SCH 4U - Factors Affecting Rates of Reaction

Factors Affecting Rates of Reaction

Collision Theory – for a reaction to occur, a collision between reactants must take place.  So, there must be an increase the number and/or effectiveness of collisions to increase the reaction rate.

Five Factors Affecting Reaction Rate

(a)   Presence of a Catalyst

• recall that a catalyst is a substance that is involved in a reaction, but it is regenerated, so it is neither a reactant nor a product
  
• a catalyst makes the collisions that do take place, more effective and thus, increases the reaction rate
• many catalysts work by providing a surface for the reactant particles to perch on and interact, through a process called adsorption
  
• common catalysts are finely powdered Pt or Pd (catalytic converters in car exhaust systems)
• biologists, call catalysts enzymes

(b)   Temperature

• as temperature increases, the rate increases because the particles move faster
• there are more collisions and harder collisions

(c)   Concentration of Reactants

• as the concentration of the reactants increases, the rate increases
• there are more particles which can cause more collisions
• the concentration of gaseous or aqueous reactants can be changed, but liquid or solid reactants cannot be altered

(d)   Surface Area

• there are two types of reactions: homogeneous (all reactants in the same state) and heterogeneous (the reactants are in different states)
• the reaction rate can be increased by crushing solids into small pieces, by shaking immiscible (completely insoluble) liquids or using a beaker instead of a test tube to hold reactants
• all the above will increase the area of interface (the area over which reactants are in contact) between the reactants

(e)   Chemical Nature of the Reactants

• the chemical nature of the reactant affects the reaction rate
• for instance, even if all the conditions surrounding the reaction are the same, some substances naturally react more quickly than others – for instance, sodium is much more reactive than gold when both are placed in water
• in homogeneous reactions, electron transfers between ions are quite quick, while reaction between molecules are much slower (since bond rearrangement occurs)

Predicting the Relative Rate of Reaction

Place the following reactions in order of increasing rate:

(a)   C₈H₁₈(l)  +  25/2 O₂(g)  →  8CO₂(g)  +  9H₂O(g)   

-  hetero, small area of interface  -  SLOWEST

(b)   3Fe²⁺(aq) + NO₃^-(aq) + 4H⁺(aq) →  3Fe³⁺(aq) + NO(g) + 2H₂O(g)   

- homo, bond rearrangement  -  MEDIUM

(c)   Cr²⁺(aq)  +  Fe³⁺(aq)  →   Cr³⁺(aq)  +  Fe²⁺(aq) 

– homo, e transfer  -  FASTEST

The following is a list of reaction characteristics, from fastest reaction type to slowest:

(a)   e transfer (homo*)

(b)   collisions between 2 reactants (homo*)

(c)   collisions between multiple reactants (homo*)

(d)   bond rearrangements (homo*)

(e)   heterogeneous* reactions (small interface site)

* homogeneous reactions are typically assumed to be faster than heterogeneous ones.   

*****Looking for more info?  This video is quite good.  Note that we will only refer to concentration (regardless of whether we are studying a solution or a gas) and not bother with pressure.*****

Homework # 7-10