CHEMISTRY 102B Hour Exam II. Dr. D. DeCoste T.A.

Transcription

1 CHEMISTRY 102B Hour Exam II March 19, 2015 Dr. D. DeCoste Name Signature T.A. This exam contains 22 questions on 9 numbered pages. Check now to make sure you have a complete exam. You have one hour and thirty minutes to complete the exam. Determine the best answer to the first 20 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and provide complete answers to questions 21 and 22. The answers must be kept within the spaces provided (40 pts.) 21 (22 pts.) 22 (18 pts.) Total (80 pts) Useful Information: Always assume ideal behavior for gases (unless explicitly told otherwise). PV = nrt {R = Latm/molK} Root mean square velocity = 3RT M = s m ΔT q K = C E= λ hc ΔE = x J [ 2 2 ] n f n i c = x 10 8 m/s 1 nm = 10-9 m h = x Js

2 Hour Exam II Page No You have neon gas in the left side, and helium gas in the right side, of a two-bulb container connected by a valve as shown below. Initially the valve is closed. Ne He 3.00 L 1.00 L 1.00 atm 3.00 atm The left bulb has a volume of 3.00 L and the Ne gas is at a pressure of 1.00 atm. The right bulb has a volume of 1.00 L and the He gas is at a pressure of 3.00 atm. After the valve is opened, what is true about the relative partial pressures of helium and neon? Assume constant temperature. a) The partial pressure of neon is 3.00 times as great as the partial pressure of helium. b) The partial pressure of neon is 1.33 times as great as the partial pressure of helium. c) The partial pressure of helium is 3.00 times as great as the partial pressure of neon. d) The partial pressure of helium is 1.33 times as great as the partial pressure of neon. e) The partial pressures of helium and neon are equal. 2. For which of the following constant volume cases is the number of collisions of gas particles with the walls of the container the greatest? Volume is the same in all cases. a) 1.0 mol of helium gas at 25 C. b) 1.0 mol of helium gas at 125 C. c) 1.0 mol of oxygen gas at 25 C. d) 1.0 mol of oxygen gas at 125 C. e) Choices a and b have equal collision rates, and these are higher than those in c and d. 3. For metals that form a 2+ charge in an ionic compound, the general equation for such a metal reacting with hydrochloric acid is M(s) + 2HCl(aq) MCl 2 (aq) + H 2 (g) 3.72 g of a metal is reacted completely with an excess of HCl and all of the hydrogen gas produced is collected in a balloon at 25 C and 1.00 atm. The volume of the balloon is measured and found to be 3.74 L. Identify the metal that was added to the HCl. a) Ni b) Ba c) Ca d) Zn e) Mg

3 Hour Exam II Page No What mass of helium gas will exert twice the pressure of g of nitrogen gas at the same conditions of volume and temperature? a) g b) g c) g d) g e) g 5. For a particular process, q = 31 kj and w = 25 kj. How many of the following statements must be true? I. Heat flows from the surroundings to the system. II. The surroundings do work on the system. III. E = 6 kj IV. H = 31 kj a) 0 b) 1 c) 2 d) 3 e) 4 6. A 1.00-g sample of potato chips is burned in a bomb calorimeter. The heat capacity of the bomb calorimeter is 11.3 kj/ C and the temperature rises by 1.98 C. If a serving of potato chips is considered to be 28.4 g, determine the caloric content of 1 serving of potato chips. 1 food Calorie = 4184 J a) 5.35 Cal. b) 84.2 Cal. c) 134 Cal. d) 152 Cal. e) 268 Cal. 7. The goal of the Chemistry 103 lab this week was to determine the identity of an unknown salt by determining the ΔH value when it dissolved in water. For this problem, you are to determine the final temperature of the solution when you dissolve a known salt in water. Consider the dissolution of ammonium nitrate: NH 4 NO 3 (s) NH 4 + (aq) + NO 3 (aq) ΔH = 25.7 kj A g sample of NH 4 NO 3 (s) is dissolved in ml of water with both substances initially at room temperature (24.7 C). Calculate the final temperature of the solution. You may make the following assumptions: no heat loss to the surroundings; the specific heat capacity of the solution is 4.18 J/g C; and the density of the solution is 1.00 g/ml. a) 17.0 C b) 17.7 C c) 24.7 C d) 31.7 C e 32.4 C 8. Use the following enthalpies of combustion to calculate the enthalpy of formation, ΔH o f, of methanol (CH 3 OH) from its elements. 2CH 3 OH(l) + 3O 2 (g) 2CO 2 (g) + 4H 2 O(l) C(graphite) + O 2 (g) CO 2 (g) 2H 2 (g) + O 2 (g) 2H 2 O(l) ΔH rxn = kj ΔH rxn = kj ΔH rxn = kj a) kj/mol b) kj/mol c) kj/mol d) kj/mol e) kj/mol

