c6 rates of reaaction

• What is activation energy?

  The minimum amount of energy needed for a reaction to take place
• The collision theory

  • The more successful frequent collisions there are, the higher the rate of reaction
  • If particles collide within enough energy, they will overcome the activation energy and react
• What is collision frequency?

  How often particles collide
• Pressure and conc vs rate of reaction

  • Increasing pressure/ conc increases rate of reaction
  • Because there are more reactant particles in a given space
  • Which means there are more frequent, successful collisions
  • So rate of reaction increases
• Temperature vs rate of reaction

  • Increasing temp increases rate of reaction
  • Because the particles gain thermal energy which transfers to kinetic energy
  • More energy causes particles to move faster and have more frequent collisions
  • More frequent, successful collisions = faster rate
• SA vs rate of reaction

  • Increasing SA increases rate of reaction
  • Because breaking up a solid increases its SA:volume ratio
  • So there is more area for reactant particles to collide with
  • More frequent, successful collisions = faster rate
• Catalyst vs rate of reaction

  • Adding a catalyst increases rate of reaction
  • Because catalysts provide a path with lower activation energy for the reaction
  • So particles can collide with more energy
  • More successful, frequent collisions = faster rate
• What is rate of reaction?

  How long it takes for reactants to turn into products
• How can you measure rate of reaction?

  EITHER

  Measure quantity of reactant used up over time

  OR

  Measure quantity of product formed over time
• What can you measure rate with?

  1. Gas syringe
  2. Mass balance - calculate change in mass
  3. Colour change / precipitation
  4. Measuring cylinder
• Measuring rate with gas syringe

  • Measure vol of gas produced over time
  • More gas given off = faster reaction
  • + Easy to take regular readings
  • + Easy to plot on graphs
  • -- Reaction can break the syringe - not safe
• Measuring rate with mass balance

  • Calculate change in mass/time if a gas is produced
  • Mass will decrease as gas is released to the surroundings
  • Add cotton wool to conical flask to prevent chemicals leaving the flask
  • + Easy to plot mass and time on a graph
  • + Most accurate method
  • + Highest resolution
  • -- SAFETY ISSUE of gas being released into the room
• Measuring rate with a measuring cylinder

  • Measure the volume of gas produced/time
  • More gas produced = faster reaction
  • + Easy to take regular readings
  • + Easy to plot on graph
• Measuring rate by colour change/precipitation

  • Record the visual change during a reaction
  • ONLY if the reactants are transparent and the product is a precipitate that will cloud the solution (or vice versa)
  • Put conical flash on a black cross, time how long it takes to disappear (if product is a precipitate and reactants are transparent)
  • OR time how long it takes for cross to be visible (if product is transparent and reactant is coloured)
  • -- Results are subjective (people might not agree over when the mark disappears)
  • -- Can't plot a graph
• Mean rate of reaction = quantity of product formed / time
• What is mean rate of reaction measured in?

  g/s

  g/cm^3

  mol/s
• What is time measured in?

  Seconds
• What are reactants and products measured in?

  Grams

  cm^3

  mol
• Mean rate of reaction = quantity of reactant used / time
• Rate of reaction GRAPHS

  • Steeper line = faster rate
  • Over time lines will plateau because reactants are used up
• Lines 2 + 3 = increased rate of reaction due to increased SA, catalyst or increased temp

  Line 4 = increased rate due to increased conc (results in more reactant particles so more product can be made)

  
• Reversible reactions

  • Shown by a double arrow in equations - reaction goes both ways
  • Reactants form products
  • Products form reactants
  • Reaction reaches equilibrium when 1. the rate of backwards and forwards reaction is the same and 2. the conc of product and reactant remains constant
• What is equilibrium?

  When the rate of both forward and backward reactions are the same
• When will a reversible reaction reach equilibrium?

  In a closed system

  Must be optimum conditions
• What is Le Chatelier's Principle?

  If you change the conditions of a reversible reaction at equilibrium, the reaction will shift forwards or backwards to make a new equilibrium
• Factors affecting Le Chatelier

  • Pressure
  • Temperature
  • Concentration
• Pressure and Le Chatelier (only in gases)

  • Increased pressure = reaction shifts to the side with fewer molecules
  • Fewer collisions = pressure is decreased
  • Decreased pressure = reaction shifts to the side with more molecules
  • More collisions = brings pressure back up
• Temperature and Le Chatelier

  • Increased temperature = reaction shifts in the endothermic direction
  • So takes IN heat energy and lowers temp (more products for endothermic)
  • Decreased temp = reaction shifts in exothermic direction
  • So releases heat energy OUT and raises temp (more products for exothermic)
• Concentration and Le Chatelier

  • Increased conc of reactants = reaction shifts FORWARDS to make more products
  • Decreased conc of products = reaction shifts BACKWARDS to decrease the amount of reactants