Kinetics

Introduction

• Kinetics: study the rate at which a chemical process occurs.
  • Also studies the reaction mechanism
  • Reaction mechanism: how the reaction occurs

Factors That Affect Reaction Rates

• Physical state of the reactants
  • In order to react, molecules must come in contact with each other.
  • The more homogeneous the mixture of reactants, the faster the molecules can react.
• Concentration of reactants
  • As the concentration of reactants increases, so does the likelihood that reactant molecules will collide.
  • Since during a reaction, concentrations are constantly changing, the rate of a chemical reaction is SELDOM constant
• Temperature
  • At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy.
• Presence of a catalyst
  • Catalysts speed up reactions by changing the mechanism of the reaction.
  • Catalysts are not consumed during the course of the reaction.

Rate Expressions

• Rate of a reaction: the change in the amount of a reactant or product per unit time. 

  • It’s often convenient to express amount in terms of concentration.

• Concentration is used because the effect of concentration on the rate is generally predictable. 

  • As concentration decreases, rate decreases. 

• Rate expression: mathematical representation of rate of reaction. 

• Reaction rates are always positive quantities.

• Concentration is most often expressed in Molarity.  

• Rate expressions can be written in terms of reactant or product concentration

Average vs Instantaneous React Rates

• Average reaction rate: the rate at which a reaction  proceeds over a time period. 
  • Calculated using concentrations at the beginning and end of a time period.
• Instantaneous reaction rate: the rate at which a reaction is proceeding at a specific time and/or conc. 
  • Calculated using a graph (Conc. vs. time) and a slope of a straight line tangent to the curve at that specific time. 
  • Or by determining the avg. rate of reaction over a very short time period. 
• Initial reaction rate: the instantaneous reaction rate at “time zero”. 

Relative Rates of Reaction

• The rate of a reaction may be expressed in terms of the change in concentration of any reactant or product. 
• Stoichiometric factors derived from a balanced equation may be used to relate reaction rates.

Rate and Collision

• Chemical reactions occur as the result of collisions between reactant molecules.
• The higher the concentration of reactant molecules, the more likely molecules will collide and react, and therefore the faster the reaction rate.  
• As reactants get consumed, collisions happen less frequently and the reaction rate decreases.

Rate Constant, k

• The rate constant, k is specific for a particular reaction at a particular temperature. 
• k is independent of reactant concentration. 
• k must be determined experimentally. 

Rate Laws

• Experiments must be done to determine the rate law for a reaction.
  • Can’t just use a balanced chemical equation. 
• Elementary step: a single event in the course of a reaction
• The rate is directly proportional to the product
• To get the rate law from an elementary step, we simply multiply k by the concentrations of all of the molecules……..IF they all have coefficients of 1. 
• The rate law for an elementary step always = k times the product of each component, raised to the power of its coefficient.
• Rate order: the sum of the exponents in the rate law
• For a reaction with a single reactant:
  • A → products
  • rate = k[A]^m
  • m is the order of the reaction
  • m must be determined experimentally (not from balanced chemical equations). 
  • Reaction orders are usually positive integers (0, 1, 2…) but can also be fractions or negative numbers. 
• Most reactions involve more than one reactant.
• A + B → products
• rate = k[A^]m[B]^n
• There are three orders:
  • m is the order with respect to A
  • n is the order with respect to B
  • The overall order of the reaction = m + n

Order with Respect to a Reactant

• The **order with respect to a reactant **indicates the dependence of the reaction rate on the concentration of that particular reactant. 
• The **overall order **gives an understanding of how all the reactants contribute to the rate of a reaction.  
  • Also, dictates the units of the rate constant for that reaction. 

Determination of a Reaction’s Rate Law

• One way to determine the rate law of a reaction is to measure the initial rate.
  • Initial rate: the rate at time zero.
• If the initial rate is measured with a number of different initial reactant concentrations then the rate law can be determined.