4 Hour Exam II Page No What color of light is emitted when an excited electron in the hydrogen atom falls from the 4 th energy level to the 2 nd energy level? Visible light spectrum red (625 nm to 700 nm) orange (585 nm to 625 nm) yellow (565 nm to 585 nm) green (470 nm to 565 nm) blue (420 nm to 470 nm) a) Red b) Orange c) Yellow d) Green e) Blue 10. How many of the following statements is/are true? I) Bohr s atomic model works for hydrogen and helium, but no other atoms. II) An excited atom can return to its ground state by absorbing electromagnetic radiation. III) The 1s orbitals for different elements have different radii. IV) Each energy level has only one type of orbital. V) The ground state phosphorus atom has one unpaired electron. a) 1 b) 2 c) 3 d) 4 e) A ground state electron in the hydrogen atom absorbs enough energy to get to n=2. Which orbital will the electron occupy? a) The 2s orbital. b) The 2p x orbital. c) The 2p y orbital. d) The 2p z orbital. e) Each of the above is equally likely. 12. How many electrons can be described by the quantum numbers n = 3, l = 1? a) 0 b) 2 c) 6 d) 10 e) Which of the following statements (a-c) is false? a) An electron configuration for an excited state of the carbon atom could be 1s 2 2s 2 2p 3. b) The ground state electron configuration for the most stable ion of sodium in a compound is 1s 2 2s 2 2p 6. c) The ground state electron configuration for the valence electrons of the halogens (Group 7A) is ns 2 np 5. d) At least two of the above statements (a-c) are false. e) All of the above statements (a-c) are true.

5 Hour Exam II Page No Which of the following concerning second ionization energy values of K and Ca is true? a) The second ionization energies are equal for K and Ca since the elements are in the same row of the periodic table. b) That of Ca is higher than that of K because as you go across a row on the periodic table ionization energy increases. c) That of Ca is lower than that of K because Ca wants to lose the second electron, so it is easier to take the second electron away. d) That of Ca is higher than that of K because the Ca atom has one more proton than the K atom and thus has greater attraction for the electrons. e) That of Ca is lower than that of K because the second electron taken from K is from a lower energy level. 15. Which of the following best evaluates the statement The 1st ionization energy for an oxygen atom is lower than the 1st ionization energy for a nitrogen atom? a) It is consistent with the general trend relating changes in ionization energy across a period from left to right because it is easier to take an electron from an oxygen atom than from a nitrogen atom. b) It is inconsistent with the general trend relating changes in ionization energy across a period from left to right and due to the fact that oxygen has one doubly occupied 2p orbital and nitrogen has all unpaired electrons. c) It is consistent with the general trend relating changes in ionization energy across a period from left to right because it is harder to take an electron from an oxygen atom than from a nitrogen atom. d) It is inconsistent with the general trend relating changes in ionization energy across a period from left to right and it is due to the fact that the oxygen atom has two doubly occupied 2p orbitals and nitrogen has only one. e) The given statement is incorrect. 16. Many decades ago, a chemist at UIUC reported the discovery of a new element and named it Illinium. Unfortunately, he could not substantiate its existence and many years later another chemist claimed it and it is now named Promethium, Pm. What is the expected ground state electron configuration for the element formerly known as Illinium? a) [Xe] 6s 2 5d 10 4f 4 b) [Xe] 6s 2 5d 1 2f 4 c) [Xe] 6s 2 5d 1 4f 4 d) [Xe] 6s 2 6d 1 5f 4 e) [Xe] 6s 2 6d 1 6f Which of the following correctly ranks the atoms from smallest ionization energy to greatest ionization energy? a) Li, B, C, N, Na b) Fr, N, P, O, F c) K, Na, S, Cl, F d) Be, Mg, Ca, Sr, Cs e) At least two of the above (a-d) correctly ranks the atoms from smallest ionization energy to greatest ionization energy.

