c6

• 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
• Reversible reaction

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


  Reaction can be changed by changing conditions (e.g. use hot conditions for forwards, cool for reverse)
• Finding rate of reaction graphically

  1. Draw tangents to curves

  2. Calculate gradient of tangent as measure of rate of reaction at specific time
• Example of reversible reaction

  • Hydrogen + Nitrogen ⇌ Ammonia (Haber Process)
• Factors affecting rate of chemical reactions

  • Concentration
  • Pressure
  • Surface area
  • Temperature
  • Catalysts
• 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 (just lost for one, gained for the other)
• 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
• Collision theory

  Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
• If a change is made to the conditions of a system at equilibrium

  The system responds to counteract the change (Le Chatelier's principle)
• If the concentration of one of the reactants or products is changed

  The system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again
• Activation energy

  Minimum amount of energy particles must have to react
• If the concentration of reactants is increased


  The position of equilibrium shifts towards the products so more product is produced until equilibrium is reached again
• If the concentration of products is increased


  The position of equilibrium shifts towards the reactants so more reactant is produced until equilibrium is reached again
• If temperature is increased

  Equilibrium moves in the direction of the endothermic reaction
• Increasing concentration, pressure, surface area


  Increases frequency of collisions and rate of reaction
• If temperature is decreased

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

  • Exothermic: Increase in temperature decreases yield, Decrease in temperature increases yield
  • Endothermic: Increase in temperature increases yield, Decrease in temperature decreases yield
• Increasing temperature


  Increases frequency and energy of collisions, increasing rate of reaction
• In gaseous reactions, if pressure is increased

  The equilibrium shifts to the side of the equation with the least number of molecules
• If pressure is increased


  Equilibrium shifts to the side of the equation with fewer moles of gas
• If pressure is decreased


  Equilibrium shifts to the side of the equation with more moles of gas
• Catalysts


  Substances that speed up chemical reactions without being changed or used up
• Effect of pressure on yield for reactions with different gas volumes

  • Reaction produces larger volume of gas (more moles): Increase in pressure decreases yield, Decrease in pressure increases yield
  • Reaction produces smaller volume of gas (fewer moles): Increase in pressure increases yield, Decrease in pressure decreases yield
• 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
• If the units used to measure time are minutes, the units for the rate would be g/min rather than g/s
• Precipitate


  A solid that is formed in a solution during a chemical reaction
• Turbidity


  The cloudiness of a solution
• Ways to measure the formation of products

  • Precipitation
  • Change in mass
  • Volume of gas given off
• Measuring the rate of a precipitation reaction


  1. Solution starts clear

  2. Precipitate forms

  3. Mark disappears
• Precipitation: This method is simple and easy but very subjective, and you can't plot a rate of reaction graph
• Measuring the rate using change in mass

  1. Place reaction vessel on balance

  2. Add reactants to flask

  3. Gas released, mass decreasing measured
• Measuring change in mass: This is the most accurate method but releases the gas into the room
• Measuring the rate using volume of gas produced

  1. Use gas syringe to measure volume of gas given off

  2. More gas given off, faster the reaction
• Gas syringes are quite sensitive but you have to be careful not to blow the plunger out
• Rate of reaction


  Shown by the gradient (steepness) of the line on a graph of product formed or reactant left against time