# Enthalpy of Reaction and Calorimetry worksheet

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1 Enthalpy of Reaction and Calorimetry worksheet 1. Calcium carbonate decomposes at high temperature to form carbon dioxide and calcium oxide, calculate the enthalpy of reaction. CaCO 3 CO 2 + CaO 2. Carbon tetrachloride can be formed by reacting chlorine with methane, calculate the enthalpy of reaction. CH Cl 2 CCl H 2 3. When potassium chloride reacts with oxygen under the right conditions, potassium chlorate is formed: 2 KCl + 3 O 2 2KClO 3 Given that the enthalpy of formation of potassium chloride is kj/mol and the enthalpy of formation of potassium chlorate is kj/mol, determine the enthalpy of reaction. Calorimetry Practice 4. Compound A is burned in a bomb calorimeter that contains 2.50 liters of water. If the combustion of moles of this compound causes the temperature of the water to rise C, what is the molar heat of combustion of compound A? 5. Compound B is burned in a bomb calorimeter that contains 1.50 liters of water. When I burned 50.0 grams of compound B in the calorimeter, the temperature rise of the water in the calorimeter was C. If the heat of combustion of compound B is 2,150 kj/mol, what is the molar mass of compound B? 6. The molar heat of combustion of compound C is 1,250 kj/mol. If I were to burn moles of this compound in a bomb calorimeter with a reservoir that holds 2.50 L of water, what would the expected temperature increase be?

2 INV MOLAR ENTHALPY of SOLUTION - MEDICAL COLD PACKS Part 2 1. Describe the difference between heat and temperature. 2. For the compounds used in this experiment ( Nelson Chemistry 12 p. 347): (a) Calculate the number of moles present in a g sample. (b) Calculate the molar enthalpy (c) Write the thermochemical chemical equation for your compound's dissociation in water. 3. (a) When a substance dissolves its heat content, or enthalpy, will either increase or decrease. This change in enthalpy is observed as a change in temperature of the water in the calorimeter. Sketch a graph showing the change in enthalpy for the dissolving of an ionic compound which is: (b) For your compound, state whether the dissolving was an endothermic or exothermic process. (c) Will the sign for _H in your case be positive or negative? 4. Describe two factors that affect the strength of an ionic bond in a compound. 5. (a) Define crystal lattice energy and hydration energy. (b) Which of the properties in part (a) reduces the solubility of an ionic compound? (c) Which property, when increased, increases the solubility? 6. For an ionic solid to melt, external energy must be applied to overcome the attractive forces holding its ions together. For example, sodium hydroxide must be heated to temperatures exceeding 318 C for it to melt. Despite such a high melting point, sodium hydroxide readily and exothermally dissolves and dissociates into its ions in water at room temperature. (a) What is the source of the energy required to separate the ions from the solid sodium hydroxide? (b) Why does the resulting sodium hydroxide solution feel hot? 7. Ammonium chloride is also soluble despite having a melting point of 320 C. Why does an ammonium chloride solution feel cold to the touch? 8. Magnesium oxide, MgO, and magnesium chloride, MgCl 2, are very similar, white, ionic solids with the following properties: Compound Melting Point Solubility MgO 2800 C insoluble MgCl C very soluble (a) Give the formula of the ions of each compound. (b) Account for the drastic difference in physical properties. 9. Consider the following data from the Group 1 chlorides. These compounds are similar in structure, each being formed from a + cation and the -1 chloride ion. However, they are quite different with respect to lattice energy, hydration energy, and enthalpy of solution. Explain these differences. Substance Lattice Energy (kj/mol) Hydration Energy (kj/mol) Enthalpy of Solution (kj/mol) LiCl NaCl KCI

