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3.1 - 3.8 Energetics

 3.1 Energetics

An exothermic reaction

 Students should:

  • 3.1 know that chemical reactions in which heat energy is given out are described as exothermic, and those in which heat energy is taken in are described as endothermic

    Every chemical reaction has an accompanying change in energy.

    Some reactions release energy to the surroundings and are known as Exothermic.

    Reactions which take in energy from their surroundings are known as Endothermic

    When a chemical reaction releases energy in a useful form the substance involved can be regarded as a fuel 

    We use a technique called calorimetry to find out the energy change associated with a chemical change. A measured amount of reactant is used to produce a temperature change in a vessel ( calorimeter). Beakers can be used as calorimeters but heat losses are a problem.

     3.2 Activity 2. Coke can calorimetry

     This experiment uses a diet coke can as a calorimeter

     Students should:

    • 3.2 describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation

    ​Watch this very brief video and note down the key measurements taken:

    1. mass of water heated 
    2. initial mass of nut
    3. ​final mass of nut ( after burning)
    ​4. mass change (3-2)
    ​5. initial temperature of water  / 'C
    6. ​final temperature of water  / 'C
    ​7. temperature change  (6-5)
    ​1. mass of water heated  ​100ml
    ​2. initial mass of nut ​6.75g
    ​3. ​final mass of nut ( after burning)​5.79g
    ​​4. mass change (3-2)​0.96g
    ​​5. initial temperature of water  / 'C​22.0oC
    ​6. ​final temperature of water  / 'C​50.0oC
    ​​7. temperature change  (6-5)​28.0oC

     3.2 Activity 3 - Candle power

    simple calorimetry with a baked bean can
    how it's done

     3.3 Activity 3. Do the maths

    ​Using the data from Mr Pauller's experiment to calculate the heat produced

     Students should:

    • 3.3 calculate the heat energy change from a measured temperature change using the expression Q = mcΔT
    This equation can be used to calculate how much energy has been released into the water.

     3.4 Activity 4. How much per mole?

     Students should:

    • 3.4 calculate the molar enthalpy change ("delta"H) from the heat energy change, Q

    The energy produced by a particular reaction can be found out using calorimetry . If we measure the mass of fuel before and after the burning we can find out the energy released per gram by using the following calculation: 

    calculate the energy per g of fuel

     3.5 Activity 1. Exo and Endo

    In an exothermic reaction the energy of the products is less than the energy of the reactants. Energy is given out to the surroundings.

     Students should:

    • 3.5 draw and explain energy level diagrams to represent exothermic and endothermic reactions

    ​When a reaction takes in energy from the surroundings it is known as an endothermic reaction. 

    Use this idea and the exothermic reaction energy profile to draw an energy profile for an endothermic reaction.

    In an endothermic reaction the energy of the products is greater than the energy of the reactants. Energy is taken in from the surroundings.

     3.6 Activity 5. Profit and loss

     Students should:

    • 3.6C know that bond-breaking is an endothermic process and that bond-making is an exothermic process
    • 3.7C use bond energies to calculate the enthalpy change during a chemical reaction

    Use the animation to see how the formation of a covalent bond releases energy and is therefore an exothermic process.  Notice that the energy released in this case is 432 kJ per mol.

    That same amount of energy (if supplied) would be sufficient to break the bond. 

    We can use the fact that individual bonds have specific amounts of energy associated with them to calculate an approximate value for the energy change for a chemical reaction: 

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