Reaction Rates and Collision Theory

This is part of Year 11 HSC Chemistry course under the topic of Rates of Reactions

HSC Chemistry Syllabus

  • Conduct a practical investigation, using appropriate tools (including digital technologies), to collect data, analyse and report on how the rate of a chemical reaction can be affected by a range of factors, including but not limited to:
    • temperature 
    • surface area of reactant(s)
    • concentration of reactant(s)
    • catalysts (ACSCH042)
  •  Investigate the role of activation energy, collisions and molecular orientation in collision theory
  • Explain a change in reaction rate using collision theory (ACSCH003, ACSCH046)

      Reaction Rates and Collision Theory

      This video discusses how reaction rates can be affected by a variety of different factors such as surface area, concentration, and temperature. It also explores how reaction rates can be explained in terms of collision theory,  

      What is Reaction Rate?

      The reaction rate is a measure of how fast a chemical reaction occurs. It can be understood in two ways: 

      • The speed at which products are formed
      • The rate at which reactants are depleted

      Consider two reactions taking place simultaneously: one where the product appears rapidly (a quick reaction) like combustion, and one where the product forms slowly (a slow reaction) like the oxidation (rusting) of iron. 

      By graphing these reactions, with the product amount on the y-axis and time on the x-axis, we get a clear visual of their speeds with a steeper gradient indicating a quicker reaction. 

      Measuring Reaction Rates

      There are several methods which can be used to measure the reaction rate:

      • Volume of gas produced: A syringe setup can measure the gas volume which is produced from combustion and acid-metal carbonate reactions. 
      • Color changes: Spectrophotometry, or colourimetry, measures how much light a solution is able to absorb over time to indicate concentration changes. 
      • Temperature changes: As reactions release or absorb energy, the surrounding temperature changes. The rate of temperature change can indicate the progress of a reaction.  

      Factors Affecting Reaction Rate

      Several variables play a crucial role in determining how quickly reactions proceed: 

      • Concentration of reactants
      • Pressure and volume
      • Temperature
      • Surface area of reactants
      • Presence of catalysts

      What is Collision Theory?

      Collision theory is a concept which is used in chemistry to predict and explain the rate of chemical reactions. It is based on the kinetic theory of gasses and asserts that for a reaction to occur, reactant particles (molecules, atoms, ions) must collide. However, not all collisions lead to a chemical reaction; certain criteria must be met for the reactants to transform into products effectively. The theory states that the rate of reaction is dependent on three factors: 

      • Rate of collision: How often molecules collide
      • Activation Energy: The energy threshold that must be reached for a reaction to occur. 
      • Molecular Orientation: The specific alignment needed for molecules to react upon collision. 

      For a more detailed explanation of collision theory and its applications in understanding reaction rate, check out our video on collision theory in equilibria.  

      The role of collision rate:

      An increased rate of molecular collisions typically leads to a higher reaction rate. Factors influencing this include: 

      • Concentration: More particles in a volume mean more frequent collisions
      • Pressure/Volume: For gases, increasing pressure (or decreasing volume) raises collision frequency. 
      • Temperature: Higher temperatures increase kinetic energy and collision rate. 
      • Surface area of solids: More exposed particles increase the likelihood of collisions. 

      Activation energy and reaction rate

      Activation energy is the energy that is needed to initiate a reaction. According to collision theory, if molecules do not collide with sufficient energy to overcome the activation energy, reactions do not occur. Catalysts can be used to lower the activation energy of a reaction, offering an alternative pathway for the reaction to occur, thus speeding up the reaction rate. The Maxwell-Boltzmann curve depicts how energy is distributed across all the reactant particles. It helps visualise the proportion of particles that is available to react upon collision. This is the region to the right of the activation energy line. Notice how adding a catalyst decreases the activation energy and shifts it leftwards, increasing the area on the right.


      Reaction rate can similarly be increased by increasing the temperature of a system. Increasing the temperature raises the proportion of molecules with enough energy to overcome the activation barrier, thus speeding up the reaction. Conversely, lowering temperature has the opposite effect. 



      Molecular Orientation

      The orientation of molecules during a collision is a factor that remains unaffected by the other variables like concentration or temperature. In collision theory molecules must align in a correct orientation in order for particular reactions to occur. This ‘correct’ orientation is one that is favourable, or conducive, to forming the product. The example below demonstrates how carbon dioxide cannot from a collision between carbon monoxide `CO` and oxygen gas `O_2` if the oxygen gas does not contact the carbon monoxide particle on the carbon face. This is because the `C=O` double bond needs to form on the carbon face.


      While molecular orientation can be manipulated using some catalysts like platinum in catalytic converters, this aspect is not be explored in the NESA NSW HSC Chemistry Syllabus.