3 Heats of Reaction Using the Standard values of Η f, calculate the heats of reaction, ΔΗ rxn, or heats of formation, ΔΗ f All answers must include the intermediate steps with the formation reaction from the elements and their manipulation to form the overall reaction. For #1-6, the equation: ΔHo rxn = ΣnΔHo products ΣnΔHo reactants eg. SO 2(g) + H 2 O (l) H 2 SO 4(aq) H kj (1) 1/8 S 8 + O 2 _ SO (2) H 2 + 1/2 O 2 _ H 2 O (3) H 2 + 1/2 O 2 + 1/8 S 8 _ H 2 SO Then multiply or reverse, whatever is needed so that they add to the original reaction. 1. C 6 H 6 (l) + 15/2 O 2 (g) 6 CO 2 (g) + 3 H 2 O (g) CANNOT BE USED HNO 3 (aq) + NO (g) 3 NO 2 (g) + H 2 O (l) 3. C 2 H 2 (g) + C 2 H 6 (g) 2 C 2 H 4 (g) 4. PbO 2 (s) + CO (g) PbO (s) + CO 2 (g) 5. Al 2 (SO 4 ) 3 (s) + 3 H 2 O (l) Al 2 O 3 (s) + 3 H 2 SO 4 (aq) 6. 3 SO 2 (s) + 2 HNO 3 (aq) + 2 H 2 O (l) 3 H 2 SO 4 (aq) + 2 NO (g) 7. Given that H rxn = kj for the following, calculate the H f for ClF 3 (g). 2 ClF 3 (g) + 2 NH 3 (g) N 2 (g) + 6 HF (g) + Cl 2 (g) 8. Given that H rxn = kj for the following, calculate the H f for HNO 2 (g). HNO 2 (g) + 1/2 O 2 (g) HNO 3 (aq) 9. Given that H rxn = kj for the following, calculate the H f for Fe(CO) 5 (g). Fe 2 O 3 (s) + 13 CO (g) 2 Fe(CO) 5 (g) + 3 CO 2 (g) 10. Given that H rxn = kj for the following, calculate the H f for Fe 3 O 4 (s). 2 Fe 3 O 4 (s) + CO 2 (g) 3 Fe 2 O 3 (s) + CO (g)

4 Practice Thermochemistry Questions 1. a) Urea (from urine) hydrolyzes slowly in the presence of water to produce ammonia and carbon dioxide. What is the standard H, S and G for this reaction when 1 mole of urea reacts with water? b) What is the spontaneity of this reaction? 2. What is G for the combustion of liquid ethyl alcohol (C 2 H 5 OH) to give CO 2(g) and H 2 O (g)? Is the reaction spontaneous? 3. Calculate H and S for the following reaction and decide in which direction each of these factors will drive the reaction. N 2(g) + 3 H 2(g) 2 NH 3(g) Use the data given below to answer your questions: Compound H (kj/mol) S (J/(mol K)) G (kj/(mol K)) CO(NH 2 ) 2(aq) H 2 O (l) CO 2(g) NH 3(g) C 2 H 5 OH (l) H 2 O (g) N 2 (g) H 2 (g)

