6-The rate and extent of chemical exchange

• measuring mean rate of chemical reactions

  • rate of reaction = quantity of products formed divided by time taken
  • rate of reaction = quantity of reactants used divided by time taken
  • units for quantity - grams (g) or moles (mol) or cm cubed (cm³) or decimetres cubed (dm³)
  • units for time - seconds (s) or minutes (min)
• rate of chemical reaction - products formed

  1. at first the line is steep as products are being produced quickly because there is a high concentration of reactants
  2. then the line starts getting less steep as products are being produced more slowly due to the decrease in concentration of the reactants
  3. the line then starts plateauing and no more products are being produced because there is no reactants left

  
• rate of chemical reaction - reactants being used

  1. at first, the line steeply falls as the reactants have a high concentration and are reacting quickly to produce products
  2. the line then starts getting less steep as the reactants slow down reacting with each other as they have a lower concentration so produce products less quickly
  3. the line then starts plateauing as there are no more reactants left to react and no more products being produced

  
• collision theory states that in order for particles to react they have to collide with sufficient energy (activation energy)
• collision theory - rate of reactions depend on:

  • the amount of energy the particles have - the more energy they have, the more energy they can transfer and the more likely they are to surpass the activation energy
  • the frequency of the collisions - even though not all collisions are successful, the more often they collide, the more successful collisions there will be overall
• factors that affect the rate of reaction

  • concentration/ pressure
  • presence of a catalyst
  • temperature
  • surface area

  rate of reaction - you must link these factors to how they affect energy and frequency of collisions
• factors of ROR - temperature

  • as temperature increases - particles gain more energy which makes them move faster so they will collide more frequently - they will also collide with more energy each time so they are more likely to surpass the activation energy
  • this will end up in a higher rate of successful collisions and therefore a higher rate of reaction
• factors of ROR - concentration/pressure

  • when concentration or pressure is increased it means that there will be more particles per unit volume and the particles will be closer together which makes collisions more frequent so increases the rate of reaction
• factors of ROR - surface area

  • a higher surface area would increase the frequency of collisions as there would be more space for the particles to collide with which increases the chance of successful collisions and therefore the rate of reaction
• factors of ROR - presence of a catalyst

  • the catalyst lowers the activation energy by providing an alternative reaction pathway which increases the chance of successful collisions - such as cobalt, nickel, enzymes
• the minimum amount of energy required for a reaction to take place is called the activation energy
• rate of reaction graphs

  A) activation energy

  
• forward and backward reactions in reversible reactions can happen at different rates
• equilibrium

  • when equilibrium is reached in a reversible reaction, it will seem as if they are not reacting as they will be happening at the same time and cancel each other out
  • forward and backward reactions in reversible reactions can happen at different rates but then their rates will even out and reach equilibrium
  • there is no overall change in concentrations of reactants or products
• system in equilibrium

  • it requires a closed system - they can escape
  • the concentrations of the reactants and products remain constant - they do not have the same concentrations
  • the forward and backward reactions occur at the same rate
  • the position of equilibrium can change and shift to the left or right (lies to the...) more products means equilibrium lies to the right
• factors affecting the position of equilibrium

  • pressure
  • concentration
  • temperature
• Le Chatelier's Principle

  • it states that if the conditions of a reversible reaction are changed, the position of equilibrium will shift to try and counteract that change
• if the forward reaction is negative then it is exothermic