3.2.5 and 3.3.5 - Maxwell-Boltzmann Distribution and Rate of reaction

Maxwell - Boltzmann Distribution Graph

Rate of reaction

Change in amount of concentration of a reactant or product over time (speed at which reactants get used up and reactants start to form)

Factors affecting rate of reaction

• temperature

• Concentration

• Pressure

• Catalysts

Temperatures affect on rate of reaction

• Heat substance → thermal energy transferred to it → thermal energy converted to kinetic energy → ∴ molecules move faster → ↑ movement ↑ frequency of collisions with energy ≤ to Ea in a given time → ∴ ROR ↑

• Cool substance → thermal energy ↓ → ∴ molecules have less kinetic energy → so molecules move slower → ↓ movement → ↓ frequency of collisions with energy ≤ to Ea in a given time → ∴ ROR ↓

Concentration affect on rate of reaction

• when conc of sample ↑ the no. of molecules within a fixed volume ↑ → particles more packed closer together → because of smaller distance between particles they collide more frequently → ↑ ROR as there’s an ↑ in no. of successful collisions

Pressure affect on rate of reaction

↑ pressure ↑ no. of particles within fixed volume → particles packed closer together ∴ distance between molecules is small → ↑ frequency of collisions → ↑ ROR as there’s an ↑ in no. of successful collisions

Catalyst affect on rate of reaction

↑ ROR without being used up → works by providing alternative reaction pathway which requires a lower Ea for reaction to occur

Heterogeneous Catalyst

• usually solid catalysts and gaseous reactants

• Heterogeneous (diff state / phase from reactant)

1. Reactants adsorb (sticks onto surface) onto surface onto active site

   • weakens bonds

   • Brings molecules closer → more frequent collision

   • More favourable orientation will hit the correct angle = more successful collisions

2. Reactions take place

3. Products are described (leave the surface)

Conditions for heterogenous catalysts

• large surface area

• Spread thinly over ceramic honeycomb

• In catalytic converter ceramic honeycomb structure counted with with rhodium and platinum

Poisoned catalyst

• some substances block the active site (they adsorb and don’t come off)

Required Practical 3 - Measuring Rate Of Reaction

• • Measure 10cm3 of sodium thiosulfate using a pipette or measuring cylinder and transfer it to the tube labelled sodium thiosulfate

• • Measure 2cm3 of 1 mol Hydrochloric Acid using a pipette or measuring cylinder and transfer it to the tube labelled hydrochloric acid

• • When using water baths begin from the coldest temperature to the hottest allowing the water baths to heat up

• • using a 400 ml beaker fill it (using the tap) with cold water and place the tubes in leave them in

• • after min 5 mins use the thermometer to see if both tubes are at the same temperature if not place them back into their water baths

• • Once temperatures are the same record the starting temperature on your graph drawn

• • Then proceed to add the HCL into the sodium thiosulfate tube and begin the timer  swirl it with cap on then remove the cap to observe when the cross disappears

• • Once the black cross disappears stop the stopwatch and record the time it took for the cross to disappear

• • Using the thermometer measure the ending temp and record it on your table with results

• • Repeat this with the 3 water baths and room temperature tubes

• • Once you have recorded the starting and ending temp you calculate the mean temperature (this is going to be the x axis on the graph you have drawn)

• • You then use the time taken for the cross to disappear you calculate the rate of reaction by using 1000/Time (this is going to be your y axis)