rates and equilibria

• rate of reaction is how fast a reaction takes place it is measured in cm3/s or g/s
• instantaneous rate of reaction is the rate of reaction at a particular point in time
• a catalyst is a substance which increases the rate of a chemical reaction but remains chemically unchanged at the end of the reaction
• collision theory is the number of collisions taking place per second between reactant particles
• a successful collision is an energetic collision between two particles which leads to a chemical reaction, there must sufficient energy in the collision to break the chemical bonds
• activation energy is the minimum amount of energy needed to start a reaction by breaking the chemical bonds in the reactant molecules
• boltzmann distribution is a histogram showing the distribution kinetic energies in a gas
• homogeneous catalyst is a catalyst which is in the same phase as the reactants
• heterogeneous catalyst is a catalyst which is in a different phase from the reactants
• the area under a Maxwell-Boltzmann distribution graph is the total number of particles
• the axes on a distribution graph are the number of particles the y-axis and kinetic energy on the x-axis
• the average energy is higher than the most probable energy as the curve is asymmetric
• the graph does not touch the x-axis as there is no maximum energy a particle can have
• when the temperature increases the distribution graph changes shape it peaks lower and further to the right
• at a higher temperature a greater proportion of particles have energies in excess of the activation energy
• the total area under the curves of a distribution graph remain constant as the number of particles remains the same
• on distribution graphs a catalyst increases the number of particles that have more energy than the activation energy as it provides an alternative activation energy pathway
• catalysts do not destroy the original pathway when they supply and alternative pathway the old pathways still remains
• how heterogenous catalysts work:

  • less energy needed to break bonds as weak bonds are formed between surface and molecules at the expense of existing bonds
  • reaction takes place at surface of catalyst
  • when two molecules collide on the surface of the catalyst the chemical bonds in the molecules break more easily
  • activation energy ins lower to a more successful collisions take place which increases rate of reaction
• if concentration is increased the number of reactant particles increases for the same volume, therefore collisions between reactant particles occur more frequently
• if temperature is increased the reactant particles have more kinetic energy, so they collide more frequently and more particles exceed the activation energy so more collisions between particles will be successful
• how to calculate initial rate of reaction:

  • sketch graph of time on x-axis and volume of gas on y-axis
  • initial concentration is used as that is the only point where concentration is known
  • repeat with varying concentrations and calculate each initial rate
• when pressure is increased the equilibrium will shift to the side of the equation where there are fewer gas phase molecules to reverse the change
• if the temperature is decreased the system will shift to favour the exothermic reaction to increase the temperature back up
• the equilibrium law:

  $Kc =$$[C]^c[D]^d/ [A]^a [B]^b$
• the powers in the equilibrium law come from the balancing numbers in front of each substance in the chemical reaction
• the square brackets in the equilibrium law represent concentration in moldm^-3
• C and D in the equilibrium constant represent the products of the reaction and A and B represent the reactants
• a big Kc value indicates the position of equilibrium lies to the right hand side
• a small Kc value indicates the position of equilibrium lies to the left hand side
• ways catalysts make chemical processes more sustainable:

  • reduce need for high temperatures, reduces energy consumption and reduces greenhouse gas emissions as less fossil fuels burnt
  • reduce amount of toxic waste products so they don’t need to be disposed of
  • improves the atom economy of the reaction
• equilibrium is achieved when the shape of the graph plateaus
• Le Chatelier’s Principle is that when a chemical system is at equilibrium and a change is made to some of its conditions the system responds to oppose these changes
• catalysts have no effect on the position of equilibrium
• when the position of equilibrium is shifting to the left of right the system is not at equilibrium
• Kc is only constant at a particular temperature so the convention is to quote temperature alongside Kc value
• the concentrations in the Kc expression are the concentrations at equilibrium
• catalysts have no effect on the Kc value
• the only factor that changes the value of Kc is temperature