Topic 6

• Rate of reaction


  How fast the reactants are changed into products
• Reactions with different rates


  • Rusting of iron (slow)
  • Chemical weathering (slow)
  • Metal reacting with acid (moderate)
  • Burning (fast)
  • Explosive reactions (very fast)
• Graphs for rate of reaction


  • Steeper line = faster reaction
  • Flat line = reaction finished
  • Faster reactions have steeper lines
• Factors affecting rate of reaction


  1. Particles must collide with enough energy

  2. Activation energy needed to start reaction

  3. Factors that increase collisions or collision energy
• Increasing temperature


  Particles move faster, more frequent collisions, more collisions have enough energy
• Increasing concentration or pressure


  More particles in same volume, more frequent collisions
• Increasing surface area


  More area for particles to collide with
• Using a catalyst


  Provides alternative reaction pathway with lower activation energy
• Enzymes are biological catalysts
• Measuring rate of reaction

  1. Observe change in product or reactant over time

  2. Use mass balance to measure gas produced

  3. Use gas syringe to measure volume of gas produced
• Reactions used to measure rate


  • Magnesium and HCl producing H2 gas
  • Sodium thiosulfate and HCl producing cloudy precipitate
• Higher acid concentration


  Faster rate of reaction
• Need to consider safety precautions when doing rate experiments
• Finding rate of reaction from a graph

  1. Remember a rate of reaction graph shows amount of product formed/reactant used up on y-axis and time on x-axis

  2. To find mean rate, calculate change in y-value and divide by change in x-value (time)

  3. To find rate at a particular point, draw a tangent to the curve and calculate the gradient
• Sodium thiosulfate and HCl produce a cloudy precipitate of sulfur when reacted
• Investigating effect of acid concentration on reaction rate

  1. Add set volume of sodium thiosulfate to flask

  2. Add HCl and start stopwatch

  3. Time how long it takes for black cross to disappear through cloudy solution

  4. Repeat with different HCl concentrations
• Increasing HCl concentration


  Decreases time taken for mark to disappear, increases reaction rate
• This reaction doesn't produce a set of graphs, just a table of times for different HCl concentrations
• Make sure you can clearly see the cross through the flask at the start of the experiment
• There are other methods besides this one for investigating reaction rate
• Reversible reactions


  • Ammonium chloride ⇌ ammonia + hydrogen chloride
• Reversible reactions


  • Forward and reverse reactions happen at the same time
  • Reach a dynamic equilibrium where forward and reverse rates are equal
• Position of equilibrium


  Depends on temperature, pressure, and concentrations of reactants and products
• Heating ammonium chloride reaction


  Moves equilibrium to the right (more ammonia and hydrogen chloride)
• Cooling ammonium chloride reaction


  Moves equilibrium to the left (more ammonium chloride)
• Endothermic and exothermic reversible reactions


  • If one direction is endothermic, the other is exothermic
  • Energy transferred in one direction equals energy transferred in the other
• Le Chatelier's Principle


  Reversible reactions try to counteract changes to conditions to maintain equilibrium
• Decreasing temperature


  Equilibrium moves in exothermic direction to produce more heat
• Increasing temperature


  Equilibrium moves in endothermic direction to absorb heat
• Increasing pressure


  Equilibrium shifts to side with fewer gas molecules
• Decreasing pressure


  Equilibrium shifts to side with more gas molecules
• Increasing reactant concentration


  Equilibrium shifts to produce more products
• Decreasing product concentration


  Equilibrium shifts to produce more products
• Rate of reaction


  How fast the reactants are changed into products
• Slow reactions


  • Rusting of iron
  • Chemical weathering-like acid rain damage to limestone buildings
• Moderate speed reaction


  • Metal magnesium reacting with an acid to produce a gentle stream of bubbles
• Fast reactions


  • Burning
  • Explosions
• Graphs for the rate of reaction

  • Steeper line = faster rate of reaction
  • Line becomes less steep as reactants are used up
  • Quickest reactions have the steepest lines and become flat in the least time
• Increasing the amount of product formed


  Increases the speed of the reaction
• Graphs 1, 2 and 3 all converge at the same level, showing that they all produce the same amount of product although they take different times to produce it</b>