topic c6- the rate and extent of chemical change

• the rate of a chemical reaction

  how fast the reactants are changed into products
• slow reactants

  • chemical weathering
  • rusting of iron
• fast reactants

  • burning
  • explosions
• factors affecting rates of reaction

  • temperature
  • surface area
  • concentration
  • catalysts

  they all lead to more particle collisions
• temperature- factors affecting rates of reaction

  • higher temperature means higher energy in particles
  • particles move faster
• surface area- factors affecting rates of reaction

  • higher surface area to volume ratio
  • gives particles more surfaces to work with
• concentration- factors affecting rates of reaction

  • higher concentration
  • more particles
• catalyst- factors affecting rates of reaction

  • reduces activation energy to start a reaction
  • e.g. biological catalysts (enzymes) breaks substrates into smaller pieces
• rate of reaction

  amount of reactant used or product formed / time
• you can measure the rate of reaction- doing these tests

  • precipitation and colour change
  • volume of gas produced
  • change in mass
• precipitation and colour change

  • how long for a visual change
  • this could be
  • colour change
  • transparent to opaque
• volume of gas produced

  • using a gas syringe
  • more gas, faster reaction
  • record in intervals to graph the process
• change in mass

  • using a mass balance (scales)
  • quicker the reading changes, faster the reaction
• using graphs- to measure the rate of reaction

  
• rate (of reaction)

  change in y/ change in x
• using graphs- to measure the rate of reaction

  • draw a tangent on the curve at desired points
  • find two points that are far apart on your tangent
  • calculate the change in x and y along the tangent line
  • calculate rate by using the rate of reaction equation on the other flashcard
• reversible reactions

  always reach equilibrium in a closed system
• closed system

  no changes to the system while active
• reversible reaction- with symbol

  A + B  ⇌ C + D
• until- you reach- equilibrium

  • forward reaction begins, reactants are used up as product is created
  • forward reaction slows as there is more product
  • backward reaction begins, product is used up and reactant is made
  • backward reaction slows as there is more reactant again
• le chateliers principle

  if you change the conditions of a reversible reaction, the system will try and counteract this change
• le chateliers principle- pressure

  if you increase the pressure on one side, the equilibrium will shift to reduce the pressure to the other side
• le chateliers principle- temperature

  if you decrease the temperature, the equilibrium will shift to produce more heat
• le chateliers principle- concentration 


  if you change the concentration, the system will no longer be in equilibrium
• two rate experiments

  • Mg + HCl -> H2 gas
  • sodium thiosulfate + HCl -> cloudy sulfur
• Mg + HCl -> H2 gas

  • measure mass loss over time using a balance
  • repeat with different HCl concentrations (keep Mg + volume constant)
  • higher concentration means a faster reaction
• sodium thiosulfate + HCl -> cloudy sulfur

  • time how long a black cross disappears
  • repeat with varying HCl concentrations (keep volume constant)
  • higher concentration means a faster reaction
• key points of two rate experiments

  • increasing concentration means a faster reaction
  • control variables- volume, temperature, and reactant amounts