Chemistry 1A Spring 2009 Exam 2 Key Chapters 4-6

Transcription

1 Chemistry 1A Spring 2009 Exam 2 Key Chapters -6 You might find the following useful kj ΔH = ΔE + ( Δn)RT R = K mol kj q = - C cal + m w Δ T K mol ΔH rxn = Σ ΔH f (products) Σ ΔH f (reactants) Substance Density in g/ml battery acid 1.29 benzene, C 6 H blood (whole) 1.05 bromine, Br chocolate 1.75 copper, Cu 8.9 chromium(iii) oxide, Cr 2 O ethanol, C 2 H 5 OH(l) gasoline gold, Au 19.0 Iron, Fe 7.87 magnesium, Mg 1.7 nitrobenzene, C 6 H 5 NO oleic acid phosphorus oxychloride, POCl sulfuric acid (concentrated) 1.8 water at 20 C

2 For each of the following, write the word, words, letter, letters, or number in each blank that best completes each sentence. (2 points per blank) 1. A(n) combustion reaction is a rapid oxidation accompanied by heat and usually light. 2. A(n) strong electrolyte is a substance that ionizes or dissociates completely in an aqueous solution. 3. Ionize means to form ions (often as a substance dissolves in water).. Dissociate means he separation of ions (often as a substance dissolves in water). 5. A(n) weak base is a substance that produces fewer hydroxide ions in water solution than particles of the substance added. 6. A(n) amphoteric substance is a substance that can act as either a Brønsted-Lowry acid and a Brønsted-Lowry base, depending on the circumstances. 7. A(n) oxidizing agent is a substance that gains electrons, making it possible for another substance to lose electrons and be oxidized. 8. A(n) endothermic change is a change that leads the system to absorb heat energy from the surroundings. 9. Specific heat capacity is the heat necessary to increase the temperature of one gram of pure substance by one kelvin (or one degree Celsius). 10. Of the two forms of radiant energy, ultraviolet radiation and microwaves, ultraviolet radiation has the higher energy photons. 11. Write a complete, balanced equations for the combustion of butanethiol, C H 9 SH(l). (6 points) C H 9 SH(l) + 15/2 O 2 (g) CO 2 (g) + 5H 2 O(l) + SO 2 (g) Or 2C H 9 SH(l) + 15O 2 (g) 8CO 2 (g) + 10H 2 O(l) + 2SO 2 (g) 12. Write the complete, complete ionic, and net ionic equations for the precipitation reaction that occurs when water solutions of magnesium nitrate and sodium phosphate are combined. (7 points) 3Mg(NO 3 ) 2 (aq) ) + 2Na 3 PO (aq) Mg 3 (PO ) 2 (s) + 6NaNO 3 (aq) 3Mg 2+ (aq) + 6NO 3 (aq) + 6Na + (aq) + 2PO 3 (aq) Mg 3 (PO ) 2 (s) + 6Na + (aq) + 6NO 3 (aq) 3Mg 2+ (aq) + 2PO 3 (aq) Mg 3 (PO ) 2 (s) 2

3 13. Write the complete, complete ionic, and net ionic equations for the acid-base reaction that occurs when solid iron(ii) sulfide and aqueous hydrochloric acid are combined. (7 points) FeS(s) + 2HCl(aq) FeCl 2 (aq) + H 2 S(aq) FeS(s) + 2H + (aq) + 2Cl (aq) Fe 2+ (aq) + 2Cl (aq) + H 2 S(aq) FeS(s) + 2H + (aq) Fe 2+ (aq) + H 2 S(aq) 1. Complete the following table. (1 point each box) Substance Strong, weak, or nonelectrolyte? Strong acid, weak acid, strong base, weak base, or neutral? HClO Strong electrolyte Strong acid HNO 2 Weak electrolyte Weak acid CH 3 OH Nonelectrolyte Neutral NaNO 2 Strong electrolyte Weak base HF(aq) Weak electrolyte Weak acid LiHSO Strong electrolyte Weak acid NaNO 3 Strong electrolyte Neutral 15. Determine the oxidation number for each atom in the equation below, and identify which substance is oxidized, which substance is reduced, the oxidizing agent, and the reducing agent. (8 points) Cr 2 O 7 2 (aq) + 6Cl (aq) + 1H + (aq) 2Cr 3+ (aq) + 3Cl 2 (g) + 7H 2 O(l) Cl in Cl is oxidized Cr in Cr 2 O 2 7 is reduced Cl is the reducing agent 2 Cr 2 O 7 is the oxidizing agent. 3

