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Kinetics Review. rate of reaction – decrease in concentration of reactants per unit time, or increase in concentration of products per unit time as the reaction proceeds Reaction rate is often measured in units of mol dm –3 s –1. Kinetics Review.
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Kinetics Review • rate of reaction – decrease in concentration of reactants per unit time, or increase in concentration of products per unit time as the reaction proceeds • Reaction rate is often measured in units of mol dm–3 s–1.
Kinetics Review • Reaction rate is concerned with how fast a reaction reaches a certain point (not to be confused with how far a reaction goes [i.e. – equilibrium] ). • Reaction rate graphs will generally show time on the X-axis and some measure of how far the reaction has gone (concentration, volume, mass, pH, etc.) on the Y-axis.
Kinetics Review • Note in these graphs that the rates of reaction usually decrease with time as the reactants are used up.
Kinetics Review • The kinetic theory (also called kinetic-molecular theory, and usually applied specifically to gases) describes the behavior of what we call ideal gases. These are several assumptions we make about gas molecules in order to simplify our physical understanding of them, and especially the math that describes their behavior.
Kinetics Review 1.Gases are “point masses” with negligible volume. (The volume of a gas molecule is so small compared to the distance between neighboring gas molecules that it can be ignored) 2.Gas particles are in constant motion, moving rapidly in straight lines in all directions. Thus they have KE. 3.Collisions between gas particles, or between gas particles and container walls, are elastic no KE lost in collisions.
Kinetics Review 4.There are no forces of attraction or repulsion between gas particles. 5.The average KE of a gas is directly proportional to the Kelvin temperature of the gas. ·When T decreases, KE decreases. ·When T increases, KE increases. ·(IB deems it important for you to describe kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in kelvins.)
Kinetics Review • There are three factors that determine if a collision between reactant molecules will be successful (lead to product formation): 1) Collision frequency of reactant molecules. 2) Number of particles with EEa. (Reactants must possess some minimum KE – activation energy – when they collide to “stick together” & form activated complex.) 3) The number of particles with appropriate collision geometry, or orientation.
Kinetics Review • There are five factors you need to understand that affect reaction rate: • Particle Size • Concentration • Temperature • Pressure (gases only) • Catalysts
Kinetics Review • Some catalysts also speed up reactions by facilitating the correct orientation of colliding molecules. • The rate of any chemical reaction depends solely upon the concentration of the reactants (only reactants at the beginning of a reaction – only products).
Kinetics Review • Maxwell-Boltzmann Energy Distribution Curves
Kinetics Review • Maxwell-Boltzmann Energy Distribution Curves
Kinetics Review • The Maxwell-Boltzmann curves are considerably different for a gas at 80 K or 300 K. When a gas becomes colder, not only does its peak shift to the left (lower temperature = slower molecules), but its peak also becomes higher and much more narrow. At lower temperatures there is less energy available, so the gas molecules tend to have weak collisions and to shuffle around at speeds that don't vary too much from each other.
Kinetics Review • At higher T, the curves flatten out. More energy available more energetic collisions that could propel a molecule much faster (or slower) than the average speed. The probability of a T=80 molecule moving at 1000 m/s is essentially zero. But at T=300, the probability is small but noticeable. Even though the number of molecules at that speed is still small, the ratio of the probabilities between the T=80 and T=300 gases is quite large, because dividing by the T=80 case is almost like dividing by zero. Even small temperature increases in a gas can multiply the number of extremely hot, low-probability molecules by hundreds or thousands of times.
