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    5.2 Hess's Law

    Understandings

    1. The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps.

    Applications and skills

    1. Application of Hess’s law to calculate enthalpy changes.
    2. Calculation of H reactions using H f data. Guidance: Enthalpy of formation data can be found in the data booklet in section 12.
    3. An application of Hess’s law is Hreactions = (H f (products)) − (H f (reactants))
    4. Determination of the enthalpy change of a reaction that is the sum of multiple reactions with known enthalpy changes.

    Inquiry questions

    5.2 How can Hess's law be used to determine the enthalpy of reaction?

    Introduction of Hess's law using energy cycle

    Task 1:
    1. Watch the video "8 Hess's Law calculation (using cycles)"
    2. Solve the 2 problems below 
    3. Choose one of the problems add some explanation
    4. take a picture of your calculations and explanations and use flipgrid to share it  (If you want to, you can make a video but sharing the picture would be fine) 
    5. Take a look at your classmates work and peer assess it on: 
                 - accuracy 
                 - quality of explanation

    Problems: 
    1. Calculate the enthalpy change for the following reaction: 

    2NO2(g) -> N2O4(g)

    use the following data:​
    1/2N2(g) + O2(g) -> NO2(g)         H = +33.2 kJ mol–1
    N2(g) + 2O2(g) -> N2O4(g)         H = +9.16 kJ mol–1

    2. Calculate the enthalpy change for the following reaction: 

    CO2(g) + H2(g)  -> CO(g) + H2O(l) 

    use the following data:​
    CO(g) + 1/2O2(g)  -> CO2(g).      H1 = –283 kJ mol–1
    2H2(g) + O2(g)  -> 2H2O(l)           H2 = –572 kJ mol-1



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    Hess's law - standard enthalpy of formation (using calculation shortcut)

    Task 2:
    1. Watch the video and take notes 
    2. Complete the Check comprehension question. Use the equation below  

    Standard enthalpy change - Calculations

    Task:
    Complete the exercises 25-28 on page 230 of the Pearsons textbook 

    After you have completed the calculation use the video to check your answers for 25 and 28 to check you are on the right path. 

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