Ch101 notes

Ch101 notes - 9.8: -The thermodynamics of the reaction is...

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9.8: -The thermodynamics of the reaction is concerned only with the difference between the initial (reactants) and final (products) states. The properties of interest are H o , S o , and G o . Although all of these properties can be determined from tables, our goal is to estimate their sign and whether they are large or small. - Kinetics is the study of the rates and mechanisms of reactions. In other words, kinetics looks at the reaction between the initial and final states. To do this, we define a reaction coordinate, which is normally a complicated combination of intermolecular distances and molecular structural changes. In order to form the C-O bond, the OH 1- ion must attack the carbon atom along the line through the center of the plane formed by the three H atoms. As the C-O distance decreases, the three H-C-H angles all increase from their original 109 o as the H atoms separate, and the C-I bond elongates. Thus, the reaction coordinate involves changes in all of these parameters. As the C-O distance decreases, the H-C-H bond angles continue to increase, and the C-I bond continues to lengthen. The system reaches an energy maximum when the H-C-H bond angles are 120 o and the CH 3 group is planar. The species that is formed at this point is called the transition state : the species that the reactants must pass through in their transition to the products. It is at the energy maximum, so it immediately changes into one of the species at lower energy. If the C-O bond shortens and the C-I bond breaks, the transition state leads to the products, but if the C-O bond breaks and the C-I bond shortens, the transition state leads to the reactants. Either action is possible. Figure 9.4 Reaction Coordinate and Transition State -Reactants must go through a transition state to reach the products, and the energy required to reach the transition state is called the activation energy -E a (f) is the activation energy for the forward reaction, while E a (r) is the activation energy for the reverse reaction. Activation energies are always positive. Note that H = E a (f) - E a (r), so E a (f) < E a (r) for exothermic reactions, but E a (f) > E a (r) for endothermic reactions
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rate of reaction- how fast the concentrations change with time -In order for two molecules to react, they must collide. Thus, the rate of reaction is proportional to the frequency at which the reactants collide. -The collision frequency is the number of collisions per unit volume per unit time, and is proportional to the product of the molar concentrations of the colliding particles. Recall that the molar concentration of particle A is the number of moles of A in a liter. -Only a fraction of colliding particles are oriented properly for reaction -Energy Effect Only a fraction of colliding particles have sufficient energy to react. -A collision must have enough energy to overcome the activation energy of the reaction.
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This note was uploaded on 04/21/2008 for the course CH 101 taught by Professor Bigham during the Spring '08 term at N.C. State.

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Ch101 notes - 9.8: -The thermodynamics of the reaction is...

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