CHEMISTRY 30 Assessment Enthalpy Change and Calorimetry Formative Record all responses in this book. Keep this question book and the answer key as part of your notes.
1. Open and closed systems can be compared by the way matter and energy transfer. How do matter and energy transfer in a closed system? Matter is transferred, energy is not transferred Matter is not transferred, energy is transferred Neither matter nor energy are transferred Both matter and energy are transferred 2. The DH value for an endothermic energy change is positive. This means that reactants have less potential energy than products products have less potential energy than reactants potential energy is decreasing in the system kinetic energy is increasing in the system 3. A common flaw in calorimetry experiments to determine the molar enthalpy of dissolving is to have a mass of solute that is inappropriate for the volume of water used resulting in a temperature change that is too small or too large. A temperature change of 5 C to 10 C is a good range. Calculate the mass of NaOH (molar enthalpy of dissolving is 44.51 kj/mol ) that should be dissolved into 250 ml of water in order to produce a temperature increase of 7.00 40.0 g 6.59 g 4.12 g 0.165 g Chemistry 30 Enthalpy Change and Calorimetry Formative Page 1
Use the following information to answer the next two questions A student using a computer-based laboratory (CBL) temperature probe attempted to determine the amount of energy released by a commerical heat pack. The student activated the heat pack and placed it in an insulated calorimeter containing 1.00 kg of water at 11.30 A graph of the results obtained is given below. 4. To more accurately determine the total heat released by the heat pack, the student should use more water use a larger heat pack start with colder water collect results for a longer period of time 5. If the energy change of the plastic container is not considered, the calculated energy change for the water from 0 s to 200 s is 13.0 kj 14.4 kj 60.3 kj 838 kj Chemistry 30 Enthalpy Change and Calorimetry Formative Page 2
6. In experiments that use a simple calorimeter, the main assumption is that distilled water is always used heat is not transferred to the surroundings the starting temperature is always room temperature the final temperature is always above room temperature 7. The molar enthalpy of neutralization for NaOH(aq) is 59 kj/mol. In an experiment, 8.0 g of NaOH(s) is dissolved in water and the solution is then allowed to reach room temperature. The NaOH(aq) is then added to a strong acid solution which was also initially at room temperature. The final volume of solution was 500 ml and the temperature increased from 20.0 C to a different temperature. Using this data, what was the final temperature of the water? 5.6 C 14 C 26 C 32 C 8. Chromium reacts with oxygen to produce chromium (III) oxide. Which is the correct potential energy graph for this reaction? Use the standard molar enthalpies of formation table on page 4 & 5 of your data book to help answer this question. Chemistry 30 Enthalpy Change and Calorimetry Formative Page 3
9. Use the graph to answer the question. For the reaction in the potential energy graph above, the reactants have i potential energy than the products and the reaction is ii. The statement above is completed by the information in row i Less More Less More ii exothermic exothermic endothermic endothermic 10. Consider the following enthalpy diagram. The true statement concerning the accompanying diagram is: DH is positive. the system is endothermic. the system releases heat to the surroundings. the enthalpy of the products is greater than the enthalpy of reactants. Chemistry 30 Enthalpy Change and Calorimetry Formative Page 4
11. When solid ammonium chloride, NH 4 Cl, is added to water, the temperature of the solution decreases. Which statement best describes this observation? The reaction is exothermic. NH 4 Cl (s) does not dissolve in water. NH 4 Cl (s) + H 2 O (l) NH 4 Cl (aq) + 33.6 kj NH 4 Cl (s) + H 2 O (l) NH 4 Cl (aq) DH=+33.6kJ 12. Much of the lead used for batteries and ammunition during the First and Second World Wars came from galena, PbS (s). The following equations represent the reactions that are involved in refining galena to produce solid lead. Equation I 2 PbS (s) + 3 O 2 (g) 2 PbO (s) + 2 SO 2 (g) DH = -827.4 kj Equation II 2 C (S) + O 2 (g) 2 CO (g) Equation III PbO (s) + CO (g) Pb (s) + CO 2 (g) In equation I, the reactants have i energy than the products and if energy were included as a term in the equation, it would be a ii. The statement above is completed by the information in row i less less more more ii reactant product reactant product Chemistry 30 Enthalpy Change and Calorimetry Formative Page 5
