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    5.3 Bond enthalpies

    Understandings

    1. Bond forming releases energy and bond breaking requires energy.
    2. Average bond enthalpy is the energy needed to break one mole of a bond in a gaseous molecule averaged over similar compounds.

    Applications and skills

    1. Calculation of the enthalpy changes from known bond enthalpy values and comparison of these to experimentally measured values.
    2. Bond enthalpy values are given in the data booklet in section 11.
    3. Average bond enthalpies are only valid for gases and calculations involving bond enthalpies may be inaccurate because they do not take into account intermolecular forces.
    4. Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic.
    5. Discussion of the bond strength in ozone relative to oxygen in its importance to the  atmosphere.

    Inquiry question 

    5.3 Why is there a difference in enthalpy in reaction when using Hess's law compared to using bond enthalpies?

    Additional Resources

    Live cooking lab recording - distance learning - Introduction to the Maillard reaction 
    Link to student cooking labs that can be completed from home
    Link to student cooking labs completed in school 

    Student cooking lab - Maillard reaction - cooking steak 

    The Maillard reaction is one of the main chemical reaction that takes place in every kitchen. 
    Use the poster below to understand the Maillard reaction and what chemistry is happening when you cook a steak. 


    Be sure to check out the video below

    Maillard reaction - cooking steak (video)

    Lab Instructions

    1. Use either the regular cooking method or cook the steak sous vide, but this time try to cook the steak either medium or welldone. 
    2. Calculate the enthalpy needed to cook the inside of the steak just right. 
    3. Calculate the enthalpy needed for the 2nd step of  the Maillard reaction using average bond enthalpies (use the DP Chemistry Data booklet to look up bond enthalpies) 

    Bond enthalpy - calculations 

    Task:
    1. Use the bond enthalpies in the IB Data booklet (Section 11)  to calculate the standard enthalpy change for the reactions:

    Reaction 1
    2H2(g) + O2(g) 2H2O(g)
    Reaction 2
    H2C=CH2 + H2 ->H3C−CH3
    Reaction 3
    The hydrogenation of the alkene double bond in unsaturated oils is an important reaction in
    margarine production. Calculate the enthalpy change when one mole of C=C bonds is hydrogenated
    from the bond energy data in the data booklet. 


    2. Choose one of the reactions to explain to your classmates. Ensure that your calculation is clearly readable. 
    3. Either add written explanation to your work and take a picture of it using Flipgrid or explain your calculation orally  showing your work 
    4. Peer assess one of your classmate's calculation and explanation ​
    Picture
    Blended learning

    Ozone depletion 

    Task:
    1. Watch the video 
    2. Read page 235-7 in the Pearson Textbook 
    3. Identify the following:
    • What wavelength is needed to break O2 (Oxygen molecules) apart (homolytic fission)
    • What wavelength is needed to break O3 (ozone molecules) apart (homolytic fission)
    • Why do we required different amounts of energy to break the bonds in O2 compared to O3
    4. Produce a visual poster (please use a colour pen/pencils and paper so you are not all the time on your device) of your explanation for the question - use diagrams and other drawings 
    5. Record your visual and oral explanation using Flipgrid
    6. Give peer feedback on:
    • use of visual 
    • Accuracy 
    • Clarity 

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