topic c6- the rate and extent of chemical change

• rates of reaction

  how fast the reactants are changed into products
• slow reactions

  • rusting
  • chemical weathering
• fast reactions

  • burning
  • explosions
• rate of reaction

  • the quickest reactions have the steepest lines that become flat in the least time
  • particles must collide with enough energy to react
• rates of reaction

  • the more collisions there are the faster the reaction is
• activation energy

  the minimum amount of energy particles need to react
• rates of reaction

  
• factors affecting rates of reaction- temperature

  • temp increases
  • particles move faster
  • particles have more energy
  • more collisions
• factors affecting rates of reaction- concentration or pressure

  • if the solution is more concentrated
  • more particles in the same volume of water
  • more frequent collisions
• factors affecting rates of reaction- surface area

  • increasing SA to volume ratio means
  • the particles will have more area to move
  • so more frequent collisions
• factors affecting rates of reaction- catalysts

  • decreases the activation energy
  • so the reaction speeds up
  • remains unchanged at the end of the reaction
  • e.g. enzymes, cobalt + nickel
• collision theory

  • to react
  • need sufficient energy
  • activation energy= minimum
• rate of reaction

  amount of reactant used or amount of product formed/ time
• measuring rates of reaction- precipitation + colour change

  • place a flask containing a transparent solution on a black mark
  • add precipitate to the solution and time how long it takes for the cross to disappear
• measuring rates of reaction- change in mass

  • reactions that produce a gas can be recorded using a mass balance
  • as the gas is released the mass will decrease
• measuring rates of reaction- volume of gas given off

  • use a gas syringe to measure the volume of gas given off
• two rate experiment

  effect of concentration on the rate of reaction
• magnesium and HCl react to produce H2 gas

  1. add 10cm^3 of dilute hydrochloric acid to a conical flask + place on balance
  2. add magnesium to the acid and quickly plug the flask with cotton wool
  3. start the stopwatch and record the mass on the balance
  4. take readings of the mass at regular intervals
  5. plot the results in a table and work out the mass lost
  6. plot a graph of time and mass lost
  7. repeat the experiment with more concentrated acid solutions
• you can calculate the mean reaction rate from a graph

  change in y/ change in x
• draw a tangent to find the rate of reaction at a particular point

  • draw a tangent to the graph
  • work out the gradient
• how to calculate gradient

  change in y/ change in x
• reversible reactions

  forwards + backwards reactions take place
• reversible reactions

  • the products can react to form the reactants again
  • equilibrium can only be reached when a reversible reaction takes place in a closed system (where nothing enters or leaves)
  • A + B ⇌ C + D
• equilibrium can be on the right or the left

  • if it's on the right, concentration of products is greater than the reactants
  • if it's on the left, concentration of reactants is greater than the products
• ammonium chloride ⇌ ammonia + hydrogen chloride

  • cooling favours the left
  • heating favours the right
• hydrated copper sulfate ⇌ anhydrous copper sulfate + water

  -> endothermic= taken in (-> exo)

  <- exothermic= given out (<- endo)

  • the same amount of energy is transferred in each direction
• Le Chatelier's Principle

  • if the conditions of a reversible reaction at equilibrium are changed, the system tries to counteract that change
  • dynamic equilibrium
• le chatelier's principle- temperature

  • increases- the equilibrium shifts to favour the endothermic direction to take in heat energy- right
  • decreases- the equilibrium shifts to favour the exothermic direction to release heat energy- left
• le chatelier's principle- pressure

  • increases- the equilibrium shifts to favour the side with fewer molecules of gas to reduce the pressure- left
  • decreases- the equilibrium shifts to favour the side with more molecules of gas to increase the pressure- right
• le chatelier's principle

  • if concentration of reactant is changed
  • the system will respond to reverse the change
• le chatelier's principle- concentration

  • if the concentration of reactants increases the system responds to make more products
  • if the concentration of reactants decreases the system responds to make more reactants
• le chatelier's principle

  catalyst makes it get to equilibrium faster