Chapter 4- Reaction Rates

what is activation energy?

• the energy needed for a reaction to happen

  • activation energy is used to break bonds in the reactants before new bonds can form in the products

what is the maxwell-boltzmann distribution of energies?

• a graph showing the energy distribution of all the molecules in a gas

  • the number of molecules is on the y-axis

  • the kinetic energy of the molecules is on the x-axis

maxwell- boltzmann distribution: why does the curve pass through the origin?

• because the molecules have zero energy

maxwell- boltzmann distribution: why is there a peak in the middle?

• it represents the most likely energy of any molecule

• there are more molecules with this energy than with any other energy

maxwell-boltzmann distribution: what is the mean energy?

• it is to the right of the peak, and it divides the area below the graph into 2 equal halves

maxwell- boltzmann distribution: what is the area under the graph showing?

• the total number of molecules

how is the curve asymptotic?

• it doesn’t reach the x-axis

• this is because there can be no maximum value for the kinetic energy of a molecule

maxwell- boltzmann distribution: where is the activation energy marked?

this allows us to view the number of molecules with the energy to react

• on the left hand side of the activation energy mark, molecules have less energy than the activation energy and so they can’t react

• on the right hand side, molecules have more energy than the activation energy and so they can react

maxwell- boltzmann distribution: effect of heating on the maxwell-boltzmann distribution of energies?

• it raises the average energy but lowers the peak of the graph

• it shifts to the right because a greater proportion of molecules have greater kinetic energy

• therefore, a greater proportion of molecules will have energy greater than or equal to the activation energy

maxwell-boltzmann distribution: effect of concentration, pressure and surface area?

• increasing the conc, pressure or surface area of reactants make successful collisions occur more frequently

• however, they don’t change the energy of the individual particles

• therefore, shape of the maxwell-boltzmann distribution doesn’t change

what are the 3 main methods for changing reaction rates?

• changing temperature

• changing concentration

• changing pressure

how does increasing concentration increase rate of reaction?

• increasing concentration increases number of collisions that happen between the reactants

• there will be more successful collsions

• therefore, the rate will increase

how does changing pressure increase rate of reaction?

• increasing pressure reduces volume of the gas

• this means molecules are closer together

  • if the molecules are closer together, they will collide more often

• therefore, more successful collisions and the rate will increase

how does adding a catalyst increase the rate of reaction?

• it offers an alternative pathway with lower activation energy

• therefore, a greater proportion of molecules will have the energy greater than the activation energy

• so, there will be more successful collisions

catalysts on a maxwell-boltzmann graph?

• catalytic pathway is shifted to the left and can be used by more molecules

• however, curve is unchanged in shape

  • more molecules have enough energy to react via the catalysed pathway

why is it necessary to heat the calcium carbonate strongly to achieve decomposition?

• to overcome activation energy

• reaction is endothermic

• to break bonds