Chemical Reactions

• Chemical Kinetics is the study of how fast reactions happen
• Rate of reaction

  The change in concentration per unit of time
• units for rate of reaction: mol dm^-3 s^-1
• Graphing rate of reaction

  Looking at the average rate of reaction, knowing that reaction rates slow down as a reaction proceeds, using a secant line for the average rate, using a tangent line for the instantaneous rate
• Initial rates of reaction

  Taking the tangent of the curve at time=0s to get the initial rate of reaction, where the initial rate is the fastest rate and decreases from there
• Reaction rates slow down as a reaction proceeds
• There are six different ways to measure the rate of a reaction:
• Ways to measure the rate of a reaction

  • Change in the volume of gas produced
  • Change in mass (allowing gas to escape)
  • Change in transmission of light (change in color)
  • Change in concentration or pH measured by titration
  • Change in pH or conductivity
  • Reaching a benchmark in a reaction ("clock" reactions)
• Volume of gas measurement

  Using a gas syringe or an inverted burette to measure the volume of gas produced over time
• Change in mass measurement

  If one of the products is a gas and is allowed to escape, measuring the change in the mass of the reaction vessel over time
• Change in color measurement

  Using a colorimeter or spectrophotometer to measure the change in color or intensity of light transmitted through the solution
• Titration measurement

  Quenching the reaction before titrating to determine the concentration of a reactant or product remaining
• Conductivity measurement

  Measuring the change in pH using a pH probe or measuring the electrical conductivity of the solution using a conductivity meter if the reactants and/or products are electrolytes
• Reaching a benchmark measurement (Clock reactions)

  Measuring the end point of a reaction and calculating the average rate of reaction
• Temperature

  A measure of the average kinetic energy of particles
• Kinetic energy

  Directly related to its absolute temperature (measured in Kelvin)
• Maxwell-Boltzmann distribution curve

  Shows the range of values of kinetic energy at a given temperature
• The area under the curve represents the total number of particles
• Collision theory

  Three things must occur for a reaction to happen: particles must collide, have enough energy, and have the right orientation
• Most collisions are unsuccessful
• Geometry

  Particles will only react if they collide in a certain orientation
• Activation energy (Ea)

  Minimum value of kinetic energy a particle must have for a collision to lead to a reaction
• Reactions with high activation energy tend to proceed slower than reactions with low activation energy
• Factors that can affect the rate of a chemical reaction

  • Temperature
  • Concentration
  • Particle Size
  • Catalysts
• Increasing temperature

  • Increases the average kinetic energy of the particles, leading to more particles having enough energy to overcome activation energy
  • Results in more collisions happening
• Maxwell-Boltzmann Distribution Curve

  • Shows the range of values of kinetic energy at a given temperature
  • Area under the curve represents the total number of particles
• Graphing Maxwell-Boltzmann Curve


  • Area under both curves is the same, but the curve shifts at higher temperatures
  • More particles have higher kinetic energy at higher temperatures, leading to a higher number of effective collisions
• Many reactions double their reaction rate for every 10˚C increase in temperature
• Increasing concentration

  • Increases collisions and the number of successful collisions
  • As the reaction progresses, the concentration of reactants decreases
• Increasing pressure (gases only)

  • Decreases volume of a gas, increasing concentration
• Decreasing particle size

  • Increases surface area, exposing more particles for collisions
  • Important for reactions in different phases like solid in gas or solid in liquid
• Catalysts

  • Provide an alternate pathway for a reaction, requiring less energy
  • Increase the rate of a chemical reaction without undergoing permanent chemical change
• Enzymes are biological catalysts that allow reactions to happen in living organisms
• Catalysts

  • KI
  • FeCl3
  • MnO2
• Activation energy (Ea) is the minimum energy required for successful collisions leading to a reaction
• By decreasing activation energy, a catalyst increases the rate of a chemical reaction without being permanently chemically changed
• Applications and Skills

  • Species react due to collisions of sufficient energy and proper orientation
  • Rate of reaction is expressed as the change in concentration of a reactant/product per unit time
  • Activation energy is the minimum energy needed for successful collisions
  • Catalysts increase reaction rate without being permanently changed
  • Description of kinetic theory, analysis of graphical and numerical data, explanation of factors affecting rate of reaction, construction of Maxwell–Boltzmann energy distribution curves, investigation of rates of reaction experimentally, sketching and explanation of energy profiles with and without catalysts
• How concentration affects rate

  1. Increasing concentration increases the collision frequency

  2. Empirical evidence shows that the effect of concentration on rate is not always straightforward

  3. Some reactants may increase rates, while others may not

  4. The degree to which concentration affects rate is known as the rate order

  5. Rate order can only be determined empirically
• Units for rate should always be mol dm–3 s–1
• Units for k will change depending on rate order and what is needed to make the rate units mol dm–3 s–1