Rate of reaction 1

• Rates of Reactions


  The rates of a reaction at different times can be compared by considering the slope of a graph of quantity or concentration of reactant or product against time
• Draw and interpret graphs representing changes in quantities or concentration of reactants or products against time

  Science Understanding
• Rate of Reaction


  A rate is a change over time
• Rate of Reaction


  • The rate of formation of a product
  • The rate of consumption of a reactant
• Variables measured to determine reaction rate


  • Concentration (mol L-1)
  • Mass (g)
  • Amount (mol)
• Units for reaction rate


  • mol L-1 s-1
  • g s-1
  • mol s-1
• Variables that can be used to determine reaction rate include; concentration, pH, volume or pressure of a gas, colour/light intensity, mass
• Graphing Rate of Reaction

  • The time taken for a reaction to reach a specified point is an indication of the rate of the reaction
  • The rates of a reaction at different times can be compared by considering the slope of a graph of a quantity (or molar concentration) of reactant or product against time
  • A steeper slope indicates faster rate of reaction
• Calculation of rate


  Quantity of reactants may be measured by a change in: pH, volume or pressure of a gas, colour intensity, mass of solid etc
• Calculating Rate of Reaction


  1. Calculation of rate: gas formation

  2. Calculation of rate: mass loss
• Exothermic & Endothermic Reactions


  • Chemical reactions involve breaking and making bonds
  • Bond breaking uses energy
  • Bond making releases energy
  • In all chemical reactions there will either be a net release of energy into the surrounding or a net absorption of energy from the surrounds
• Enthalpy (H)


  The total energy/heat of the system at constant pressure and volume
• Enthalpy change


  The quantity (amount) of heat released when a certain amount of substances react at a constant pressure
• Change in enthalpy over a course of a reaction


  ΔH= H products-H reactants
• Activation energy (Ea)


  The minimum amount of energy the reactants must gain before reacting to form products
• Endothermic reaction


  • Ba(OH)2.8H2O(s) + 2NH4Cl(s) → 2NH3(g) + 10H2O(l) + BaCl2(s)
  • ΔH = +164 kJ/mol
• Exothermic reaction


  • C12H22O11 → 12C + 11H2O
  • ΔH = -5640.9 kJ/mol
• Rate of Reaction


  Rates of reaction can be influenced by a number of factors, including the presence of inorganic and biological catalysts (enzymes)
• Predict and explain, using collision theory, the effect on rates of reaction due to changes in

  1. Concentration

  2. Temperature

  3. Pressure (for reactions involving gases)

  4. Surface area

  5. Catalyst
• Collision theory


  Can be used to explain and predict reaction rate
• Collision theory


  • Chemical reactions involves collisions between reacting particles
  • Particles are constantly moving around and colliding
  • The minimum energy for the collision to be productive is called the activation energy
• Effective collisions


  When reactant particles collide with sufficient energy and the correct orientation, reaction occurs
• Rate of Reaction


  Topic 2 - Managing Chemical Processes
• Higher concentration


  More chance of collision, more chance of reaction, faster rate of reaction
• For reactions involving gases, higher pressure


  Particles closer together, more chance of collision, more chance of reaction, faster rate of reaction
• Higher temperature


  Faster movement of particles, more chance of collision, more energy for reaction, more chance of reaction, faster rate of reaction
• More subdivided reactants


  More surface area available, more chance of collision, more chance of reaction, faster rate of reaction
• Adding a catalyst to a reaction


  Provides alternate pathway with lower activation energy, more chance of reaction, faster rate of reaction
• Energy profile diagrams


  • Alternate reaction pathway with lower activation energy
  • Notice ∆H is unchanged by the presence of a catalyst
  • You can see the reaction pathway on the energy diagram
• Energy profile diagrams can be used to represent the relative enthalpies of reactants and products, the activation energy, and the enthalpy change for a chemical reaction
• Bond breaking and making


  1. Bonds in the reactants are broken, absorbing energy (upward arrows)

  2. New bonds form the products, releasing energy (downward arrows)

  3. Enthalpy is shown as delta H
• Energy profile diagrams


  • Show the reaction process for a productive collision
  • The horizontal line on the left shows the energy contained in the reactants
  • The peak in the centre shows the activation energy
  • The horizontal line on the right shows energy contained in the products
  • Note: these are diagrams of the process, not graphs against time. Slopes are NOT related to the rate of reaction.