Rates of Reactions (Topic 5)

• The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time


  Mean rate of reaction = quantity of reactant / time

  
• The quantity of reactant or product can be measured by the mass in grams or by a volume in cm3 .


  Moles = mass/Mr moles = grams/rams
• 
  

  Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts.
• Collision Theory:

  chemical reactions can occur only when reacting particles collide with each other and with sufficient energy (activation energy)
• Activation energy:

  The minimum amount of energy that particles must have to react
• when these factors increase , how does this affect the rate of reaction?

  It increases the rate of reaction
• How does increasing concentration affect the rate of reaction? Why?

  • increases rate of reaction,
  • with increase in conc, more particles causing more frequent successful collisions
• How does increases temperature affect the rate of reaction?

  • Increased temp increases energy in particles, more energetic
  • This means the particles will more faster and have more frequent collisions
  • this INCREASES the rate of reaction
• How does increasing pressure affect the rate of reaction? WHY?

  • Increased pressure means more particles per cm3
  • This causes more succseful frequent particle collisions
  • resulting in an INCREASED rate of reaction
• How does increasing surface area affect rate of reaction? WHY?

  • surface area is increased by making reactant a powder or crushed into fine pieces
  • This causes for the same volume of reactant, more frequent collisions
  • this INCREASES the rate of reaction
• What is a catalyst?

  A catalyst is a substance that increases the rate of a chemical reaction without being used up. Catalysts work by providing a different

  pathway for the reaction that has a lower activation energy. (decreasing activation energy)
• What is an example of a biological catalyst?

  Enzymes (lipase, amylase, protease)
• What is the reaction profile for a reaction with a catalyst?

  
• How does the presence of a catalyst affect the rate of reaction?

  it increases the rate of reaction
• How can rate of reaction be studied & measured?

  • precipitation & colour change(black X disappears)
  • change in mass (gas released)
  • volume of gas released (use gas syringe)
• What chemicals are used in the rate experiment required practical?

  Sodium thiosulfate and Hydrochloric acid (HCl)

  • both clear solutions, when react form yellow precipitate
  • place beaker over black X and time how for X to disappear
• How does the RP for rates change to measure the affect of acid conc?

  • by repeating with solutions at different concentrations
  • (only change concentration of 1 reactant, keep 1 the same)
• How can rates be calculated from graphs?

  calculate mean rate of reaction or draw tangent
• When do reversible reactions reach equilibrium?

  When the rate of forwards reaction equals the rate of backwards reaction
• What conditions have to be inplace for a reversible reaction to reach equilibrium?

  Closed system
• What is the equation for a reversible reaction?

  A+B <=> C+D
• What is changed to change the direction of a reversible reaction?

  the conditions
• If the forward reaction of a reversible reaction is exothermic, the backwards reaction will be ...

  endothermic
• What does it mean is a reaction is endothermic

  energy is taken in (temperature decrease)
• What does it mean is a reaction is exothermic?

  Energy is released (temperature increase)
• How is energy affected in a reversible reaction? (closed system)

  the same amount of energy is transferred in each direction, the overall amount of energy remains the same
• What is Le Chatelier's principal?

  the idea that is you change the conditions of a reversible reaction at equilibrium, the system will counteract that change
• What direction will equilibrium shift to if the temperature is DECREASED

  Will move to exothermic as more energy will be released causes the system to heat up again to return to normal conditions
• What direction will equilibrium shift to if temperature is INCREASED?

  will shift to endothermic so more energy taken in so temp will decreases to return to normal condition
• What direction will equilibrium shift to if pressure is INCREASED?

  will move to the side with fewer molecules (moles) until equilibrium is reached
• What direction will equilibrium shift to if pressure is DECREASED?

  Equilibrium will shift to the side with most molecules (moles) until equilibrium is reached again
• What will happen if the concentration of products or reactants is in/decreased?

  The system will no longer be at equilibrium
• What will happen is the increase the conc of reactants of a reaction at equilibrium?

  Equilibrium will shift to the products until equilibrium is reached again
• What will happen as the increases of products in a reaction at equilibrium?

  Equilibrium will shift to the reactants until equilibrium is reached again