Energetics flashcards

Energetics explores the heat changes that accompany chemical reactions. Students must understand enthalpy change (delta H) as the heat exchanged at constant pressure, and distinguish between exothermic reactions (negative delta H) and endothermic reactions (positive delta H). Standard enthalpy changes of combustion, formation, and neutralisation are key definitions to master. Hess's law states that the total enthalpy change for a reaction is independent of the route taken, allowing calculation of enthalpy changes that cannot be measured directly. Bond enthalpy calculations provide an alternative method using average bond energies. Students must also apply calorimetry calculations using q = mc delta T to experimental data. Understanding activation energy and its relationship to enthalpy change, as well as drawing and interpreting energy profile diagrams, completes the core knowledge required for this topic.

1. Card 1

   What is enthalpy change (delta H) and what sign indicates an exothermic reaction?

   Enthalpy change is the heat exchanged with surroundings at constant pressure. A negative delta H indicates an exothermic reaction, where energy is released to the surroundings.

2. Card 2

   Define standard enthalpy change of formation.

   The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (298 K, 100 kPa).

3. Card 3

   Define standard enthalpy change of combustion.

   The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions. It is always negative.

4. Card 4

   State Hess's law.

   Hess's law states that the total enthalpy change for a chemical reaction is independent of the route taken, provided the initial and final conditions are the same.

5. Card 5

   Write the calorimetry equation used to calculate heat energy transferred.

   q = m x c x delta T, where q is heat energy in joules, m is mass in grams, c is the specific heat capacity of water (4.18 J g-1 K-1), and delta T is the temperature change.

6. Card 6

   How is bond enthalpy used to estimate the enthalpy change of a reaction?

   delta H = energy to break bonds (reactants) minus energy released forming bonds (products). Breaking bonds is endothermic; forming bonds is exothermic.

7. Card 7

   Why are bond enthalpy calculations only approximate?

   Bond enthalpy values used are average bond enthalpies taken from many different compounds, so they do not exactly represent the bond energies in the specific molecules of the reaction.

8. Card 8

   What is the standard enthalpy change of neutralisation?

   The standard enthalpy change of neutralisation is the enthalpy change when one mole of water is formed by the reaction of an acid and an alkali under standard conditions.

9. Card 9

   What does an energy profile diagram show, and what is activation energy?

   It shows the energy of reactants and products along a reaction pathway. Activation energy is the minimum energy required for reactants to undergo a successful collision and react.

10. Card 10

    What is the standard enthalpy of formation of any element in its standard state?

    By definition, the standard enthalpy of formation of any element in its standard state is exactly zero, as no formation reaction is taking place.

11. Card 11

    How do you calculate delta H using standard enthalpies of formation?

    delta H reaction = sum of delta Hf of products minus sum of delta Hf of reactants, with each value multiplied by its stoichiometric coefficient from the balanced equation.

12. Card 12

    Why does a simple calorimetry experiment often give a less negative value than the true delta H?

    Heat is lost to the surroundings (the calorimeter, air, etc.) rather than all being absorbed by the water, causing the measured temperature rise to be smaller than expected.

• Students often reverse the Hess's law formula and subtract products from reactants instead of reactants from products when using standard enthalpies of formation, giving the wrong sign for delta H.
• Many students confuse activation energy with enthalpy change, incorrectly believing that a more negative delta H always means a lower activation energy, when in fact the two quantities are independent of each other.
• A common error in calorimetry is using the mass of the fuel burned rather than the mass of the water being heated in the equation q = mc delta T, leading to a completely incorrect value for heat transferred.

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