5 SUPPLEMENTARY ENTHALPY PROBLEMS 1. Construct an enthalpy diagram showing the enthalpy changes for a one step conversion of germanium, Ge(s), into GeO2 (s) and a two step conversion - first to the monoxide, GeO(s) followed by the oxidation to GeO2 (s). The relevant thermochemical equations are as follows; Ge(s) + 1/2 O2 (g) GeO(s) H = -255 kj Ge(s) + O2(g) GeO2 (s) H = kj Using this diagram, determine the value of H for the reaction GeO(s) + 1/2 O2 (g) GeO2 (s) 2. Nitrogen monoxide oxidizes to the toxic red brown gas nitrogen dioxide in air. Construct an accurate enthalpy diagram for this reaction. Use the diagram to explain which of the oxides of nitrogen has stronger covalent bonds. 2 NO(g) + O2 (g) 2 NO2 (g) 3. One of the building blocks for proteins such as those in muscles and in sinews, is an amino acid called glycine, C2H5NO2. The equation for its combustion is 4 C2H5NO2 (s) + 9 O2 (g) 8 CO2 (g) + 10 H2O(l) + 2 N2 (g) The value of its H comb is kj/mol. Calculate H f for glycine. 4. On Easter Sunday, April 3, 1983, nitric acid spilled from a tank car near Denver, Colorado. The spill was neutralized with sodium carbonate. The reaction is 2 HNO3 (aq) + Na2CO3 (s) 2 NaNO3 (aq) + H2O(l) + CO2 (g) (a)calculate H for this reaction. H f for NaNO3 (aq) is -467 kj/mol, H f for HNO3 (aq) is kj/mol, H f for Na2CO3 (aq) is kj/mol] (b) Approximately 9.1 x 10 4 L of nitric acid was spilled. Assuming a molar concentration of 15.4M, how much sodium carbonate was required for complete neutralization and how much heat was evolved. (c) According to the Denver Post for April 4, 1983, authorities feared a volatile reaction might occur during the neutralization. Considering the magnitude of H, what was their major concern? The large amount of heat energy released in this neutralization reaction could cause the decomposition of the nitrate compounds into toxic oxides of nitrogen.

6 Supplementary Thermochemistry Problems 1. a) Calculate the enthalpy change, ΔH rxn, for the following reaction using equations 1, 2 and 3. Given: FeO + CO Fe + CO 2 ΔH rxn (kj) 1) Fe 2 O CO 2 Fe + 3 CO ) 3 Fe 2 O 3 + CO 2 Fe 3 O 4 + CO ) Fe 3 O 4 + CO 3 FeO + CO Calculate the enthalpy change, ΔH rxn, for the following reaction using equations 1, 2 and 3. Given: 3 N 2 H ClF 3 3 N HF + 2 Cl 2 ΔH rxn (kj) 1) 2 ClF NH 3 N HF + Cl ) N 2 H 4 + O 2 N H 2 O ) 4 NH O 2 2 N H 2 O Calculate the enthalpy change, ΔH rxn, for the following reaction using equations 1, 2 and 3. Given: 2 H 3 BO 3 B 2 O H 2 O ΔH rxn (kj) 1) H 3 BO 3 HBO 2 + H 2 O ) 2 B 2 O 3 + H 2 O H 2 B 4 O ) H 2 B 4 O 7 + H 2 O 4 HBO Using the average bond energies, N N (941 kj/mol), N=N (418 kj/mol), N-N (160 kj/mol), N-H (391 kj/mol) and H-H (432 kj/mol) estimate the ΔH f for hydrazine, N 2 H The ΔH rxn of the gaseous reaction: XeF 2 + H 2 2 HF + Xe is found to be 430 kj. Use this value and average bond energies [H-F (565 kj/mol)] to determine the average bond energy of the Xe-F bond.

7 1. For the following reaction, KINETICS Rate Mechanisms and the RDS NO (g) + O 3 (g) NO 2 (g) + O 2 (g) Given: E a(forward) = 10.0 kj/mol and ΔH (forward) = kj/mol a) Draw the energy level diagram for this reversible reaction. b) Calculate the E a(reverse) and the ΔH (reverse). For the reaction the following data was obtained: Trial [O 3 ] i (mol / L) [NO] i (mol / L) Initial Rate of Formation of NO 2 (mol / L s) x x x c) What is the rate law for the reaction? d) The proposed mechanism is 3-step. The 1 st is the RDS producing O and NO 3, which are both rxn intermediates. Derive the other 2 elementary steps for the mechanism. 2. Propose a mechanism for the chain reaction: H 2 (g) + I 2 (g) 2 HI (g) Where, I, is a catalyst. Label each step as either chain initiation, propagation or termination. 3. For the reaction: 2 NO (g) + Br 2 (g) 2 NOBr (g) the following data was obtained: Trial [NO] i (mol / L) [Br 2 ] i (mol / L) Initial Rate of Formation of NOBr (mol / L s) x x x 10-4 a) Write the rate law equation that is consistent with the data. b) Calculate the value of the specific rate constant, k, and specify units. c) The following mechanism was proposed for the reaction: Br 2 (g) + NO (g) NOBr 2 (g) slow NOBr 2 (g) + NO (g) 2 NOBr (g) fast Is this mechanism consistent with the given experimental observations? Justify your answer. 4. Propose a mechanism for the chain reaction: H 2 (g) + I 2 (g) 2 HI (g) Where, I, is a reaction intermediate. Label each step as either chain initiation, propagation or termination.

