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1.3 Collision Theory Collisions and Concentrations

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1 1.3 Collision Theory Collisions and Concentrations
The relationship between concentration of particles colliding and rate is: OR Rate Law The relationship above is called a rate law. The proportionality constant in this case is called a rate constant. Every reaction has its own unique rate law and its own unique rate constant. For many reactions, the exponents, x and y, are each equal to 1. The values of x and y are called reactant orders. Text Pages

2 1.3 Collision Theory Distribution of Collisions
A Pool Table as a Model The break at the start of the game is probably the best model of the distribution of collision energies involved between the reacting particles in a chemical reaction (Figure 1.3.2). Some of the particles move quickly and collide directly with others to transfer a great deal of energy. Others barely move at all or collide with a glancing angle that transfers very little energy. In addition, we must remember that real reactant collisions occur in three dimensions, while our pool table model is good for two dimensions only (Figure 1.3.3). Text Page 31

3 1.3 Collision Theory Collision Theory
Text Page 31

4 1.3 Collision Theory Effective Collisions
Requirements There are a tremendous number of collisions in a reacting sample that results in a wide range of velocities and kinetic energies. Only a small percentage of the molecules in a given sample have sufficient kinetic energy to react. Only collisions between particles having the threshold energy of Ea are energetic enough to overcome the repulsive forces between the electron clouds of the reacting molecules and to weaken or break bonds, resulting in a reaction. Text Pages

5 1.3 Collision Theory Effective Collisions - 2
Temperature, Number of Molecules and Kinetic Energy Note that in both cases, with and without a catalyst, the number of particles capable of colliding effectively (as represented by the shaded area under the curve) increases at the higher temperature. Text Page 33 Ea1 with catalyst Ea2 without catalyst

6 1.3 Collision Theory Effective Collisions - 3
Shape and Orientation of Molecules Particles having the activation energy associated with a given reaction don’t always react when they undergo a collision. Even very high-energy collisions may not be effective unless the molecules are properly oriented toward one another. SUMMARY Increased temperature, concentration, and surface area all increase the frequency of collisions leading to an increase in reaction rate. The presence of a catalyst and temperature both increase the fraction of collisions that are successful. Text Page 34

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