4 For the following numerical problems, be sure to show your work and put a box around your answer. Remember that there will be part credit, so even if you cannot complete a problem, be sure to set up as much of the problem as possible. 16. Potassium permanganate, KMnO, a common oxidizing agent, is made from various ores that contain manganese(iv) oxide, MnO 2. The following equation shows the net reaction for one process that forms potassium permanganate. 2MnO 2 (s) + 2KOH(aq) + O 2 (g) 2KMnO (aq) + H 2 (g) a. What is the maximum mass, in kilograms, of KMnO that can be made from the reaction of g of an ore that contains 56.% MnO 2 with 23.2 L of 6.00 M KOH and excess oxygen gas? (6 points) 56. g MnO g KMnO 1 kg? kg KMnO = g ore g ore g MnO2 10 g = kg KMnO 6.00 mol KOH 2 mol KMnO g KMnO 1 kg? kg KMnO = 23.2 L KOH soln 3 1 L KOH soln 2 mol KOH 1 mol KMnO 10 g = 22.0 kg KMnO b. Explain why the oxygen gas is in excess. ( points) (1) Because air contains oxygen gas, the oxygen is less expensive than the other reactants, (2) the oxygen is less important than the manganesecontaining compounds, (3) oxygen is very safe, so we don t mind having it left in the product mixture, and () because the O 2 is a gas, it is easily separated from the product mixture. 17. How would you make milliliters of M sulfuric acid from 18.0 M H 2 SO? (8 points) 3? ml conc. soln = ml dil. soln mol HCl 10 ml conc. soln 10 3 ml dil. soln 18.0 mol HCl or M V D D MCV C = MDV D V C = = MC ( ) M ml 18.0 M = 6.9 ml conc. soln Add 6.9 ml 18.0 M H 2 SO to about ml water, swirl, cool, and dilute with water to yield a total volume of ml.

5 18. Consider the reaction of nitrogen dioxide and water to form aqueous nitric acid and nitrogen monoxide gas. 3NO 2 (g) + H 2 O(l ) 2HNO 3 (aq) + NO(g) H =? a. Calculate the H for this reaction of nitrogen dioxide and water, using the following equations and their H s. (6 points) 2NO(g) + O 2 (g) 2NO 2 (g) H = 173kJ 2N 2 (g) + 5O 2 (g) + 2H 2 O(l ) HNO 3 (aq) H = 255 kj N 2 (g) + O 2 (g) 2NO(g) H = 181 kj 3NO 2 (g) 3NO(g) + 3/2O 2 (g) H 1 = 3/2( 173kJ) N 2 (g) + 5/2O 2 (g) + H 2 O(l ) 2HNO 3 (aq) H 2 = ½( 255 kj) 2NO(g) N 2 (g) + O 2 (g) H 3 = (181 kj) 3NO 2 (g) + H 2 O(l ) 2HNO 3 (aq) + NO(g) H rxn = H 1 + H 2 + H 3 = 9 kj b. How much heat is evolved or absorbed when ml of 3.50 M HNO 3 is formed? (If you are unable to get an answer to part a, you should still set up this part.) (6 points) 3.50 mol HNO 9 kj 3? kj = ml soln = 3 10 ml soln 2 mol HNO3 6 kj 5

6 Answer the following in short answer form. (8 points) 19. Consider the neutralization reaction between the strong acid nitric acid, HNO 3 (aq) and the strong base sodium hydroxide, NaOH(aq). a. Describe the mixture of the two solutions before the reaction. Your answer should include mention of the particles in solution, the attractions between these particles, and a description of what these particles are doing. At the instant that the solution of nitric acid is added to the aqueous sodium hydroxide, there are four different ions in solution surrounded by water molecules,h 3 O +, NO 3, Na +, and OH. The oxygen ends of the water molecules surround the hydronium and sodium cations, and the hydrogen ends of water molecules surround the nitrate and hydroxide anions. b. Describe the changes that take place during the reaction. When H 3 O + ions and OH ions collide and form water molecules. c. Describe the mixture after the reaction. Your answer should include mention of the particles in solution, the attractions between these particles, and a description of what these particles are doing. The sodium and nitrate ions are unchanged in the reaction. They were separate and surrounded by water molecules at the beginning of the reaction, and they are still separate and surrounded by water molecules at the end of the reaction. 6

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