The rate and extent of chemical change

• the rate of a chemical reaction is

  how fast the reactants are changed into products
• the rate of a chemical reaction depends on

  the collision frequency of reacting particles

  the energy transferred during a collision
• activation energy


  the minimum amount of energy that particles need to react

  particles need this energy to break the bonds in the reactants and start the reaction
• the rate of reaction depends on

  temperature

  the concentration and pressure of a solution

  surface area

  the presence of a catalyst
• temperature 


  when the temperature increases, the particles all move faster

  if the particles move faster, the collide more frequently

  the faster they move, the more energy they have so more of the collisions will have enough energy to make the reaction happen
• concentration and pressure


  if a solution is more concentrated, there are more particles

  when the pressure of the gas is increased, the space the particles occupy is decreased

  this makes collisions between the reactant particles more frequent
• surface area


  if one of the reactants is a solid, breaking it up into smaller pieces will increase its surface area to volume ratio

  this means for the same volume of solid, the particles around it ill have more area to collide, making collisions more frequent
• catalyst


  a catalyst is a substance that speeds up a reaction, without being used up itself

  catalysts decrease the activation energy needed for the reaction to occur- they provide an alternative energy pathway with a lower activation energy
• rate of reaction 


  amount of reactant and amount of product formed / time
• visual changes 


  can be measured through time
• change in mass


  can be measured using a mass balance- as a gas is released, the mass goes down

  the quicker the reading goes down, the faster the reaction
• the volume of gas 


  a gas syringe can be used to measure the volume of gas given off

  the more gas given off in a given time, the faster the reaction
• reversible reactions will reach equilibrium


  as the reactants react, their concentrations fall so the forward reaction will slow down. as more products are made, their concentrations rise, speeding up the backward reaction

  after a while, the forward reaction will be going at the same rate as the backward reaction- the system is at equilibrium
• equilibrium is only reached if the reversible reaction takes place in a closed system


  a closed system means that none of the reactants and products can escape and nothing else can get in
• if the equilibrium lies to the right, the concentration of the products is greater then that of the reactants 


  if the equilibrium lies to the left, the concentration of reactants is greater than the products
• reversible reactions can be exothermic and endothermic


  if the reaction is endothermic in one direction, it will be exothermic in the other direction

  the energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction
• reversible reactions try to counteract changes 


  le chatelier's principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract the change
• changes in temperature 


  if the temperature is decreased, the equilibrium will move in the exothermic direction to produce more heat- there will be more products for the exothermic reaction and fewer products for the endothermic reaction

  if the temperature is increased the equilibrium will move to the endothermic direction to try and decrease it
• changes to pressure


  changing the pressure only affects the equilibrium involving gases

  if the pressure increases, the equilibrium tries to reduce it and shifts to the direction where there are fewer molecules of gas

  if the pressure decreases, the equilibrium tries to increase it and shifts to the direction where there are more molecules of gas
• changes to concentration 


  if you change the concentration of either the reactants or the products, the reaction will no longer be at equilibrium

  if you increase the concentration of the reactants the system tries to decrease the it by making more products