3.1.5 Kinetics Flashcards Preview

A-level Chemistry > 3.1.5 Kinetics > Flashcards

Flashcards in 3.1.5 Kinetics Deck (26):

What is a reaction rate?

Change in concentration (or amount) of reactant or product over time


State the formula for a rate of reaction


Name the 2 conditions needed for particles to react with one another when they collide

  • Collide in right direction 
    • Need to be facing each other in right way
  • Collide with certain minimum amount of kinetic energy


What is meant by activation energy?

Minimum (amount of kinetic) energy needed for a reaction to occur


Why do particles need a minimum amount of kinetic energy for them to react?

Need to have this much energy to break their bonds & start reaction


Molecules in gas don't all have the...

same amount of energy


What does a Maxwell-Boltzmann distribution show?

Number of molecules in a gas with different kinetic energies


Draw a Maxwell–Boltzmann distribution

  • (y axis) %/fraction of particles/molecules/atoms
  • (x axis) energy


Maxwell–Boltzmann Distribution

What does the peak of the curve represent?


Maxwell–Boltzmann Distribution

Where is the mean (average) energy of all molecules on the curve? Label it on your curve.


Maxwell–Boltzmann Distribution

At what speed are most molecules moving at?


Maxwell–Boltzmann Distribution

Shade the area where there's molecules that can react


What is the area under a Maxwell–Boltzmann distribution curve equal to?

Total number of molecules


Why does the Maxwell–Boltzmann distribution curve start at (0,0)?

Because no molecules have 0 energy


Describe the effect of temperature on reaction rate

  • ↑ temp of reaction = particles have more kinetic energy ∴ move faster
  • Greater proportion of molecules E ≥ Ea  & able to react (have at least activation energy)
  • More frequent successful collisions in given time
    • Another reason why ↑ temp = reaction faster


Describe and illustrate the effect of an increase in temperature on Maxwell–Boltzmann distribution curve

Changes shape of Maxwell–Boltzmann distribution curve → shifts to the right 


Why does a small increase in temperature create a large increase in reaction rate? (2)

  • Many more molcules have E ≥ Ea (energy greater than the activation energy)
  • Far more successful collisions per second


Describe the effect of concentration on reaction rates

  • ↑ concentration of reactants in solution = particles will be closer together
  • Closer = collide more often
  • Collisions occurring more frequently = more chances to react
  • Explains why ↑ concentration = ↑ reaction rate


Describe the effect of pressure on reaction rates

  • Gas particles closer together
  • ↑ collsion frequencies = reaction rate ↑


What is a catalyst?

  • Substance that increases the rate of reaction by providing an alternative reaction pathway with lower activation energy
  • Catalyst is chemically unchanged by end of the reaction


Catalysts take part in reactions but...

 are remade at end


Draw a Maxwell–Boltzmann distribution showing the effect of a catalyst work


Describe how you can measure reaction rates

Measure how fast reactants are used up or how fast products are formed


Name 3 ways to measure reaction rates

  1. Timing how long a precipitate takes to form
  2. Measuring a decrease in mass
  3. Measuring the volume of gas given off


Measuring reaction rates

Describe "measuring a decrease in mass"


  1. When one or more products is gas = measure rate of formation using mass balance
  2. As gas given off, mass of reaction mixture decreases
  3. Method is accurate & easy to do
    • Usually done in fume cupboard 


Measuring reaction rates

Describe "measuring the volume of gas given off"

  1. Uses gas syringe to measure volume of gas being produced
  2. Use it when 1 or more products is gas
  3. Method is accurate