13. Identify the exothermic reaction 1 CO (g) + 111 kj C (s) + 2 O 2 (g) H 2 (g) + I 2 (g) 2 HI (g) D H = +52 kj 2 H 2 (g) + O 2 (g) 2 H 2 O D H = -486 kj 14. When 1.30 mol of HF(g) is dissolved in a large amount of water, the temperature increased from 20.0 C to 27.6 The molar enthalpy of solution for HF(g) is 61.5 kj/mol. From the information, what was the mass of water? 9.54 x 10 2 g 1.49 x 10 3 g 2.51 x 10 3 g 5.23 x 10 3 g Chemistry 30 Enthalpy Change and Calorimetry Formative Page 6
Use the information to answer the next 2 question(s). A simple calorimeter is made of two nested polystyrene cups. A thermometer, with a mass of 22.46 g and a specific heat capacity of 0.865 J/g C, is used to measure the temperature change. Two solutions are mixed in the calorimeter resulting in a total of 125 ml of water. The increase in temperature recorded was 2.00 15. The quantity of heat absorbed by the thermometer in the above investigation is 1.73 C 2.00 C 19.4 J 38.9 J 16. The error caused by the heat absorbed by the thermometer in the above investigation is zero too small to be measured usually assumed to be negligible accounted for in the calculations in most high school experiments 17. Living plants produce glucose in the process of photosynthesis according to the equation that follows. 6 CO 2 (g) + 6 H 2 O (l) + energy C 6 H 12 O 6 (s) + 6 O 2 (g) This reaction is i, and the value of DH is ii. The statement above is completed by the information in row i ii Exothermic positive Exothermic negative Endothermic positive Endothermic negative 18. Write the chemical equation which best represents the expression that follows. D f H = 90.8 kj/mol of mercury (II) oxide 2 Hg (l) + O 2 (g) 2 HgO (s) + 90.8 kj 2 Hg (l) + O 2 (g) + 90.8 kj 2 HgO (s) f HgO 2 Hg (l) + O 2 (g) 2 HgO (s) D H = 90.8 kj/mol 2 Hg (l) + O 2 (g) 2 HgO (s) DH = +90.8 kj Chemistry 30 Enthalpy Change and Calorimetry Formative Page 7
For the following numercial response questions, record your answers on the separate numerical response answer sheet. 1. NUMERICAL RESPONSE Commercial drain cleaners typically contain sodium hydroxide and aluminum. When the solid cleaner is poured down the drain and water is added, the reaction that occurs is represented by the equation 2 NaOH (s) + 2 Al (s) + 2 H 2 O (l) 2 NaAlO 2 (aq) + 3 H 2 (g) DH = -850.0 kj In the production of 4.00 mol of NaAlO 2 (aq), the heat released is MJ (Record your three-digit answer on your response sheet) 2. NUMERICAL RESPONSE Consider the following information. Calorimetric Data 1 maximum temperature change of water 2 mass of aluminum calorimeter 3 mass of aluminum calorimeter and water 4 initial temperature of aluminum calorimeter 5 maximum temperature change of KOH (s) 6 mass of KOH (s) used The data required to determine the molar enthalpy of dissolving for KOH (s), listed in numerical order, are,, and. (Record your four digit answer in numerical order on your response sheet) Chemistry 30 Enthalpy Change and Calorimetry Formative Page 8
3. NUMERICAL RESPONSE Consider the following reaction: 2 C 2 H 2 (g) + 5 O 2 (g) 4 CO 2 (g) + 2 H 2 O (g) DH = -2 511.0 kj The amount of energy released by the combustion of 100 g of C 2 H 2 (g) is MJ (Record your three-digit answer on your response sheet) 4. NUMERICAL RESPONSE In an experiment, a student heated 500 g of water from 25.0 C to 91.0 C using 0.133 mol of ethanol. If it is assumed that all the heat energy was absorbed by the calorimeter water, the experimental molar enthalpy of combustion for ethanol was +/- MJ/mol. (Record your three-digit answer on your response sheet) 5. NUMERICAL RESPONSE Consider the following information. Equations 1 C 8 H 18 (l) + 25 2 O 2 (g) 8 CO 2 (g) + 9 H 2 O (g) + 5 074.1 kj 2 C 3 H 8 (l) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (g) + 2 043. 9 kj 3 C 2 H 5 OH (l) + 3 O 2 (g) 2 CO 2 (g) + 3 H 2 O (g) + 1 234.8 kj 4 CH 4 (l) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) + 802.5 kj When ranked in order from the reaction that has the smallest enthalpy change per mole of carbon dioxide to the reaction that has the largest enthalpy change per mole of carbon dioxide, the reactions are,, and smallest largest (Record all four digits of your answer on your response sheet) Chemistry 30 Enthalpy Change and Calorimetry Formative Page 9
6. NUMERICAL RESPONSE A student uses a simple "coffee cup" calorimeter to determine the molar enthalpy of solution for solid ammonium nitrate. The experimental evidence collected was recorded below: Mass of calorimeter Mass of calorimeter and water Mass of ammonium nitrate Initial temperature of water Final temperature of water 25.45 g 175.45 g 1.68 g 22.30 o C 20.98 o C According to the above data, the molar enthalpy of solution for ammonium nitrate is kj/mol (Record your answer to three digits on your response sheet) 7. NUMERICAL RESPONSE Consider the following equation for the combustion of hydrogen: H 2 (g) + O 2 (g) H 2 O (g) + 243 kj In order to produce 1215 kj of thermal energy g of H 2 must be burned. (Record your three-digit answer on your response sheet) You have completed the assessment! Please ensure that any separate written response questions are completed. Chemistry 30 Enthalpy Change and Calorimetry Formative Page 10
