Lesson 8: Factors that Affect Reaction Rates

• Factors that Affect Reaction Rate

  • Pressure
  • Catalyst
  • Temperature
  • Quantity (how much is available, higher concentration, more collisions)
• Nature of Reactants

  • State of Matter
  • Bond Type
  • Number of Bonds/Molecular Size
  • Strength of Acid
  • Reactivity of Metal
• State of Matter

  • For molecular compounds, gases tend to react faster than liquids or solids
  • For ionic compounds, simple aqueous ions tend to react almost instantaneously
• State of Matter

  • C3H8(l) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) fast
  • C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) very fast
• In order to burn liquid propane, the liquid must first be vaporized before it reacts with oxygen
• Bond Type


  Reactions involving ionic species tend to proceed faster than reactions involving molecular compounds
• Bond Type

  • 2 CO(g) + O2(g) → 2 CO2(g) very slow
  • 5 Fe2+(aq) + MnO4 - (aq) + 8 H+ (aq) → 5 Fe3+(aq) + Mn2+(aq) + 4 H2O(l) very fast
• Number of Bonds/Molecular Size

  Reactions involving the breaking of fewer bonds per reactant proceed faster than those involving the breaking of a larger number of bonds per reactant
• Number of Bonds/Molecular Size

  • C5H12(l) + 8 O2(g) → 5 CO2(g) + 6 H2O(g) very fast
  • C9H20(l) + 14 O2(g) → 9 CO2(g) + 10 H2O(g) fast
• If there is a larger compound, it will react more slowly because there are more bonds to break, so there are more IMFs between them. If you have a longer chain, it can impact the speed of the reaction due to the IMFs needed to be broken
• Strength of Acid

  Strong acids which are completely ionized in solution react faster than weak acids which are only partially ionized in solution
• Strong Acids

  • HCl(aq)
  • HBr(aq)
  • HI(aq)
  • H2SO4(aq)
  • HNO3(aq)
  • HClO4(aq)
• Strength of Acid

  • 2 HCl(aq) + Zn(s) → H2(g) + ZnCl2(aq) very fast
  • 2 CH3COOH(aq) + Zn(s) → H2(g) + Zn(CH3COO)2(aq) fast
• Reactivity of Metal

  Metals with low electronegativity react faster than metals with higher electronegativity because the lower the value the more easily a metal will give up electrons
• Reactivity of Metal

  • 2 Li(s) + 2 HOH(l) → H2(g) + 2 LiOH(aq) fast
  • 2 K(s) + 2 HOH(l) → H2(g) + 2 KOH(aq) very fast
• Electronegativity values: Li = 1.0 K = 0.8
• Surface area of Solid Reactants

  • An increase in the surface area of a solid reactant increases the reaction rate
  • The greater the exposed matter of a solid the better the chance of collision. The more collisions per unit of time, the faster the reaction.
• The reason why many solids are powdered using a mortar and pestle before being used in a reaction is to increase surface area
• Surface area of Solid Reactants

  • active metals with acids, e.g. HCl with zinc
  • coal dust with oxygen gas
  • grain dust with oxygen gas
• Note: for solids, it is surface area, not concentration that affects reaction rate
• Concentration of Reactants

  • An increase in the concentration in one or more of the reactants will increase the reaction rate (as long as the reactant is in the rate-determining step)
  • An increase in the number of particles in a given volume of space (mol/L) will result in a better chance of collision. The more collisions per unit of time, the faster the reaction.
• Concentration of Reactants

  • 2 HCl(aq) + Zn(s) → H2(g) + ZnCl2(aq) 2.00 g of Zinc will react faster in 6.00 mol/L HCl than the same mass of Zinc in 2.00 mol/L HCl
• Changing Concentration

  • To change the concentration of an aqueous species: more solute can be added to make the solution more concentrated, more solvent can be added to make the solution more dilute (less concentrated)
  • To change the concentration of a gas: Adding more gas to a fixed volume or increasing the pressure by decreasing the volume of the container will increase the concentration. Removing some gas from a fixed volume or decreasing the pressure by increasing the volume of the container will decrease the concentration
• Temperature of the Reaction System

  • In general, as temperature increases, reaction rate increases
  • At higher temperatures, particles have greater kinetic energy so they are moving faster. Particles will thus collide more frequently and with greater intensity. Therefore more particles will overcome the activation energy barrier to form products.
• Catalyst

  • A catalyst is a substance that speeds up the reaction rate without being used up in the process
  • It provides a new pathway (reaction mechanism) to form products, one that requires less energy
  • It lowers the activation energy for the reaction, therefore more particles have the required energy to overcome the activation energy barrier and form products
• Increasing the temperature, concentration or surface area has no effect on activation energy. Only a catalyst can lower EA.
• liquid propane is liquid because of its intermolecular forces, so London dispersion forces have to break to create a gas, higher the activation energy, the longer the reaction
• applications:

  • bath bombs: bigger bath bomb means longer time to dissolve because of its surface area
  • medicine: softer medicines are faster than capsule or hard medicines because of its surface area
  • tea: hot water infuses tea faster because the hot water has more kinetic energy so it has more chances of collision