Ans_Ex5 - Nielsen CHM1041 Answer to Exercise Set # 5...

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Nielsen Bhaskar/PC/1713-I/A 2/05 1 CHM1041 Answer to Exercise Set # 5 Kinetics 1. Why are some collisions between molecules of reactants not effective? A collision between reactant molecules must pass through a transition state or activated complex to proceed to the products of any single step. In the transition state, chemical bonds are both breaking and forming, consequently the molecular alignments must be specific. In addition, a minimum energy is required (E act ) to achieve the process of bond reorganization. Only a fraction of the reactant molecules possess this energy. 2. Describe with the aid of a diagram a possible transition state for the single step reaction between CH 3 Br and OH -1 to give methyl alcohol, CH 3 OH, and Br -1 . -1 -1 33 HO: + CH -Br HO-CH + :Br Bond forming Bond breaking δ - δ - 3 HO --- CH --- Br ⎡⎤ ⎢⎥ ⎣⎦ transition state δ Partial charge or a fraction of unit charge; here, the negative charge is building up on bromine while negative charge is decreasing on oxygen. The sum of both δ - Energy charges in this case equals –1 -1 3 HO + CH Br -1 3 HOCH + Br Reaction Progress
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Nielsen Bhaskar/PC/1713-I/A 2/05 2 3. In order to get charcoal to burn, it must first be heated. Why? Would you consider this reaction exothermic or endothermic? Explain. Is H is - ) and heat is released when the product, CO 2 , is formed. Some of this liberated heat is recycled to allow the remaining atoms and molecules the energy needed to achieve the transition state, and the reaction proceeds until the reactants are consumed. 4. The single-step reaction, () 2 gg 2 g NO Cl + NO NO + ONCl g is reversible; E act (forward) is 28.9 KJ and E act (reverse) is 41.8 KJ . Draw a potential energy diagram for the reaction, and indicate both energies of activation and the position of the transition state. Indicate H and deduce its magnitude and sign. Is this an endothermic or exothermic reaction? E act E act forward reverse Energy H = -12.9 kJ exothermic Reaction Progress 5. The following statements are sometimes encountered with reference to catalysis, but they are not stated as carefully as they might be. What modifications should be made in each of them? a) A catalyst is a substance which speeds up a chemical reaction but does not take part in the reaction. A catalyst speeds up the reaction rate by providing a new reaction pathway of lower energy leading to the final products. The catalyst does take part in the reaction but is regenerated chemically unaltered before or at the time the final products are formed. It may then be recycled.
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This note was uploaded on 12/12/2011 for the course CHM 2041 taught by Professor Staff during the Fall '09 term at Santa Fe College.

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Ans_Ex5 - Nielsen CHM1041 Answer to Exercise Set # 5...

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