Chemistry 30 Enthalpy change & calorimetry formative assessment answer key. Highlight each question number that was incorrect on your test (Note that a question may apply to more than one concept). The concept(s) with the most questions highlighted is/are the one(s) that need to be reviewed thoroughly. This is part of your responsibility as a student in this course. Questions Concept Outcomes 8, 9, 10, 13, 17, 18, NR 5 2, 12, NR 1, NR 3, NR 7 Communicating enthalpy change Enthalpy change recall the application of Q = mcd to the analysis of heat transfer. explain, in a general way, how stored energy in the chemical bonds of hydrocarbons originated from the sun. define enthalpy and molar enthalpy for chemical reactions. write balanced equations for chemical reactions that include energy changes. compare energy changes associated with a variety of chemical reactions through the analysis of data and energy diagrams. identify that liquid water and carbon dioxide gas are reactants in photosynthesis and products of cellular respiration and that gaseous water and carbon dioxide gas are the products of hydrocarbon combustion in an open system classify chemical reactions as endothermic or exothermic, including those for the processes of photosynthesis, cellular respiration and hydrocarbon combustion. use and interpret DH notation to communicate and calculate energy changes in chemical reactions. 1, 3, 4, 5, 6, 7, 11, 14, 15, 16, NR 2, NR 4, NR 6 calorimetry use calorimetry data to determine the enthalpy changes in chemical reactions perform calorimetry experiments to determine the molar enthalpy change of chemical reactions use thermometers or temperature probes appropriately when measuring temperature changes Chemistry 30 Lesson 1-07 Page 1 2012 T. de Bruin
Question Answer Explanation 1 B Basic understanding/definition of open, closed & isolated systems. 2 A Endothermic reactions (positive DH) require an input of energy. This input adds to the potential energy of the system. Thus endothermic reaction products have more potential energy than the reactants. 3 B Basic calorimetry calculation using = then = 4 D You needed to examine the graph and realize that the temperature had not yet peaked. Thus the final temperature was taken too early. 5 A Use = 6 B Standard assumptions in calorimetry. The calorimeter is considered to be an isolated system. 7 C Use = and solve for 8 D 9 C The DH is negative thus the reaction is exothermic. The enthalpy change graph must thus go down. This is a formation reaction so the product is written to the right of the enthalpy change arrow in the graph. If an enthalpy change graph goes up, the reaction is endothermic, absorbing energy and increasing the overall potential energy. 10 C If an enthalpy change graph goes down, the reaction is exothermic releasing energy. 11 D 12 D In calorimetry, if the temperature decreases, the chemical reaction/process is absorbing energy (endothermic). Equation I has a negative DH, thus the reactants have more potential energy than the products. A negative DH is included in a balanced equation as a product. 13 D Choice D is the only negative DH, which is exothermic. 14 C Use = and solve for 15 D Use = 16 C Standard assumption in calorimetry that the calorimeter materials absorb/release a negligible amount of energy. 17 C You need to know that photosynthesis is an endothermic process with a positive DH. 18 C A negative DH is exothermic and the value would be included as a product in relation to the amount of moles of the chemical stated. As an enthalpy statement the amount of energy must be in relation to the amount chemical specified. Chemistry 30 Lesson 1-07 Page 2 2012 T. de Bruin
Question Answer Explanation Use a ratio to calculate the enthalpy change for a different number of moles: NR 1 1.70. = Solve for. You need to note that the answer must be in MJ. NR 2 1236 (any order) This knowledge was gained from the calorimetry labs. You need to subtract #2 from #3 in order to determine the amount of water in the calorimeter. Use a ratio to calculate the enthalpy change for a different amount of C 2 H 2 :. =. / Solve for. NR 3 4.82 NR 4 1.04 You need to note that the answer is required to be in MJ. You could also use = then =. / =. 1255.5 = 4.82 = 1255.5 / Use = and solve for. You must note that the answer must be recorded as MJ/mol. NR 5 NR 6 3124 in order 39.5 kj/mol You must calculate = for each reaction, where is the number of moles of CO 2 in each balanced equation. Once that is done, rank in order from smallest to largest. Use = and solve for. Note the mass of water is calculated by subtracting the mass of the calorimeter from the mass of the calorimeter and water. Use a ratio to calculate the enthalpy change for a different number of moles: NR 7 10.1 g =. / Solve for. You could also use the second method as in NR 3. Chemistry 30 Lesson 1-07 Page 3 2012 T. de Bruin