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    16.1 Rate expression and reaction mechanism

    Understanding ​
    1. Reactions may occur by more than one step and the slowest step determines the rate of reaction (rate-determining step/RDS). Guidance: Use potential energy level profiles to illustrate multi-step reactions; showing the higher Ea in the rate-determining step in the diagram.
    2. The molecularity of an elementary step is the number of reactant particles taking part in that step. Guidance: Catalysts are involved in the rate-determining step.
    3. The order of a reaction can be either integer or fractional in nature. The order of a reaction can describe, with respect to a reactant, the number of particles taking part in the rate-determining step.
    4. Rate equations can only be determined experimentally. Guidance: Any experiment which allows students to vary concentrations to see the effect upon the rate and hence determine a rate equation is appropriate.
    5. The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of the reaction.
    6. Catalysts alter a reaction mechanism, introducing a step with lower activation energy.

    Applications and skills
    1. Deduction of the rate expression for an equation from experimental data and solving problems involving the rate expression.
    2. Sketching, identifying, and analysing graphical representations for zero, fi rst, and second-order reactions. Guidance: Consider concentration–time and rate–concentration graphs.
    3. Evaluation of proposed reaction mechanisms to be consistent with kinetic and stoichiometric data. Guidance: Reactions where the rate-determining step is not the fi rst step should be considered.

    Inquiry questions 
    6.1 + 16.1 Is it important to know how to change the rate of reaction when you cook?

    Additional resources

    Introduction to rate expression 
    Picture
    Source of image: https://en.wikipedia.org/wiki/Buckminsterfullerene
    See below the experimental data of the reaction. We measured the absorbance of light of a certain wavelength over time. Please note that the absorbance of light is directly proportional to the concentration of the oxidised buckminsterfullerene.
    Task: 
    1. ​Access the google spreadsheet and make your own copy - use the link below 
    2. Use the spreadsheet functions to calculate Δ t and Δ [C60O3] - for help look at the model tab in the spreadsheet 
    3. Use the spreadsheet functions to calculate rate (rate = -(Δ [C60O3]/Δ t) - for help look at the model tab in the spreadsheet 
    4. Plot a rate-time graph - see below what it should look like ​
    5. Plot a rate-absorbance graph of the decomposition  of the oxidised buckminsterfullerene - see below what it should look like ​​
    The rate-absorbance graph demonstrates that the the rate is proportional to absorbance, which means that rate is proportional to the concentration of oxidised buckminsterfullerene. This will help us to determine the rate expression of the raction, see below:

    Using cooking lab data to determine the order of reaction when baking soda reacts with vinegar 
    Task:
    1. Watch the video 
    2. Use the data in the poster and create a table in a spreadsheet
    3. Using the volume of CO2 calculate the ∆ [NaHCO3] 
    4. Plot the  ∆ [NaHCO3]  over time 
    5. Determine the rate of reaction by drawing a tangent at at least 6 points in time 
    6. Use your data to plot the following
     - a) Rate over time 
     - b) Rate vs Concentration 
    7. Determine the order of reaction 
    ​8. Explain your working using the flipgrid below 
    ​9.  Give feedback to your peers 

    Order of reaction and rate law 

    Task:
    Watch videos and take notes
    Complete the check for comprehension tasks

    Rate constant

    More information about determining rates, reaction mechanism & rae expression

    Task:
    1. Form and pick on of the topics below
    - Determination of the order of reaction (pearson textbook page 294-5)
    - Reaction mechanism (pearson textbook page 296-8)
    - Rate expression (pearson textbook page 298-9)
    2.  Research your topic using the textbook, internet and other sources
    3. Create a presentation with the following items
    - diagram and tables to support understanding 
    - worked example 
    - one questions for students to answer about your topic

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