Rate & Extent of Chemical Changes

• rate of reaction =

  amount of reactant used OR amount of product formed / time
• how can you calculate rate of changes graphically

  • draw tangents to curve + use the slope of the tangent
  • calculate the gradient of the tangent to the curve on the graph as the measure of rate of reaction at a specific time
• which factors affect the rates of chemical reactions

  • concentration
  • pressure
  • surface area
  • temperature
  • catalysts
• what is collision theory

  chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
• what is activation energy

  the minimum amount of energy that particles must have to react
• what happens when the concentration of reactants in a solution + the pressure of reacting gases + the surface area of solid reactants is increased

  • increases the frequency of collisions
  • increases rate of reaction
• what happens when the temperature increases

  • increases frequency of collisions
  • makes collisions more energetic
  • increases the rate of reaction
• what are catalysts

  • substances that speed up chemical reactions without being changed or used up in the reaction
  • enzymes acts as catalysts in biological systems
  • catalysts are not included in equation for reaction
• how do catalyst affect the activation energy

  decreases the activation energy

  • increases the proportion of particles with energy to react
  • provides a different pathway for a lower activation energy
• what are reversible reactions

  when the products of the reaction can react to produce the original reactants

  • the direction of the reaction is changed by changing the conditions
  • an example is the haber process : hydrogen + nitrogen ⇌ ammonia
  • equals amount of energy are transferred each way (as the energy lost in one reaction is gained in the same amount in the opposite)
• what is a closed system

  reversible reactions where no reactants or products can escape

  • equilibrium is reached when reactions occur at the exact same rate in each direction
• what is the effect of changing conditions upon equilibrium

  relative amounts of all reacting substances at equilibrium are dependant on the conditions of the reaction

  • if a system is at equilibrium and there are changes to its conditions the system responds to counteract the change
  • this is called le chateliers principle
• what is the effect of changing concentration upon equilibrium

  if the conc on a reactant or product is changed, the system in no longer at equilibrium + the conc of all substances will change until equilibrium is reached again

  • if the conc of reactants increased -> position of equilibrium shifts towards products -> more products are produced until equilibrium is reached again
  • if the conc of products increased -> position of equilibrium shifts towards reactants -> more reactants are produced until equilibrium is reached again
• what is the effect of changing temperature upon equilibrium

  • if temp is increased -> equilibrium moves in the direction of the endothermic reaction
  • if temp is decreased -> equilibrium moves in the direction of the exothermic reaction
• what is the effect of changing pressure upon equilibrium


  in gaseous reactions an increase in pressure favours the reaction producing the least number of molecules

  • if pressure increases -> equilibrium shifts to equation side with fewer moles of gas -> produces more product
  • if pressure decreases -> equilibrium shifts to equation side with more moles of gas -> produces more reactant