Chemistry 211 Exam II Key Fall 2024 PDF

## Chemistry 211, Fall 2024 ### Exam II on Chapters 4 & 5 Dr. Scott Day Name: **Key** Date: ### Matching Match the words to their best definition/description. (3 pts each) | | Definition/Description | Answer | |---|---|---| | 1. | A substance that increases the hydrogen ion concentration in an...

## Chemistry 211, Fall 2024 ### Exam II on Chapters 4 & 5 Dr. Scott Day Name: **Key** Date: ### Matching Match the words to their best definition/description. (3 pts each) | | Definition/Description | Answer | |---|---|---| | 1. | A substance that increases the hydrogen ion concentration in an aqueous solution. | I: Acid | | 2. | A system that can exchange matter and energy with the surroundings. | N: Open System | | 3. | An insoluble solid formed by a reaction in solution. | P: Precipitate | | 4. | The energy of motion. | B: Kinetic Energy | | 5. | The energy used to cause an object with mass to move against a force. | O: Work | | 6. | Energy is conserved. | T: The 1st Law of Thermodynamics | | 7. | A process in which the system absorbs heat from its surroundings. | K: Endothermic | | 8. | The sum of all the kinetic and potential energy of a system. | M: Internal Energy | | 9. | Substances written on the right of the arrow in a chemical equation. | G: Products | | 10. | The gain of electrons by a substance. | S: Reduction | ### Multiple Choice Choose the best answer for each question. Write the answer in capital letters in the margin. (6 pts each) 1. What are the spectator ions in the following precipitation reaction? Mg(NO3)2(aq) + 2 NaOH(aq) → Mg(OH)2(s) + 2 NaNO3(aq) **C**. Na+ and NO3- 2. What is the oxidation number of Cl in the compound, HCIO2? **B**. +3 3. Which element is oxidized and which is reduced in the reaction 8 H2S(g) + 4 O2(g) → S8(1) + 8 H2O(g) **A**. S is oxidized and O is reduced 4. If 0.372 moles of benzene (C6H6) react with 51.6 grams of bromine (Br2), what is the theoretical yield of bromobenzene, C6H5Br? C6H6 + Br2→ C6H5Br + HBr **A**. 50.70 grams of C6H5Br 5. How many grams of Pb(NO3)2 are needed to completely precipitate all of the iodide ions from 250 mL of a 0.167 M potassium iodide solution? Pb(NO3)2 (aq) + 2 KI(aq) → PbI2 (s) + 2 KNO3(aq) **B**. 6.91 grams of Pb(NO3)2 6. The Claus process for removing hydrogen sulfide from natural gas relies on the reaction: 8 H2S(g) + 4 O2(g) → S8(1) + 8H2O(g) If the process gives a 98% yield of S8 and you began with 0.134 moles of H2S and excess oxygen, what mass of S8 was actually measured? **B**. 4.21 grams of S8 7. The internal energy of a system is always increased by **C**. adding heat to the system and having the surroundings do work on the system 8. What is the change in internal energy of a system when 0.37 J of work is done on the system and the system releases 1.97 J of heat to the surroundings? **B**. -1.60 J 9. What is the mass of an aluminum sample if 819 J caused its temperature to increase from 25°C to 51°C? [Cs = 0.90 J/g-K] **D**. 35.0 g 10. How much energy is released when 2.5 grams of F2 is reacted with excess H2 according to the following thermochemical equation? H2(g) + F2(g) → 2HF(g) ΔΗ = -537 kJ **A**. -35.3 kJ 11. Calculate the reaction enthalpy for the reaction: P4O6(s) + 2O2(g) → P4O10(s) ΔΗrxn = ? given the following thermochemical equations. P4(s) +3 O2(g) → P406(s) ΔΗrxn = -1640.1 kJ P4(s) +5 O2(g) → P4O10(s) ΔΗrxn = -2940.1 kJ **A**. -1300.0 kJ 12. What is ΔΗ°rxn for the following reaction: CH4(g) + 4 F2(g) → CF4(g) + 4 HF(g) | Compound | ΔΗ°f (kJ/mol) | |---|---| | CH4(g) | -74.8 | | F2(g) | 0 | | CF4(g) | -679.9 | | HF(g) | -268.6 | **C**. -1680 kJ ### Problems Show your work for full credit. 1. When a solution of ammonium sulfide is mixed with a solution of magnesium nitrate, a precipitate of magnesium sulfide forms. a. Write the molecular equation for this reaction. (4 points) (NH4)2S(aq) + Mg(NO3)2(aq) → MgS(s) + 2NH4NO3(aq) b. Write the total ionic equation for this reaction. (4 points) 2NH4+(aq) + S2-(aq) + Mg2+(aq) + 2NO3-(aq) → MgS(s) + 2NH4+(aq) + 2NO3-(aq) c. What are the spectator ions in this reaction? (2 points) NH4+ and NO3- d. Write the net ionic equation for this reaction. (2 points) Mg2+(aq) + S2-(aq) → MgS(s) 2. What is the molarity of a Ca(OH)2 solution if 37.5 mL is needed to neutralize 74.8 mL of a 0.519 M HCl solution? Ca(OH)2(aq) + 2 HCl(aq) → 2 H2O(l) + CaCl2(aq) 74.8mL x (1L/1000 mL) x (0.519 moles HCl/ 1 L) x (1 mole Ca(OH)2/ 2 moles HCl) = 1.94 x 10^-2 moles Ca(OH)2 M = (1.94 x 10 ^-2 moles Ca(OH)2)/ 0.0375 L = 0.518 M 3. When a 3.27 gram sample of solid sodium hydroxide is dissolved in 100.0 grams of water in a constant pressure calorimeter, the temperature of the solution increases from 22.7°C to 41.9°C. Assuming the specific heat of the solution is 4.184 J/g-K, calculate ΔH for this process. (12 points) ΔH = q = - (mass of solution) x (specific heat) x (change in temperature) ΔH = -(100.0g + 3.27g) x (4.184 J/g-K) x (41.9°C - 22.7°C) ΔH = -8296 J = -8.296 kJ 4. The fuel used in many gas grills is propane, C3H8(g). To produce a flame, the propane is combusted with oxygen from the air according to the following equation: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) ΔΗ° = -2044 kJ a. What is the heat of formation of C3H8(g)? [∆Hf° (CO2(g)) = -393.5 kJ/mol and ∆Hf° (H2O(g)) = -241.82 kJ/mol] (8 points) ΔH°rxn = ΣnΔHf°(products) - ΣmΔHf°(reactants) -2044 kJ = [(3)(-393.5 kJ/mol) + (4)(-241.82 kJ/mol)] - [(1)(ΔHf°(C3H8(g))) + (5)(0 kJ/mol)] ΔHf°(C3H8(g)) = -103.78 kJ/mol b. If the tank of propane holds 20 pounds, what is the total energy (kJ) that can be produced from propane combustion reaction? [1 pound = 453.6 g] (8 points) 20 pounds x (453.6 g/1 pound) x (1 mole C3H8/ 44.097 g C3H8) x (-2044 kJ/ 1 mole C3H8) = 4.21 x 10^5 kJ