Rate of Chemical changes

• Rate of reaction

  • Amount of reactant used / Time
  • Amount of product formed / Time
• Calculating rates of reactions

  • Quantity of reactant or product can be measured by mass in grams or volume in cm3
  • Units of rate of reaction may be g/s or cm3/s
  • Can use quantity of reactants in moles and units for rate of reaction in mol/s
• Finding rate of reaction graphically

  1. Draw tangents to curves

  2. Calculate gradient of tangent as measure of rate of reaction at specific time
• Factors affecting rate of chemical reactions

  • Concentration
  • Pressure
  • Surface area
  • Temperature
  • Catalysts
• Collision theory

  Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
• Activation energy

  Minimum amount of energy particles must have to react
• Increasing concentration, pressure, surface area

  Increases frequency of collisions and rate of reaction
• Increasing temperature

  Increases frequency and energy of collisions, increasing rate of reaction
• Catalysts

  Substances that speed up chemical reactions without being changed or used up
• How catalysts work

  • Decrease activation energy, increasing proportion of particles with energy to react
  • Provide different pathway with lower activation energy

  
• Enzymes act as catalysts in biological systems
• Catalysts are not included in the equation for a reaction
• Reversible reaction


  A chemical reaction where the products can react to produce the original reactants
• Reversible reaction


  1. Reactants

  2. Products

  3. Direction of reaction can be changed by changing conditions
• Example of reversible reaction

  • Hydrogen + Nitrogen ⇌ Ammonia (Haber Process)
• If a reversible reaction is endothermic one way

  It is exothermic in the opposite direction
• The same amount of energy is transferred each way in a reversible reaction
• Equilibrium

  When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction
• Le Chatelier's principle

  • If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change
• Effect of changing concentration

  1. If concentration of reactants is increased, position of equilibrium shifts towards products

  2. If concentration of products is increased, position of equilibrium shifts towards reactants
• If temperature is increased

  Equilibrium moves in the direction of the endothermic reaction
• If temperature is decreased

  Equilibrium moves in the direction of the exothermic reaction
• Effect of temperature on yield for exothermic and endothermic reactions

  • Exothermic: Increase in temperature decreases yield, Decrease in temperature increases yield
  • Endothermic: Increase in temperature increases yield, Decrease in temperature decreases yield
• Effect of pressure changes on equilibrium

  1. If pressure is increased, equilibrium shifts to side of equation with fewer moles of gas

  2. If pressure is decreased, equilibrium shifts to side of equation with more moles of gas
• Effect of pressure on yield for reactions with larger or smaller volume of gas

  • Larger volume of gas: Increase in pressure decreases yield, Decrease in pressure increases yield
  • Smaller volume of gas: Increase in pressure increases yield, Decrease in pressure decreases yield