8 5. The reaction: NO 2 (g) + CO (g) NO (g) + CO 2 (g) is believed to occur by the following reaction mechanism: NO 2 (g) + NO 2 (g) NO 3 (g) + NO (g) ( slow ) NO 3 (g) + CO (g) NO 2 (g) + CO 2 (g) ( fast ) a) What is the rate law according to this mechanism? b) What would be the rate law if the reaction occurred directly in a single step? 6. Draw an energy level diagram for: X + Y Z using the following information: a) The reaction is reversible b) The reaction consists of three elementary steps c) The rate determining step is the second step d) The forward reaction is endothermic e) The E a is twice the H for the forward reaction.

9 KINETICS - REVIEW 1. At high temperatures, the following reaction occurs: 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2 O (g) One experiment showed a concentration of ammonia of mol/l at 10 s and of mol/l at 55 s. Calculate the average rate, over this time interval, for each of the following: a) the rate of decomposition of NH 3. b) the rate of production of water. c) the rate of production of NO. d) the rate of consumption of oxygen. 2. For the following reactions, For the following reaction and its reverse activation energies were determined as follows: C2H4(g) + H2 (g) C2H6(g) Ea = 180 kj/mol C2H6(g) C2H4(g) + H2(g) Ea = 317 kj/mol Calculate H for the reaction of C2H4 with hydrogen 3. For the reaction: 2 A + 2 B C + D The following data about the reaction above were obtained from three experiments: Experiment [A] mol/l [B] mol/l Rate of Formation of C (mol/l min) x x x10-4 a) What is the rate equation for the reaction? b) What is the numerical value of the rate constant k? What are its dimensions? c) Propose a reaction mechanism for this reaction.

10 A + A + B A 2 B A 2 B + B A 2 + B 2 Then 2 A + 2 B A 2 (or C) + B 2 (or D) 4. The following results were obtained when the reaction represented below was studied at 25 C ClO 2(g) + F 2(g) 2 ClO 2 F (g) Experiment [ClO 2 ] i (mol/l) [F 2 ] i (mol/l) Rate of Increase of [ClO 2 F] i (mol/l s) x x x10-3 a) Write the rate law expression for the reaction above. b) Calculate the numerical value of the rate constant and specify the units. c) In experiment 2, what is the initial rate of decrease of [F 2 ]? d) Which of the following reaction mechanisms is consistent with the rate law developed in (a). Justify your choice. I. ClO 2 + F 2 ClO 2 F 2 (fast) ClO 2 F 2 ClO 2 F + F (slow) ClO 2 + F ClO 2 F (fast) II. F 2 2 F (slow) 2 (ClO 2 + F ClO 2 F) (fast) 5. Freon gases, e.g. CCl 2 F 2 (g), from leaking air conditioners and refrigeration units also make their way into the upper atmosphere.. These gases catalyze the destruction of ozone. CCl 2 F 2 (g) + O 3 (g) CClF 2 (g) + ClO (g) + O 2 (g) Where, Cl, is a reaction intermediate. Propose a mechanism for this reaction.

11 6. Draw a potential energy diagram for the reaction X + Y Z showing the following information. (a) The mechanism consists of two elementary steps. (b) The second elementary step is the rate-determining step. (c) The overall reaction is exothermic. (d) What would happen to the shape of the graph if a catalyst were added?

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