CHEM 101 Fall 08 Exam III(a)

Transcription

1 CHEM 101 Fall 08 Exam III(a) On the answer sheet (scantron) write you name, student ID number, and recitation section number. Choose the best (most correct) answer for each question and enter it on your answer sheet. Potentially useful data: Avogadro s Number: mol -1 q = c m ΔT 1. Please choose the letter a as your answer for this question. 2. The combustion of a fuel is exothermic because: a. weak bonds are formed. b. strong bonds are broken. c. the bond enthalpies of the products are greater than the bond enthalpies of the reactants. d. the bond enthalpies of both the reactants and products are large. e. the bonds in the fuel are strong. 3. A calorimeter is used to measure the heat given off by an exothermic reaction. What change is actually measured during the experiment? a. The mass of the bomb. b. The mass of the water. c. The temperature of the insulating container. d. The temperature of the water. e. The heat capacity of the water. 4. The temperature of 3.50 kg of water is raised by 1.17 C when 1.00 g of hydrazine N 2 H 4 is burned in a bomb calorimeter. The calorimeter has a heat capacity of 883 J/ C. How much heat is given off by the sample? The specific heat of water is 4.18 J/g C. a kj b kj c kj d kj e kj 5. Based on the following reaction below and the heat change listed beside it, which statement is false? N 2 (g) + 3 H 2 (g) 2 NH 3 (g) H = - 92 kj a. For the reverse process, H = + 92 kj. b. For the formation of two moles of NH 3, H = - 92 kj. c. For the reaction of two moles of N 2 and three moles of H 2, H = kj. d. Per mole of N 2, H = - 92 kj. e. Per mole of H 2, H = - 31 kj. Page 1 of 5

2 6. Determine the heat of reaction for the process C 2 H 4 (g) + 6 HCl(g) 2 CHCl 3 (g) + 4 H 2 (g) using the information given below: 2 C(s) + 2H 2 (g) C 2 H 4 (g) H = 52 kj H 2 (g) + Cl 2 (g) 2 HCl(g) H = kj C(s) + ½ H 2 (g) + 3/2 Cl 2 (g) CHCl 3 (g) H = kj a. 297 kj b. 29 kj c kj d kj e kj 7. Consider these two reactions. S(s) + O 2 (g) SO 2 (g) SO 2 (g) + ½ O 2 (g) SO 3 (g) What is ΔH for the following reaction? a. 397 kj b kj c. 833 kj d kj e kj S(s) + 3/2 O 2 (g) SO 3 (g) ΔH = 299 kj ΔH = 98 kj 8. The standard enthalpies of formation for several substances are given below: CO(g) -111 kj/mol CO 2 (g) -394 kj/mol H 2 O(g) -242 kj/mol H 2 O(l) -286 kj/mol H 2 O 2 (l) -188 kj/mol CH 3 OH(g) -201 kj/mol C 3 H 8 (g) -104 kj/mol CH 3 OH(l) -239 kj/mol Using these, determine the H for the following reaction: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) a kj b kj c. 783 kj d. 575 kj e kj Page 2 of 5

3 9. Which element will have five electrons in its Lewis symbol? a. argon b. boron c. carbon d. phosphorus e. sulfur 10. Select the element that never forms more than one bond in a Lewis structure. a. O b. C c. N d. Al e. H 11. How many valence electrons will be in the correct Lewis structure for CH 3 CH 3? a. 8 b. 10 c. 14 d. 18 e Which of the following diagrams is the best Lewis structure for that molecule? a. b. c. d. e. 13. Which of the following statements could not describe covalent bonding? a. The sharing of two electrons between two atoms. b. The sharing of four electrons between two atoms. c. An electrostatic interaction between two oppositely charged ions. d. The sharing of two or more electrons equally between two atoms. e. The sharing of two or more electrons unequally between two atoms. Page 3 of 5

4 14. How many electrons are in the correct Lewis structure for ClO 4 -? a. 16 b. 24 c. 31 d. 32 e In organic compounds, cis-trans isomerism is associated with which of the following types of bonds? a. carbon-carbon triple bonds b. carbon-carbon single bonds c. carbon-carbon double bonds d. carbon-oxygen single bonds e. carbon-oxygen double bonds 16. Which bond is longest? a. carbon-oxygen triple bond b. carbon-oxygen single bond c. carbon-carbon single bond d. carbon-carbon double bond e. carbon-nitrogen triple bon 17. Which bond is the weakest? a. H-F b. H-Cl c. H-Br d. H-I e. C=C 18. Which of these elements is the most electronegative? a. Mg b. Zn c. O d. Rh e. Si 19. Which bond is least polar? a. C-C b. C-N c. N-H d. C-F e. C-O Page 4 of 5

5 20. What is the formal charge on carbon in the Lewis structure of CO? a. -2 b. -1 c. 0 d. +1 e Which of the following species has one or more resonance structures? a. H 2 S b. CCl 4 c. PCl 3 d. NH 4 2 e. CO Write the singly bonded Lewis dot structure for BF 3. Which of the following statements best describes this structure? a. It obeys the octet rule on all atoms. b. It has less than an octet on at least one atom. c. It has a lone pair of electrons on the boron atom. d. It has less than an octet of electrons on all atoms. e. It exceeds the octet rule. 23. Write the correct Lewis structure for XeF 4. This molecule exceeds the octet rule. Determine the number of bonding pairs and lone pairs on the central atom. a. 4 bonding pairs and 1 lone pair b. 5 bonding pairs and 0 lone pairs c. 4 bonding pairs and 2 lone pairs d. 5 bonding pairs and 2 lone pairs e. 6 bonding pairs and 0 lone pairs Page 5 of 5

6 chem101f08_exam3a Answer Section SHORT ANSWER 1. ANS: A PTS: 0 MULTIPLE CHOICE 2. ANS: C PTS: 1 3. ANS: D PTS: 1 4. ANS: D PTS: 1 5. ANS: C PTS: 1 6. ANS: D PTS: 1 7. ANS: A PTS: 1 8. ANS: B PTS: 1 9. ANS: D PTS: ANS: E PTS: ANS: C PTS: ANS: A PTS: ANS: C PTS: ANS: D PTS: ANS: C PTS: ANS: C PTS: ANS: D PTS: ANS: C PTS: ANS: A PTS: ANS: B PTS: ANS: E PTS: ANS: B PTS: ANS: C PTS: 1 Page 6 of 5