Rate of Reactions

• Chemical kinetics


  The study of reaction rates or speeds at which a chemical reaction occurs, how reaction rates change under varying conditions, and what molecular events occur during the overall reaction

  
• Rate of chemical reaction

  The change in the concentration of a reactant or a product per unit time
• If reactant: [X]2 < [X]1, rate of consumption of reactant is greater than formation of product
• If product: [X]2 > [X]1, rate of formation of product is greater than consumption of reactant
• Types of rate of reaction

  • Average rate of reaction
  • Instantaneous rate of reaction
  • Initial rate of reaction
  • Relative rate of reaction

  
• Initial rate of reaction is the instantaneous rate at t=0

  
• Relative rate of reaction

  The rate at any one particular point in time, expressed as 1/time to complete reaction

  
• Rate of reaction can be expressed in terms of disappearance of any reactant or appearance of any product
• Methods of measuring rate of reaction

  • Colorimetric
  • Conductometric
  • Manometric
  • Thermometric
  • Spectrometric
  • Mass determination
  • Titrimetric

  
• Collision theory


  • Reactant molecules must collide with enough energy and proper orientation
  • Only a small fraction of collisions are effective
  • Only the most energetic molecules undergo reaction
  • Probability of collision resulting in reaction depends on orientation
• Activation energy


  Minimum energy required by reactants to overcome barrier and form products
• Transition state theory


  Emphasis is on what happens to reactant molecules when they are about to be transformed into products, at the top of the energy profile
• Transition state area or region


  The area where the reactant molecules are about to be transformed or changed into products
• Transition state


  1. Reactant molecules reach the transition state

  2. Some of the old bonds are broken

  3. New bonds are being formed

  4. The activated complex does not always change into products, it can change back into the reactant
• Activated Complex (Transition State)

  An unstable grouping of atoms that can break up to form products
• Factors affecting the rate of chemical reaction

  • Concentration of reactants
  • Temperature
  • Presence of a catalyst
  • Surface area
  • Pressure
  • Nature of reactant
• Concentration


  Increasing the concentration of a reactant molecule increases the number of particles per unit volume of the reaction vessel, hence the number of collisions and number of effective collisions per unit time increases and so the rate of reaction also increases
• In some reactions, the rate is unaffected by the concentration of a particular reactant, as long as it is present at some concentration. Also a decrease in concentration of reactant will reduce the rate of chemical reaction.
• Reaction between nitrogen (II) oxide and oxygen


  • The reaction rate doubles when the oxygen concentration doubles. However, doubling the concentration of NO quadruples the rate of reaction.
• Temperature


  An increase in temperature corresponds to an increase in the kinetic energy of the reactant molecules or particles resulting in more collisions as well as the number of effective collision per unit time, increasing the rate of chemical reaction
• With increase in temperature, the fraction of reactant molecules whose kinetic energy becomes greater than the activation energy of the reaction also increases, hence reaction rate increases.
• Maxwell-Boltzmann distribution curve


  Shows how the kinetic energies of the molecules in a gas might be distributed at different temperatures. Only a small fraction of molecules have sufficient energy to react at lower temperature, but this fraction roughly doubles when the temperature is raised by 10K.
• Surface area


  The rate of reaction of solid substance is related to its surface area. Increasing the surface area of the solid-phase reactant increases the number of collisions per unit second and effective collision per unit time, increasing the rate of chemical reaction.
• Reaction between magnesium metal and hydrochloric acid

  • When the magnesium is in one big lump, the hydrogen ions can only hit the outer layer. But when the magnesium is split into smaller bits, there are hardly any magnesium atoms inaccessible to the hydrogen ions, increasing the number of collisions per second and the rate of reaction.
• Catalyst


  A substance that increases the rate of a reaction without undergoing any chemical change during the reaction, by providing an alternative reaction pathway with a lower activation energy.
• Some substances may decrease the rate of a reaction. These are generally called inhibitors or negative catalysts. They interfere with the reaction by forming relatively stable complexes, which require more energy to break up, reducing the speed of the reaction.
• Energy profile diagram for exothermic reaction

  Shows the effect of a catalyst on the activation energy, where the catalyzed pathway has a lower activation energy (Ea') compared to the uncatalyzed pathway (Ea).
• Energy profile diagram for endothermic reaction

  Also shows the effect of a catalyst on the activation energy, where the catalyzed pathway has a lower activation energy (Ea') compared to the uncatalyzed pathway (Ea).
• Nature of reactant


  Individual properties of substances like state of matter, molecular size, bond type and bond strength can also affect reaction rates.
• Gases tend to react faster than solids or liquids because it takes less energy to separate the particles. Aqueous ions also react faster than species in other states of matter.
• Reactions involving ionic species tend to proceed faster than reactions involving molecular compounds.
• Pressure


  Increasing the pressure increases the rate of reaction for reactions with gaseous reactants, because it increases the frequency of collisions and effective collisions per unit time.
• Light


  Light-sensitive reactions like photosynthesis and photography depend on the absorption of light energy to drive the reaction.
• Rate Law


  An equation or expression that relates the rate of reaction to the rate constant and the concentration of the reactants raised to their corresponding powers.

  
• Rate constant (K)


  The constant of proportionality in the rate law equation, representing the rate when the initial concentration of all reactants are one. It varies proportionally with temperature.

  
• Order of reaction


  The sum of the powers to which the concentration of the reactants are raised in a rate law equation.

  
• Reactants


  Raised to some power
• Reaction


  • a A + b B → c C + d D
• Rate law equation


  R = K [A]x[B]y
• R


  Rate of reaction