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

the Haber process is an example of a [what] reaction?

reversible

dynamic equilibrium definition:

• the rate of the forward reaction is equal to the rate of the reverse reaction

• the concentrations of reactants and products do not change

• in a closed system

le Chatelier’s principle:

when a system in equilibrium is subjected to an external change, the system adjusts itself to minimise the effect of that change

Which experiment can be used to investigate the changes to position of equilibrium with concentration? 

the equilibrium between yellow aqueous chromate ions and orange aqueous dichromate ions 

1. add yellow potassium chromate ions to a beaker → K₂CrO₄ to a beaker 

2. add dilute sulphuric acid, H₂SO₄ dropwise until there is no further change → solution turns an orange colour 

3. add aqueous sodium hydroxide, NaOH, until there is no further colour change. The solution turns back to a yellow colour 

adding H₂SO₄ increases the concentration of H+ ions → increases the rate of the forwards reaction, causing the position of equilibrium to shift to minimise the change in H+ concentration 

position of equilibrium shifts to the right → making more products 

adding NaOH increases the number of OH- ions. position of equilibrium shifts to the left, making more the H+ reactant → a new position of equilibrium is established. 

how can we investigate changes to the position of equilibrium with temperature?

1. dissolve cobalt chloride in water in a boiling tube

2. add a small quantity of HCl

3. place the boiling tube in some iced water → solution colour is pink

4. set up a boiling water bath and transfer the boiling tube into the boiling water → solution turns a blue colour

5. transfer the boiling tube back to the iced water → solution turns pink again

what can be added to help in the colour change of dissolving Cobalt Chloride in a dynamic equilibrium? 

Hydrochloric acid → eq. shifts to the right! 

does a change in pressure affect equilibrium?

changing the pressure of a system containing gases in equilibrium may result in the position of equilibrium changing → only if there are more gaseous molecules on one side of the equation that the other!

the pressure of a gas is proportional to its…

concentration → two moles of 2NO₂ (g) will have 2x the concentration and 2x the pressure of 1 mole of N₂O_{4 (g)}

effect of changing pressure on 2NO₂(g) ⇌ N₂O₄ (g)

does adding a catalyst have an effect on equilibrium? 

no → merely speeds up the rates of the forward and reverse reactions equally. A catalyst will, however, increase the rate at which an equilibrium is established 

K_c formula: 

using the EQUILIBRIUM concentrations!

what does the K_c value tell us?

→ K_c = 1 → equilibrium is halfway between reactants + products

→ K_c > 1 → equilibrium sits towards the products

→ K_c < 1 → equilibrium sits towards the reactants