Kinetics Senior Chemistry

Particle Collisions For a reaction to proceed to products, the reactants must collide with one another. Rate of reaction The rate of a reaction is the rate that reactants disappear and products form. As the reaction proceeds and reactants are used up, the rate decreases. Since the reacting particles must collide to react, increasing the rate of collisions increases the rate of reaction. Rate of reaction: the change in concentration of reactants or products over time

Rate Equations This graph shows the decrease in concentration of the reactants in a reaction. The instantaneous rate can be determined at any point along the graph by measuring the gradient which is change in concentration over time Δ[R] Δt The average rate of reaction can be calculated by using the concentration change over the entire reaction. Rate = For you to do: Draw a graph that shows the concentration of products vs. time

Some Factors Affecting Reaction Rates Temperature Temperature Higher temperature means particles have more kinetic energy, leading to an increase in reaction rate. Higher temperature means particles have more kinetic energy, leading to an increase in reaction rate. Concentration Concentration Increasing the concentration of the reactants increases the chance of collisions, which increases the rate of reaction. Increasing the concentration of the reactants increases the chance of collisions, which increases the rate of reaction. Pressure Pressure An increase in pressure is the same as increasing the concentration of the gas particles, which increases the rate. An increase in pressure is the same as increasing the concentration of the gas particles, which increases the rate. Surface area Surface area Fine powders have more surface area than large pieces leading to more collisions. For this reason, fine powders create an explosion hazard when mixed with air Fine powders have more surface area than large pieces leading to more collisions. For this reason, fine powders create an explosion hazard when mixed with air Catalysts Catalysts Catalysts lower the activation energy, therefore increasing the rate of reaction Catalysts lower the activation energy, therefore increasing the rate of reaction

Kinetic Theory of Matter Kinetic Theory describes the movement of particles in matter. There are three basic movements: Vibrational (vibrate) Rotational (spin) Translational (move from here to there) Solids vibrate around fixed positions as the atoms are strongly attracted holding a definite shape. Liquids have particles that are still close together, but they can move around. This is why they flow. Gases have much greater movement and all three motions are possible. Source:

Kinetic Theory – Assumptions about gases Source:

Collision Theory Collision Theory states that in order for reactions to occur, successful collisions must take place between reacting particles. To collide, they must be in motion... Successful collisions result in products being formed. To increase rate, these must be increased. Unsuccessful collisions do not result in the formation of products

Collision Theory For a reaction to occur, there must be successful collisions. This means they must have: Enough energy (>Ea) Correct orientation

Collision Theory – particle size effect Explain how the rate of reaction can be affected by the change in particle size?

Collision Theory – Temperature effect Explain how the rate of reaction can be affected by the increase in temperature? Go to: to see an animation about the effect of temperature Remember, temperature is a measure of the average kinetic energy of the particles. These particles need energy to get over the Activation Energy barrier in order to go to products.

Collision Theory – Concentration effect Explain how the rate of reaction can be affected by the increase in concentration?

Collision Theory – Pressure effect Explain how the rate of reaction can be affected by the increase in pressure?

Energy Distribution Curves Particles in a sample of gas do not all have the same kinetic energy at a given temperature. The curve on the right shows that particles can have a wide range of energy values. The particles over the threshold energy, are able to react It also shows that if we increase the temperature, the number of particles over the threshold also increases. Note that the area under the two curves is the same and represents the total number of particles. The diagram on the right shows how the energy distribution curves relate to the enthalpy diagram. Notice that the threshold energy is the same as the Ea.

Maxwell-Boltzmann Energy Distribution Curves Gas particles in a container will increase their kinetic energy and their velocity with an increase in temperature. (K.E. = ½ mv 2 ) Maxwell-Boltzmann distribution curves show this relationship. For you to do: Go to m and click "Run/Save the Kinetic Theory Particle Simulations" link. Run simulations 1, 2 and 3. m 1.Enter at least 5 different values into each simulation 2.Plot the value for the maximum of each curve vs the variable you are changing (e.g. temperature) 3.Draw conclusions about what the M-B Distribution curves are showing and what your graphs are showing.

Maxwell-Boltzmann Energy Distribution Curves M-B curves can be thought of in terms of probability: The highest point in the curve represents the most probable kinetic energy value The average kinetic energy is slightly higher due to the asymmetric shape of the curve

Catalysis and energy distribution Recall that catalysts lower the Activation energy of a reaction. This means that more particles are energetic enough to react at the same temperature. See the enthalpy diagram and M-B distribution curve below.

Kinetics – 2016 R Slider