6 Hour Exam II Page No The following graph plots the first, second, and third ionization energies for Mg, Al, and Si. A B C Which of the following corrects matches the plot to the element? a) A = Mg B = Al C = Si b) A = Si B = Al C = Mg c) A = Si B = Mg C = Al d) A = Mg B = Si C = Al e) A = Al B = Si C = Mg 19. In order to remove an electron from the potassium atom, energy is, and in order to remove an electron from a chlorine atom, energy is. a) required, required b) required, released c) released, required d) released, released 20. Which of the following is true about the general trend of atomic size across a row on the periodic table? a) Atomic size increases to the right because with a greater number of electrons, there is more electron repulsion. b) Atomic size increases because with a greater number of protons, the nucleus is bigger thus the atom is bigger. c) Atomic size decreases to the right because with a greater number of protons, the electrons are more attracted to the nucleus. d) Atomic size decreases to the right because with a greater number of electrons, there is more attraction to the nucleus. e) Atomic size is constant since all atoms in a row have the same valence energy level.

7 Hour Exam II Page No Consider a steel, rigid tank at 1.00 atm and 25 C that contains equal masses of hydrogen gas and nitrogen gas. a. Circle one of the following choices: The partial pressure of hydrogen gas is greater than less than equal to the partial pressure of nitrogen gas in the steel tank. [1 point] b. Explain your answer to part a above using the ideal gas law. [3 points] c. Explain your answer to part a above using the kinetic molecular theory. [4 points] d. Determine the ratio of Partial pressure of Partial pressure of hydrogen gas. Show all work. [3 points] nitrogen gas

8 Hour Exam II Page No (con t) e. Determine the density of the mixture of nitrogen and hydrogen gases in g/l. Show all work. [4 points] f. You initiate a reaction between the nitrogen and hydrogen gases to form ammonia (NH 3 ) gas and let the reaction run to completion at constant temperature. i. Calculate the new pressure in the tank after the reaction is complete. Show all work. [4 points] ii. How does density compare to that you found in part e? Determine the ratio of density of gases after the reaction. Support your answer/show all work. density of gases before the reaction [3 points]

9 Hour Exam II Page No Consider the combustion of ethanol as represented by the following balanced equation: C 2 H 5 OH(l) + 3O 2 (g) 3H 2 O(l) + 2CO 2 (g) a. Using the enthalpies of formation given below, determine H for the combustion of ethanol in units of kj. Show all work. [3 points] H f [C 2 H 5 OH(l)] = 278 kj/mol H f [H 2 O(l)] = 286 kj/mol H f [CO 2 (g)] = 394 kj/mol b. Explain your work above. Do not just state what you did, but explain the conceptual reasons behind why what you did works. In your explanation, explain what H f values represent and why you were not given the H f value for O 2 (g). [7 points]

10 Hour Exam II Page No (con t) c. You combust 1.50 g of ethanol in an excess of oxygen and all of the energy produced as heat is transferred to g of ice [H 2 O(s)] at 10 C. Determine the final state (solid/liquid mixture, solid, or liquid) and temperature of the H 2 O. Use the information below and show all work. [8 points] H fusion (water) = 6.02 kj/mol Specific heat capacity of ice = 2.03 J/g C Specific heat capacity of water = 4.18 J/g C