Thermochemistry Specific Heat and Heat Capacity

AP Chemistry Thermochemistry over BREAK!! Terms to review: (know what they are and what units they are measured in) ● ● ● ● ● Heat Temperature Specific heat ΔHrxn q Equations to remember: ● ● ● q=mcΔT o C water= 4.18 J/ g°C o C ice= 2.03 J/ g°C o C steam= 1.97 J/ g°C ΔHvap=m(2259 J/g) ΔHfus=m(334 J/g) Phase change curve: ● Place the equations listed above on the correct position on the graph below. Complete the following two worksheets over break. I have included some notes to help remind you of your first year chemistry course. There will be a quiz on the first day back from break. Enjoy ☺ AP Chemistry PHASE CHANGE PRACTICE Use the following information to solve the phase change problems: Specific heat of water = 4.18 J/g·°C ΔHfusion = 334 J/g Specific heat of ice = 2.03 J/ g·°C = 2259 J/g ΔHvaporization Specific heat of steam = 1.97 J/g·°C Recall that it requires additional energy to change phases: SOLID LIQUID GAS ΔHfusion 🡪 ΔHvaporization 🡪 🡨 -ΔHfusion 🡨 -ΔHvaporization -------------------------------------------------------------------------------------------------------------------------------1. How much heat is required to melt 233.0 grams of ice into water, from -15.00°C to room temperature (25.00°C)? ANSWER: heating ice = 7095 J, heating water = 24349 J, heat of fusion = 77822 J, TOTAL HEAT = 110 kJ 2. How much heat is required to change 32.5 grams water into steam, from room temperature (25.0°C) to 115°C? ANSWER: heating water = 10189 J, heating steam = 960 J, heat vaporization = 73418 J, TOTAL HEAT = 85 kJ 3. How much heat is needed to melt 1.43 grams of ice into water from -5.34°C to 84.3°C? ANSWER: heating ice = 15.5 J, heating water = 504 J, heat fusion = 478 J, TOTAL HEAT = 998 J AP Chemistry 4. How much heat is needed to convert 0.232 grams water into steam, from 32.5°C to 112°C? ANSWER: heating water = 65.5 J, heating steam = 5.48 J, heat vaporization = 524 J, TOTAL HEAT = 595 J 5. How much heat do you need to add to 3.22 grams H2O to raise the temperature from -23.0°C to 152°C? ANSWER: heating ice=150 J, heating water=1346 J, heating steam=330 J, heat fusion=1075 J, heat vaporization=7274 J, TOTAL=10. kJ 6. How much heat is needed to raise the temperature of 199 grams H2O from -10.3°C to 154°C? ANSWER: heating ice=4161 J, heating water=83182 J, heating steam=21170 J, heat fusion=66466 J, heat vaporization=449541 J, TOTAL = 625 kJ AP Chemistry Calorimetry Calorimeter A device used to measure enthalpy changes in chemical rxns. Usually a thick-walled vessel that keeps the reaction system well-insulated from the surroundings, equipped with a stirrer and a thermometer. In our case, we will use high-tech ultra-expensive styrofoam cups with cardboard covers. Using a calorimeter, you can measure the temperature change (ΔT) of the rxn, and relate this to the enthalpy (ΔH) of the rxn. Calories: Relating Energy to Food The food you consume contains energy that your body needs to survive. Food energy is measured in calories, which are really kilocalories in the United States. Potato chips that have 180 calories actually have 180 kcal, or 180,000 calories! 1 calorie = 4.18 joules How much energy does a 235 calorie candy bar have? Remember that in the United States, one calorie is actually 1 kilocalorie. Theory of Calorimetry Calorimetry is based upon of conservation of energy, the First Law of Thermodynamics. Consider an exothermic rxn run in a calorimeter. Since the system is closed (completely isolated from the surroundings), all heat evolved by the reaction will be absorbed by the calorimeter. Thus: ΔHrxn = -ΔHcalorimeter The magnitude of the observed temperature change depends on the heat capacity of the calorimeter. So what’s heat capacity anyways? Heat Capacity and Specific Heat Heat Capacity (Cp) The heat capacity of any substance is the amount of energy (heat) required to raise the temperature of that substance 1°C (or 1°K). Heat capacity thus measures the substance’s ability to absorb energy as heat. Water, for example, has a relatively high specific heat. What do you think has a greater heat capacity, oven mitts or a piece of metal? How do you know this? The units for heat capacities are: kJ (heat absorbed) Cp = ----K (increase in temperature) In a calorimetry experiment, we can say that: ΔHrxn = Cp(calorimeter) ∙ ΔT Specific Heat (Csp) If we want to get more specific, we can speak of a substance’s specific heat. This is the amount of heat required to raise the temperature of 1 gram of that substance 1°C. Units for specific heat are usually: kJ Csp = --------K∙g The heat involved in any thermal process can be calculated using this formula: q = m ∙ Csp ∙ ΔT q = heat m = mass (moles for Molar Specific Heat) Csp = Specific Heat – pay attention to the units ΔT = Change in temperature Sample Problems Example: When 1000 J of heat is added to 50.5 g of ammonia, the temperature increases by 9.8°C. What is the