc T H= Hproducts Hreactants Where, c is the specific heat capacity, C is the heat capacity, T is the temperature, E is the total energy (system + surroundings), Q is the energy in the system, p is the Pressure, V is the volume, m is the mass, Hvap is the enthalpy of vaporization, Hf is the enthalpy of formation, and is the change (Final minus initial) Supercharged Science 1

2 1. Explain the difference between temperature and heat. 2. Explain the difference between heat capacity and specific heat capacity. 3. Explain the triple point of a phase change graph. 4. If water has an Hvap of 2,260 J/g, what is the heat capacity of 250 g steam that reaches 175 C? 5. An unknown substance has an Hf of 260 kj/g when 5 kj of energy are added. How many grams of the substance are in your sample? 2017 Supercharged Science 2

3 6. What is the heat capacity of a substance that has an increased total energy of 42.5 kj at 1.5 atm, when the volume increases by 0.5L after warming the substance by 27.3 C? 7. Compare and contrast Formation Enthalpy (Hf) and Vaporization Enthalpy (Hvap). 8. Calculate the mass of a substance that has a specific heat of 0.473J/g C and releases 1,523 J of energy when its cools from 50 C to 17 C. 9. You have decided to use system enthalpy in an attempt to identify an unknown substance. After carefully measuring, it is determined that the system contains 300 J of energy when the pressure and volume are held constant at 1 atmosphere of pressure and 2.3 L. What is the enthalpy of the system? 2017 Supercharged Science 3

4 10. Using your answer to #9, how would the enthalpy change is the pressure was increased to 2.5 atmospheres? Explain. Hint: It may help to review Boyle s Law from Chapter If PCl3 has a specific heat of J/g C, how many moles will be warmed from 23 C to 57 C with 4kJ of heat energy? 12. How many moles of ethanol (C2H5OH) will condensate if it s Hf is -277 kj/mol and 275 J of energy are released? 2017 Supercharged Science 4

5 13. How much heat energy must be applied to water to cause 15 moles of it to vaporize if the Hvap for water is 2260 J/g? 14. Using the following chemical reaction, what is the total change in H? The enthalpy of vaporization of (methane) CH4 is kj/mol. CH4 C + 2 H2 15. What is the heat capacity of a solution that releases 450kJ of energy when it cools by 37 C? 2017 Supercharged Science 5

6 1. Explain the difference between temperature and heat. Heat is a type of energy that is added to a system, whereas temperature is the measure of heat. This means that temperature is a way to quantify the amount of energy. 2. Explain the difference between heat capacity and specific heat capacity. These are very similar in nature. The difference is that that heat capacity applies to a substance in general. Specific heat capacity is how much energy is required to raise 1 gram of a pure substance by 1 C. Here is an example: Water has a certain heat capacity. It is undoubted that there would take a lot more heat energy to boil a 20 gallon pot of water than a 5 gallon pot of water. Thus, the heat capacity varies by how much is present (20 or 5 gallons), and how much heat is required to raise each by 1 C. The 20 gallon obviously has a lot higher heat capacity despite both pots containing pure water. Specific heat capacity is not based on varying volumes. This references 1 gram, so continuing with water. We know that Density = mass. We also know that water s density is 1 g/ml. Volume Water s specific heat capacity is how much energy is required to raise 1 ml of water by 1 C. 3. Explain the triple point of a phase change graph. The triple point is the pressure and temperature that a substance is known to exist in solid, liquid, and gas phases. The smallest adjustment will shift the phase equilibrium to one of the phases (depending on the change). 4. If water has an Hvap of 2,260 J/g, what is the heat capacity of 250 g steam that reaches 175 C? Q= mhvap You are solving for Q. Q= 250g(2,260 J/g) = 565,000 J or 565kJ C= Q T = = 7.53 kj/g C 5. An unknown substance has an Hf of 260 kj/g when 5 kj of energy are added. How many grams of the substance are in your sample? Q = mhf Solving for m. m= Q Hf = = 1.92*10-2 grams 2017 Supercharged Science 6

7 6. What is the heat capacity of a substance that has an increased total energy of 42.5 kj at 1.5 atm, when the volume increases by 0.5L after warming the substance by 27.3 C? Q= E + p V = (1.5*0.5) = J/g C = Q T = = 1.58 J/ C 7. Compare and contrast Formation Enthalpy (Hf) and Vaporization Enthalpy (Hvap). These two are practically opposites of each other. Both are measures of energy stored in bonds, however, The enthalpy of vaporization is the amount of energy associated with breaking a bond to accomplish gas phase. Alternatively, the enthalpy of formation is the amount of energy association with making a new bond. 8. Calculate the mass of a substance that has a specific heat of 0.473J/g C and releases 1,523 J of energy when its cools from 50 C to 17 C. Q= mc T m= Q c T = 1523 ( ) = 97.6 g 9. You have decided to use system enthalpy in an attempt to identify an unknown substance. After carefully measuring, it is determined that the system contains 300 J of energy when the pressure and volume are held constant at 1 atmosphere of pressure and 2.3 L. What is the enthalpy of the system? H= E + pv = (1*2.3) = J/g 2017 Supercharged Science 7

8 10. Using your answer to #9, how would the enthalpy change is the pressure was increased to 2.5 atmospheres? Explain. Hint: It may help to review Boyle s Law from Chapter 4. Increasing the pressure in the system will cause an increase to the enthalpy, but minimally. If you plug these numbers in, you will find that at 2.5 atmospheres, J/g. What this translates into is that increasing the pressure 250% causes at 1% difference. The greatest manipulation of enthalpy values is done by changing the temperatures. This is observed in everyday life as well as the phase diagram. Remembering that enthalpy drives phase changes. If you have a gold ring and smash it with a hammer (exerting a lot of pressure), it will remain a solid, even if it breaks into pieces. On the contrary, if you bake your ring in the oven at 400%, it will melt. The small observed change in paragraph 1 is because a few of the molecules will shift phase, and remember that the enthalpy values vary depending on if the substance is melting, forming, or vaporizing. 11. If PCl3 has a specific heat of J/g C, how many moles will be warmed from 23 C to 57 C with 4kJ of heat energy? Q= mc T Solving for m. m= Q c T = 4,000 ( ) = g # Moles= Grams Given Molecular Weight = = 0.98 moles 12. How many moles of ethanol (C2H5OH) will condensate if it s Hf is -277 kj/mol and 275 J of energy are released? You should first convert kj/mol to kj/g for ethanol, but the final answer is the same. Try it. 277 kj 1 mol Ethanol 6 kj/g 1 mol Ethanol 46 g Ethanol Q= mhf Solving for m. m= Q Hf = Don t forget to convert J to kj or kj to J. = 0.05 g C2H5OH = 1.04*10-4 moles C2H5OH 2017 Supercharged Science 8

9 13. How much heat energy must be applied to water to cause 15 moles of it to vaporize if the Hvap for water is 2260 J/g? 15 moles H2O 18 g H2O 270g H2O 1 mole H2O Q= mhvap = 270g*2,260 J/g = 610,200 J 14. Using the following chemical reaction, what is the total change in H? The enthalpy of vaporization of (methane) CH4 is kj/mol. CH4 C + 2 H2 H = Hproducts Hreactants = 0 (-74.85) = kj/mole 15. What is the heat capacity of a solution that releases 450kJ of energy when it cools by 37 C? C= Q T = = kj/ C 2017 Supercharged Science 9