Kinetics

• Collision Theory


  A reaction occurs when particles collide in the right direction, right orientation and collide with a certain minimum amount of kinetic energy.

  
• Activation Energy

  The minimum amount of kinetic energy particles need for a reaction to occur.

  
  

  (minimum energy required to break bonds in reactant particles to start a reaction)
• Why don't reaction occur all the time when collisions occur

  Although collision occurs, conditions aren't met. Such as orientation, direction and kinetic energy to break activation energy threshold
• How frequent are reactions with high/low activation energies and if not how to overcome it.

  Reactions with low activation energies are frequent but reactions with high activation energy aren't frequent. To overcome this, give particles more energy by heating them
• Drawing enthalpy profiles

  
• Maxwell-Boltzmann Distribution

  Particles in a gas have different kinetic energies, therefore move with different speeds (slow/fast).

  
  

  Maxwell-Boltzmann distribution is a theoretical model that represents the kinetic energy in gas particles.

  
  

  A diagram showing that no particles at any one time have 0 energy.

  
  

  Shows that there are only a few particles that meet the activation energy and those that surpass it will be able to complete a reaction

  
• What do different areas on the curve of Maxwell-Boltzmann distribution show

  Area under curve is equal to total number of molecules

  
  

  Curve begins at (0,0) because no particles have 0 energy

  
  

  Area under curve after towards the left side shows few particles moving slowly (lower ke)

  
  

  Peak of curve shows then most likely energy of a particle

  
  

  the mean energy of particles is a bit right to the peak

  
  

  most molecules move at moderate speed=energy in middle

  
  

  Some molecules have ke higher than Ea, these are the only particles that can react

  
• The effect of temperature on reaction rate and Maxwell-Boltzmann distribution

  Increasing temp of gas causes particles to have more KE so a greater proportion of particles will have at least the activation energy and be able to react, also increases number of collisions increasing reaction rate.

  
• The effect of increasing concentration on reaction rate

  Increasing concentration of reactants causes particles to be closer together, therefore collisions are more frequent causing higher rate of reaction

  
• The effect of increasing pressure on reaction rate

  Increasing pressure causes particles to move closer together, increasing the number of particles in a given volume, increasing the frequency of collisions and therefore increasing the rate of reaction.

  
  

  (same graph as conc).

  
• Catalyst


  A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy.
• Why are catalysts beneficial

  Don't get used up during reactions, (they take part in reactions but are reformed at the end) therefore can be reused.

  
  

  Saves money as it means reactions can occur with lower temperature and pressure

  
  

  More product is formed
• How Catalysts Work

  Although molecules will have the same amount of energy, catalysts lower the activation energy so more particles will have enough energy (above the threshold) to react when they collide by providing an alternative route (with lower activation energy). Therefore more particles react and increase rate of reaction.

  
• Calculating reaction rate

  Rate of reaction is change in the amount of reactant/product over time (g/s-1 or cm3/s-1)

  
• 3 ways rates can be measured in experiments

  Time taken for precipitate to form(Disappearing cross practical)

  
  

  Change in mass

  
  

  Volume of gas produced
• Disappearing cross practical


  Stand conical flask on top of white tile with black cross.

  
  

  Add fixed volumes of reactant solutions to the flask and start stopwatch.

  
  

  Look through the solution to observe the cross. As precipitate forms, cross will become less visible.

  
  

  Stop timer when cross is no longer visible, the timer records the time taken for precipitate to form.
• Describe how to investigate effect of temp on rates of reaction using disappearing cross experiment

  (same process as before but)

  
  

  Use water bath to gently heat both reactant solutions to desired temp and add them to the conical flask together which is on top of a tile with a black cross.

  
  

  Volume and conc of solution must be the same
• Problem with disappearing cross method

  Difficult to know when exactly the cross disappears, try use the same observer to reduce errors
• Change in mass experiment

  (When product is gas, rate of formation can be measured using mass balance).

  
  

  Place reaction on balance, when reaction starts, start a timer and record mass at regular intervals, the reaction is finished when the reading on mass balance stops decreasing.

  
  

  Accurate results.

  
  

  Can repeat reaction with acids at diff temps to investigate the effect of diff temps on reaction rate, all variables must be kept the same.-mass should decrease faster.
• Problem with change in mass experiment

  Gas produced could be toxic or flammable so carry out in a fume cupboard.
• Gas volume experiment

  If gas is given off during a reaction, reaction rate can be measured by collecting gas in a gas syringe at regular time intervals. (reaction finishes when gas volume stops increasing).

  
  

  Method is accurate because gas syringes give volumes to the nearest 0.1cm3.

  
  

  Because no gases escape due to airtight seal, method can be used for reactions that produce toxic or flammable gases.

  
  

  vigorous reactions blows plunger out of syringe, therefore do rough calculation of expected gas to be produced before beginning.

  
  

  Reaction can be repeated with acids at diff temps to investigate its effect on rates of reaction-should show gas is produced quicker=faster rate of reaction.

  
• Factors Affecting Rate of Reaction

  - Increasing temperature

  - Increasing concentration/pressure (depending on the state of the molecules)

  - Increasing surface area

  -Adding a catalyst
• Activation Energy Lowers

  The affect that occurs to a Maxwell-Boltzmann curve when a catalyst is added
• Number of Particles Exceeding Activation Energy Increases

  The affect that occurs to a Maxwell-Boltzmann curve when the temperature of the particles is increased
• Increasing Surface Area


  This disperses the mass of a substance so that there are more particles in a given area without changing the mass of the substance. This increases the likelihood of successful collisions
• Successful Reaction Conditions

  The particles must have sufficient energy and must collide at the correct orientation
• D. Choose vertical lines always

  
• At given temp kinetic energy is...

  constant
• What reduces accuracy of disappearing cross experiment

  Rinsing flask with acid
• Suggest why a student records the times to the nearest second and not to the nearest 0.01 s (disappearing cross experiment)

  Hard to judge to the nearest 0.01s/exact moment cross disappears or due to reaction time
• If product of disappearing cross experiment is so2, Suggest why small amounts of reactants are used in this experiment.

  limit toxic gas (so2) formed
• Suggest why the student chose not to carry out experiments at temperatures in the range 1-10 °C

  reaction would be too slow
• If two reactants are in excess at fixed temp, why would rate of reaction be determined by concentration of another different reactant

  Excess means that the two reactants are constants therefore have zero effect on rate unlike the other reactant..
• Samples of the reaction mixture are removed at timed intervals and titrated with alkali to determine the concentration of H+(aq). State and explain what must be done to each sample before it is titrated with alkali.

  stop reaction by dilution/cooling
• Constant gradient/decrease in concentration is proportional to time

  
• What happens to maxwell-Boltzmann distribution curve when temp decreases