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173 quiz 7 - CHEM 173(07-08 QUIZ 7 May 9 2008 Name...

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CHEM 173(07-08), QUIZ 7 May 9, 2008 Name: Section (173): 25 points 1. Answer the following appropriately ( filling in the blank, circling the correct answer etc. ) a. Sketch a graph of Δ r G o - vs. T for a reaction which is spontaneous only at high temperatures. Predict the sign of Δ r H o - and Δ r S o - . ans : Spontaneous only at high temperatures means that the graph will be below the x - axis at high temperatures and above it at low temperatures. The slope( = - Δ S o - ) will be negative and the intercept ( = Δ H o - ) will be positive. So Δ S o - will be positive and Δ H o - will be positive. b. Classify as redox (R) or as acid base (A): (i) CaO(s) + 2HCl = CaCl 2 + H 2 O : ; (ii) 2H 2 (g) + O 2 (g) = 2H 2 O(l) : ; (iii) CN - + MnO - 4 -→ CNO - + MnO 2 : ans : (i) A, (ii) R, (iii) R. Check to see whether any element is undergoing a change in oxidation number. c. Which of the following is a balanced reaction representing a redox reaction taking place in acid medium : i. 2O 2 (g) + 3H + + 4Fe 2+ * ) 2H 2 O(l) + 4Fe 3+ ii. O 2 (g) + 2H 2 O(l) + 4Fe 2+ * ) 4OH - + 4Fe 3+ iii. O 2 (g) + 4H + + 4Fe 2+ * ) 2H 2 O(l) + 4Fe 3+ ans : Acid medium means that there must be H + on the left. This eliminates (ii). (i) is not
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