Kinetics and Equilibria

• what is reaction rate?

  the change in concentration (or amount) of a reactant/product over time
• rate of reaction = amount of reactant used or product formed / time
• collision theory:

  • particles in liquids/gases are always moving/colliding
  • particles don/t react every time they collide - only if conditions is right
  • reaction between two particles won't take place unless they collide in right direction and collide with at least a certain minimum amount of kinetic (movement) energy
• what is activation energy?

  the minimum amount of kinetic energy that particles need to react
• to start a reaction particles must have enough energy to break their bonds
• if temperature increases, particles will have more kinetic energy and will move faster
• greater portion of molecules will have at least the activation energy and be able to react. maxwell-boltzmann distribution curve shifts to the right
• when temperature increases, reaction is faster because particles collide more often
• if concentration increases, particles will be close together and collide more often. if collisions occur more frequently, there's more chances to react. so increasing concentration increases reaction rate
• if pressure increases, gas particles will be close together and collide more often, so more frequent collisions so reaction rate increases
• what is a catalyst?


  a substance that increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. the catalyst is chemically unchanged at the end of the reaction
• catalysts increase the rate of reaction and save money in industrial processes. they lower the activation energy so there's more particles with enough energy to react when they collide and is done by the reaction going via a different route
• one ways to measure reaction rate:

  timing how long a precipitate takes to form

  • use method when product is precipitate which clouds a solution
  • watch the mark through solution and time how long it takes to be obscured
  • if same observer uses same mark each time, can compare rates of reaction since same amount of precipitate will have been formed when mark becomes obscured - method objective
• second way to measure reaction rate:

  measuring a decrease in mass

  • can measure rate of formation using a mass balance when products is a gas
  • as gas given off, mass of reaction mixture decreases
  • method accurate
• third way to measure reaction rate:

  measuring the volume of gas given off

  • involves using gas syringe to measure volume of gas produced
  • method only used when one or more products is a gas
  • gas syringes give volumes to the nearest 0.1cm3
  • method is accurate
• reversible: able to be turned the other way round, goes both ways
• as reactants get used up, forward reaction slows down - as more product is formed, reverse reaction speeds up
• forward reaction goes at the same rate as the backward reaction so amounts of product/reactants don't change - dynamic equilibrium. at equilibrium, concentrations of reactants/products stats constant
• dynamic equilibrium happens in a closed system - nothing can get in or out
• if concentration, pressure/temperature changes, position of equilibrium alters and there would be different amounts of reactant/products at equilibrium
• if position of equilibrium moves to the right, the reaction is exothermic and gets more products
• if position of equilibrium moves to the left, the reaction is endothermic and get more reactants
• le Chatelier's principle?

  if a system is in equilibrium, a change in the conditions will shift the equilibrium
• le Chatelier's principle: if a reaction at equilibrium is subjected to change in concentration, pressure or temperature, position of equilibrium will move to counteract the change
• if temperature is raised, position of equilibrium shifts to cool things down
• if pressure/temperature raised, position of equilibrium shifts to try to reduce it again
• homogeneous equilibrium?


  means reactions where every species is in the same physical state
• concentration:

  • if increased in reactants: equilibrium aims to get rid of extra reactants and does this by making more products so equilibrium shifts to the right
  • if increased in products: equilibrium aims to get rid of extra products, this makes the reverse reaction go faster so equilibrium shifts to the left
  • decreasing concentrations has opposite effect
• pressure:

  • increasing shifts equilibrium to the side with fewer gas molecules which reduces pressure
  • decreasing shifts equilibrium to the side with more gas molecules which raises pressure
• temperature:

  • increasing means adding heat. equilibrium shifts in the endothermic (positive ΔH) direction to absorb heat.
  • decreasing mans removing heat. equilibrium shift in the exothermic (negative Δ) direction to produce more heat, in order to counteract drop in temperature
  • if forward reaction is endothermic, reverse reaction will be endothermic, vice versa
• catalysts have no effect on position of equilibrium. they can't increase yield - but equilibrium is reached faster
• aA + bB ⇌ dD + eE, Kc = [D]d [E]e / [A]a [B]b
• if temperature increases, equilibrium shifts in endothermic direction - Kc will change
• if temperature changes, more products at equilibrium Kc will rise but less products at equilibrium, Kc will decrease
• changing concentration of reactant/product, will not affect the value of Kc
• catalysts don't affect Kc - they'll speed up reaction in both direction by same amount, so help system reach equilibrium faster
• chemical reactions occur when particles of substances collide
• collisions must have energy > activation energy and the particle orientation must be correct for the reaction to be successful
• reaction conditions:

  • providing particles with more energy impact the collisions of the particles
  • to increase rate, conditions are changed to increase collisions occurring with sufficient energy
• maxwell-distribution: not all molecules in a substance have the same amount of energy as it's distributed in a pattern