Chapter 10: Reaction Rates, Catalysts, and The Boltzmann Distribution

rate of reaction definition:

the change in concentration of a reactant / product in a given time

what shape does a typical concentration/time graph have?

curve:

• steepest at start

• becomes less steep

• flattens off

4 factors that can change the rate of a reaction:

1. concentration (or pressure when reactants are gases)

2. temperature

3. use of a catalyst

4. surface area of solid contacts

what does collision theory state?

2 particles must collide to react

why are some collisions effective, and some ineffective?

1. particles must collide with correct orientation.

2. particles must have sufficient energy to overcome the activation energy barrier of the reaction

how does increasing concentration affect the rate of reaction?

• rate of reaction generally increases

• increases the number of particles in the same volume

• particles are closer together + collide more frequently

• therefore → more frequency + successful collisions (correct orientation + sufficient energy) → increased rate of reaction

how does increasing the pressure of a gas affect the rate of reaction?

• gas = compressed in a smaller volume → pressure of gas is increased → rate of reaction increases

• concentration of gas molecules increases as the same number of gas molecules occupy a smaller volume

• gas molecules = closer together + collide more frequently → leads to more effective collision in the same time

2 methods for following the progress of a reaction:

• monitor removal (decrease in conc.) of reactant

• monitor formation (increase in conc.) of product

what 2 methods can be used for monitoring reactions that produce gas?

1. monitor volume of gas produced at regular time intervals → gas collection

2. monitor loss of mass of reactants using a balance → mass removal

draw the equipment for gas collection:

on a concentration-time graph, what represents the rate?

the gradient

on a (gas collection or mass lost) / time graph, what represents the rate?

the gradient

3 rules for catalysts:

1. catalyst not used up in the reaction

2. may form an intermediate / provide a surface on which the reaction can take place

3. catalyst is regenerated at the end of reaction

Exo + Endothermic reaction enthalpy profile diagrams (with catalyst!)

3 things to remember when drawing enthalpy profile diagrams?

• reactants + products written out

• ∆H shown with an arrow pointing in correct direction

• activation energy shown with upwards arrow

homogeneous catalysts:

• same physical state as the reactants

• forms an intermediate → breaks down to give catalyst + product

2 reactions that use homogeneous catalysis:

1. making esters (with sulphuric acid as a catalyst)

2. ozone depletion (Cl• catalyst)

heterogeneous catalyst:

• different physical state from the reactants

• usually solid → in contact with gaseous reactants/reactants in solution

• reactant molecules are adsorbed (weakly bonded) → onto surface of catalyst → where the reaction takes place

• product leaves surface of catalyst by desorption → at the end of the reaction

lower activation energy often means a lower…

temperature → this makes using catalysts more sustainable (less electricity etc.)

what does a Boltzmann distribution graph look like?

the curve should NEVER cross [which] axis on the Boltzmann distribution graph??

the x axis

why does the curve of a Boltzmann distribution graph start at the origin?

no molecules have 0 energy

what is the area underneath the curve of a Boltzmann distribution graph equal to?

the total number of molecules

what happens to a Boltzmann distribution in changing temperature?

• temperature increases ⬆average energy of molecules increases ⬆

• more molecules have higher energy

• graph is now stretched over a greater range of energy values

• peak of the graph is now lower ⬇ on the y-axis and further along the x-axis → the peak of average energy is now higher (further towards the right!)

• number of molecules = the same → area under the curve is the same

how does a higher temperature increase reaction rate?

• more molecules surpass the activation energy

• greater proportion of collisions → will lead to reaction

• reaction rate is increased

• collisions = more frequent as molecules are moving faster → increased energy of molecules is much more important than increased frequency of collisions!

effect of a catalyst on a Boltzmann distribution:

• alternative route → lower activation energy

• compared to E_a, a greater proportion of molecules now have an energy equal to, or greater than the lower activation energy, E_c. On collision, more molecules will reacts to form products → increase in rate of reaction