# HW 3 - Homework#3 Holcombe This print-out should have 21...

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– Homework #3 – Holcombe – 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Consider the reaction CaCO 3 (s) CaO(s) + CO 2 (g) , at equilibrium at a certain temperature. Now a small amount of Ca 14 CO 3 (s) ( 14 C la- beled calcium carbonate) is added. What species will be present after equilibrium is re-established? 1. CaCO 3 (s), CaO(s), 14 CO 2 (g), CO 2 (g) 2. Ca 14 CO 3 (s), CaO(s), 14 CO 2 (g), CO 2 (g) 3. CaCO 3 (s), Ca 14 CO 3 (s), CaO(s), 14 CO 2 (g), CO 2 (g) correct 4. CaO(s), 14 CO 2 (g), CO 2 (g) 5. CaCO 3 (s), Ca 14 CO 3 (s), CaO(s), CO 2 (g) Explanation: The addition of extra reactant Ca 14 CO 3 will favor the production of more products (CaO(s) and 14 CO 2 (g)). However, since this is a dynamic equilibrium, both forward and backward reactions occur. Therefore, an equilibrium will be established where 14 C is present in both products and reactants con- taining C. 002 10.0 points In which of the following cases will the LEAST time be required to arrive at equilibrium? K below refers to the equilibrium constant. 1. K is about 1. 2. K is a very large number. 3. Cannot tell without knowing the value of K . 4. Cannot tell since the time to arrive at equilibrium does not depend on K . correct 5. K is a very small number. Explanation: The equilibrium constant K reflects the relative amounts of products and reactants present when the reaction is complete, and gives no information on the time needed. The rate constant k indicates how fast the reaction occurs. 003 10.0 points Using the law of mass action, write the equilibrium expression for the following reac- tion: 2 Cu 2+ (aq) + 4 I (aq) ←→ 2 CuI(s) + I 2 (aq) 1. K = [Cu 2+ ] 2 [I ] 4 [I 2 ][CuI] 2 2. K = [I 2 ] [Cu 2+ ] 2 [I ] 4 correct 3. K = [Cu 2+ ] 2 [I ] 4 [I 2 ] 4. K = [I 2 ][CuI] 2 [Cu 2+ ] 2 [I ] 4 Explanation: The equilibrium expression K is the ratio of products to reactants raised to their respec- tive stoichiometric coefficients. Solid species are not included in the expression. 004 10.0 points Write the reaction quotient for MgSO 4 · 7 H 2 O(s) MgSO 4 (s) + 7 H 2 O(g) 1. Q = P MgSO 4 P 7 H 2 O P MgSO 4 · 7H 2 O 2. Q = 1 P 7 H 2 O 3. Q = P 7 H 2 O P MgSO 4 · 7H 2 O

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– Homework #3 – Holcombe – ( 2 4. Q = P MgSO 4 · 7 P H 2 O P MgSO 4 · 7H 2 O 5. Q = P 7 H 2 O correct Explanation: Q = activities of products activities of reactants , where “activities” are for an ideal gas: a J = [ P J ] for a solute: a J = [J] for a pure solid or liquid: a J = 1 005 10.0 points Consider the following reactions at 25 C: reaction K c 2 NO(g) N 2 (g) + O 2 (g) 1 × 10 30 2 H 2 O(g) 2 H 2 (g) + O 2 (g) 5 × 10 82 2 CO(g) + O 2 (g) 2 CO 2 (g) 3 × 10 91 Which compound is most likely to dissociate and give O 2 (g) at 25 C?
• Fall '07
• Holcombe

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