5 - energy changes

• Rate of a chemical reaction


  Measuring the quantity of a reactant used or the quantity of product formed over time
• Calculating rates of reactions

  1. Mean rate of reaction = quantity of reactant used/time taken

  2. Mean rate of reaction = quantity of product formed/time taken
• Quantity of reactant or product

  Measured by the mass (grams) or volume (cm3)
• Quantity of reactants for rate of reaction

  In terms of moles and units for mol/s
• Factors which affect the rates of chemical reactions

  • Concentrations of reactants in the solution
  • Pressure of reacting gases
  • Surface area of solid reactants
  • Temperature
  • Presence of catalysts
• Collision Theory


  How various factors affect rates of reactions
• Activation Energy


  The minimum amount of energy that particles must have to react
• Increasing the rate of reaction (frequency of collisions)

  Increasing the concentration of reactants in solution, the pressure of reacting gases, the surface area of solid reactants
• Increasing the temperature


  Increases the frequency of collisions (makes the collisions more energetic), so increases the rate of reaction
• Catalysts


  Change the rate of chemical reactions, but are not used up (increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy)
• Different reactions need different catalysts
• Enzymes


  Act as catalysts in biological systems
• Reversible reactions


  Products of the reaction can react to produce the original reactants
• If a reversible reaction is exothermic in one direction

  It is endothermic in the opposite
• The same amount of energy is transferred in reversible reactions
• Equilibrium


  When a reversible reaction occurs in an apparatus which prevents the escape of reactants and products, and the forward and reverse reactions occur at exactly the same rate
• If a system is at equilibrium and a change is made to any of the conditions


  The system responds to counteract the change
• Le Chatelier's Principle


  Predicting the effects of changing conditions on a system at equilibrium
• If concentration of one reactant/products is changed

  The system is no longer at equilibrium, the concentrations of all the substances will change until equilibrium is reached again
• If concentration of a reactant is increased


  More products will be formed
• If concentration of a product is decreased


  More reactants will react
• If the temperature of a system at equilibrium is increased


  Relative amount of products at equilibrium increases (endothermic) or decreases (exothermic)
• If the temperature of a system at equilibrium is decreased


  The relative amount of products at equilibrium decreases (endothermic) or increases (exothermic)
• For gaseous reactions at equilibrium, if pressure is increased


  Equilibrium position shifts towards the side with the smaller number of molecules
• For gaseous reactions at equilibrium, if pressure is decreased


  Equilibrium position shifts towards the side with the larger number of molecules