ch 10: Reaction Rates in Equilibrium

Exothermic Reactions

• “Outside”
  
  

  • Heat is a product leaving the reaction.
    
    

  • Feels warm (e.g., hand warmers).
    
    

Endothermic Reactions

• “Inside”
  
  

  • Heat is a reactant, taken in or absorbed by the reaction.
    
    

  • Feels cold (e.g., instant cold pack).
    
    

Reaction Rates on the Molecular Level

Reaction Rate

• Definition: The speed at which a chemical reaction occurs.
  
  

• Ways to Measure:
  
  

  1. Changes in concentration.
     
     

  2. Changes in volume or mass.
     
     

Collision Theory

• Collisions are key for reactions:
  
  

  1. Molecules must be in close contact.
     
     

  2. Molecules must collide.
     
     

  3. Collisions must occur with sufficient energy (speed).
     
     

  4. Collisions must happen at the correct angle or orientation.
     
     

Factors Affecting Reaction Rates

1. Available Surface Area:
   
   

   • Greater surface area allows for more collisions.
     
     

   • More collisions = faster reactions.
     
     

2. Concentration:
   
   

   • Higher concentration (more solute) = more molecules.
     
     

   • More molecules = more collisions = faster reactions.
     
     

3. Temperature:
   
   

   • Higher temperatures increase molecular speed.
     
     

   • Faster molecules = more collisions = faster reactions.
     
     

4. Reactions with Gas Molecules:
   
   

   • Affected by pressure changes.
     
     

   • Higher pressure = faster reactions.
     
     

5. Catalysts:
   
   

   • Speed up the reaction without being consumed.
     
     

Le Chatelier’s Principle

Key Points

• Only applies to reversible reactions.
  
  

• Stressing a reaction causes a change in the reaction rate and equilibrium position.
  
  

• Taking away causes a shift towards what was removed.
  
  

• Adding causes a shift away from what was added.
  
  

Types of Stresses

1. Adding Reactant:
   
   

   • Causes a shift towards the product (right) to undo the change by producing more product.
     
     

2. Adding Product:
   
   

   • Causes a shift towards the reactant (left) to undo the change by using up the added product.
     
     

3. Removing Reactant:
   
   

   • Causes a shift towards the reactant (left) to replace what was removed.
     
     

4. Removing Product:
   
   

   • Causes a shift towards the product (right) to replace what was removed.
     
     

5. Changing Pressure (only affects gases):
   
   

   • Increase in pressure: Shifts towards the side with fewer moles of gas.
     
     

   • Decrease in pressure: Shifts towards the side with more moles of gas.
     
     

Temperature Changes

• Increase in temperature:
  
  

  • Causes a shift away from heat (right) in an exothermic reaction.
    
    

• Decrease in temperature:
  
  

  • Causes a shift towards heat (left) in an exothermic reaction.
    
    

Catalysts

• A catalyst always speeds up the forward reaction.
  
  

• Causes a shift to the right (towards the product) by making the reaction proceed faster.