6 - Rates of reactions

• Rate of reaction is how quickly a reaction happens,

  • High rate means the reaction happens quickly
  • Low rate means the reaction happens slowly
  • Rate of zero means the reaction is not happening or has stopped
• Particles must collide with enough energy (Activation energy) to cause a reaction.
• Exam tip : Whenever asked about explaining a rate of reaction, always link it back to the amount of collisions that have enough energy in a set time.
• 5 Factors that effect the rate of reaction : Temperature, Concentration, Pressure, Surface area and catalysts.
• Temperature

  • Increasing/decreasing temperature increases/decreases the rate of reaction as particles gain/lose more kinetic energy so move faster/slower so the chance of successful collisions increases/decreases in a set time.
• Concentration

  • Increasing/decreasing concentration increases/decreases the rate of reaction as particles gain/lose more kinetic energy so move faster/slower so the chance of successful collisions increases/decreases in a set time.
  • More or less particles in a set volume
• Pressure

  • Increasing/decreasing pressure increases/decreases the rate of reaction as particles gain/lose more kinetic energy so move faster/slower so the chance of successful collisions increases/decreases in a set time.
  • Affects volume particles are in
• Surface area

  • Affects size of pieces (large, small, powder ect.)
  • Smaller pieces = larger surface area
  • Affects number of particles on the surface which can collide
  • Increase surface area, rate increases
• Catalyst

  • Increases rate of reaction by lowering activation energy
• Rate is not measured it is calculated

  It has to be calculated from data of a reaction

  A rate can be calculated by the volume of gas being measured after being produced in a set time.

  A rate can also be calculated by measuring the loss of mass in a reaction over a set time.
• RATE EQUATIONS

  Rate(cm^3/s) = Volume of gas (cm^3) / time (s)

  Rate (g/s) = Change in mass (g) / time (s)
• A catalyst is a substance that:

  • Increases the rate of reaction An enzyme is a biological
  • Can be recycled - not used up catalyst.
  • Is specific to a reaction
  • Can be filtered out
  • Works be lowering the activation energy of the reaction by providing an alternate reaction pathway.
• Gas syringe can be used to measure amount of gas product from a reaction.

  
• An inverted measuring cylinder can also be used to collect gas produced from a reaction.

  
• Cotton wool is used to prevent any liquids/solutions splashing out of the flask, as this would lose mass.

  
• Exothermic reaction profile with and without a catalyst

  
• A - Reaction has a high rate and the rate is constant. B - Reaction has a low rate and the rate is constant. C - Reaction starts with a high rate, but slows down then stops.

  
• Rate decreases over time due to the concentration of reactants decreasing (becoming products) - lowers the chance of successful collision.
• Mean overall rate : given vol/mass/given time

  • go up from given time
  • find vol/mass
  • do calculation
• Mean overall rate example straight lines.

  (80-20)/(70-10) = (60)/60) = 1g/s

  
• Finding the rate of curved lines

  • Asked for rate at one specific time
  • Draw a tangent
  • Work out the gradient of tangent

  e.g. (80-50)/(90-30) = 30/60 = 0.5cm^3/s

  
• LEARN MEASURING RATE OF REACTION RQ
• Reactants ⇌ Products

  • Reversible reaction
  • The overall energy change of a reaction is the opposite for the reverse reaction
  • Products can react again to reform the reactants
• DYNAMIC EQUILIBRIUM

  • Equilibrium does not mean the amount of reactants/products is equal, only the rates of the reactions - so the amounts no longer change.
• Closed system - A reaction where no reactants or products can escape.

  Equilibrium - When a reversible reaction is in a closed system, the rates of the forwards and the reverse rates will be equal.

  
• Dynamic equilibrium means the position of equilibrium can change, this can affect yield.

  • Equilibrium can be on the RIGHT - Higher amount of reactants than products - the yield is low
  • Equilibrium can be on the LEFT - Higher amount of products than reactants - the yield is high
• LE CHATELIER'S PRINCIPLE : ' Whenever a change is made to a reversible reaction in dynamic equilibrium, the equilibrium will shift to try to oppose the change'

  • So whatever we do to a reaction in equilibrium, the reaction does the opposite to get back to where it was.
• EFFECT OF CONCENTRATION

  If the conc. of a substance is increased/decreased, equilibrium will shift the opposite way.

  e.g. A+B ⇋ C+D

  Increase conc. of A:

  • Opposed by increase in conc. of D
  • Favours the forwards reaction to oppose the change
  • Equilibrium shifts to the right
  • So yield increases
• EFFECTS OF TEMPERATURE

  Depends on exo/endo themic reactions. If we increase/decrease temperature the equilibrium shifts to the side that will do the opposite.

  e.g. A+B ⇋ C+D (reverse 400KJ)(forwards -400KJ)

  Increase temperature

  • Favours the endothermic reaction
  • Favours the reverse reaction
  • Equilibrium shifts to the left
  • Yield decreases
• TEMPERATURE AND CONCENTRATION

  • If conc. of reactants ↑, favours forwards, equilibrium right, yield ↑
  • If conc. of products ↓, favours forwards, equilibrium right, yield ↑
  • ↑ temperature favours endothermic, equilibrium follows the endothermic arrow, if going right yield ↑, if going left yield ↓
  • ↓ temperature favours exothermic, equilibrium follows the exothermic arrow, if going right yield ↑, if going left yield ↓
• PRESSURE

  Pressure only affects equilibrium to do with substances as gasses.

  • If pressure is increases, equilibrium shifts to the side with less moles of gas. Yield increase if right. Yield decrease if left.
  • If pressure is decreased, equilibrium shifts to the side with more moles of gas. Yield increase if right. Yield decrease if left.
• Catalysts have no effect on equilibrium.