specific heat of ammonia? AP Chemistry Example: A 117 g sample of sodium chloride is dissolved in 1 L of water in a calorimeter whose total heat capacity (water + vessel) is 1000 J/K. The temperature decreases by 12.8 K. Calculate the heat of reaction per mole for this process. Example: A 36.7 g sample of aluminum is heated to 102°C and placed in a calorimeter containing 150.0 g water at 23.4°C. After the metal cools, the final temperature of metal and water is 25.0°C. Calculate the specific heat capacity of aluminum, assuming that no heat escapes to the surroundings or is transferred to the calorimeter. The specific heat of water is 4.18 J/°C∙g Example: In a calorimeter, 50.0 mL of 0.300 M AgNO3 and 50.0 mL of 0.300 M HCl are mixed to yield the following reaction: Ag+(aq) + Cl-(aq) 🡪 AgCl(s) The two solutions were initially at 23.0°C and the final temperature is 25°C. Calculate the heat that accompanies this reaction in kJ/mol of AgCl formed. Assume that the combined solution has a mass of 100.0 g and has a specific heat capacity of 4.18 J/°C∙g Phases of Matter There are 3 common phases of matter: SOLID, LIQUID, GAS Draw pictures of a solid, liquid, and gas, on a molecular level: The types of phase changes are listed below: SOLID melting🡪 LIQUID evaporation🡪 🡨 solidification 🡨 condensation GAS Breaking bonds requires energy. Forming bonds gives off energy. Going from a solid 🡪 liquid 🡪 gas therefore requires energy. Going from a gas 🡪 liquid 🡪 solid therefore releases energy. Are the following phase changes exothermic or endothermic? a) b) c) d) ice cubes melting freezing water into ice boiling a liquid steam from your shower condensing into water droplets on the mirror A substance may never change temperature and phase simultaneously! This means that if a substance is heated, the heat may be used to do one of two things: -The temperature can increase -The phase can change (solid🡪liquid, liquid🡪gas) BUT NEVER BOTH AT THE SAME TIME! Draw a heat vs temperature graph of water being heated: The energy it takes to go from solid 🡪 liquid is called the enthalpy of fusion (ΔHfusion) Going the reverse, a liquid 🡪 solid, releases the same amount of energy (-ΔHfusion) The energy it takes to go from liquid 🡪 gas is called the enthalpy of vaporization (ΔHvaporization) Going the reverse, a gas 🡪 liquid, releases the same amount of energy (-ΔHvaporization) AP Chemistry SOLID ΔHfusion 🡪 🡨 -ΔHfusion LIQUID ΔHvaporization 🡪 GAS 🡨 -ΔHvaporization Calorimetry Worksheet HEAT GAINED BY WATER = HEAT LOST BY OBJECT Specific Heat of Water = 4.18 J/G·°C One A 32.5 g sample of tin is heated to 132°C and placed in a calorimeter containing 125 g of water at 25.00°C. After the metal cools, the final temperature of metal and water is 27.00°C. Calculate the specific heat capacity of tin. a) Calculate the amount of heat gained by the water. b) Calculate the specific heat of the tin. Remember that the heat gained by the water is equal to the heat lost by the tin. Two A 28.2 g sample of nickel is heated to 99.8°C and placed in a coffee-cup calorimeter containing 150.0 g water at 23.5°C. After the metal cools, the final temperature of the metal and water is 25.0°C. Calculate the specific heat capacity of nickel, assuming that no heat escapes to the surroundings or is transferred to the calorimeter. a) Calculate the amount of heat gained by the water. c) Calculate the specific heat of the nickel. Remember that the heat gained by the water is equal to the heat lost by the nickel. Three A 46.2 g sample of copper is heated to 95.4°C and then placed in a calorimeter containing 75.0 g water at 19.6°C. The final temperature of the metal and water is 21.8°C. Calculate the specific heat capacity of copper, assuming that no heat escapes to the surroundings or is transferred to the calorimeter. a) Calculate the amount of heat gained by the water. d) Calculate the specific heat of the nickel. Remember that the heat gained by the water is equal to the heat lost by the nickel. AP Chemistry Four A 32.55 g sample of a greenish, unknown substance is heated to 54.54°C and the placed in a calorimeter containing 100.00 g water at 10.00°C. The final temperature of the metal and water is 12.32°C. What is the specific heat capacity of the unknown substance? Five A 0.231 g piece of brass alloy is heated to 135.03°C and then placed in a calorimeter containing 50.0 g water at 25.00°C. The final temperature of the metal and water is 30.00°C. What is the specific heat capacity of the brass alloy? Six The specific heat capacity of aluminum is 0.900 J/°C·g. a) Calculate the energy needed to raise the temperature of a 850. g block of aluminum from 22.8°C to 94.6°C. b) Calculate the molar heat capacity (heat/mol) of aluminum. Seven The specific heat capacity of graphite (pure carbon) is 0.71 J/°C·g. Calculate the energy required to a) Raise the temperature of 1.0 mol graphite by 1.0°C. b) Raise the temperature of 850 g graphite by 150°C.