Energy changes flashcards

Energy changes in chemistry focuses on the heat energy transferred during chemical reactions. Reactions can be exothermic, releasing energy to the surroundings and causing a temperature rise, or endothermic, taking in energy from the surroundings and causing a temperature drop. Students must understand activation energy, which is the minimum energy needed to start a reaction, and how reaction profiles (energy diagrams) show the relative energies of reactants and products. Bond breaking always requires energy input, whilst bond forming always releases energy. The overall energy change of a reaction depends on which process dominates. Students should be able to calculate energy changes using bond energies and understand practical applications such as hand warmers (exothermic) and sports cold packs (endothermic). Catalysts lower activation energy without changing the overall energy change of the reaction.

1. Card 1

   What is an exothermic reaction?

   An exothermic reaction releases energy to the surroundings, causing the temperature of the surroundings to rise. Examples include combustion and neutralisation.

2. Card 2

   What is an endothermic reaction?

   An endothermic reaction takes in energy from the surroundings, causing the temperature of the surroundings to fall. Examples include thermal decomposition and dissolving certain salts.

3. Card 3

   What is activation energy?

   Activation energy is the minimum amount of energy that reacting particles must have in order to collide successfully and start a chemical reaction.

4. Card 4

   On a reaction profile, where is activation energy shown?

   Activation energy is shown as the difference in energy between the reactants and the peak (transition state) at the top of the energy profile diagram.

5. Card 5

   Does bond breaking require or release energy?

   Bond breaking always requires energy to be taken in. Energy must be supplied to overcome the forces of attraction holding atoms together in a chemical bond.

6. Card 6

   Does bond forming require or release energy?

   Bond forming always releases energy to the surroundings. When new bonds are made between atoms, energy is given out as the atoms become more stable.

7. Card 7

   How do you calculate the overall energy change using bond energies?

   Overall energy change = energy needed to break bonds minus energy released forming bonds. A negative result means exothermic; a positive result means endothermic.

8. Card 8

   What does a catalyst do to the activation energy of a reaction?

   A catalyst lowers the activation energy by providing an alternative reaction pathway, allowing more particles to react without changing the overall energy change.

9. Card 9

   Give one everyday application of an exothermic reaction.

   Hand warmers use exothermic reactions, such as the oxidation of iron, to release heat energy to the surroundings, warming the user's hands.

10. Card 10

    Give one everyday application of an endothermic reaction.

    Sports cold packs use endothermic reactions, such as dissolving ammonium nitrate in water, to absorb heat from the surroundings and cool an injury.

11. Card 11

    On a reaction profile for an exothermic reaction, how do the products compare to the reactants?

    In an exothermic reaction profile, the products are at a lower energy level than the reactants, showing that energy has been released to the surroundings overall.

12. Card 12

    What is the difference in energy between reactants and products on a reaction profile called?

    This difference is called the overall energy change (sometimes written as delta H). It shows whether the reaction releases or absorbs energy overall.

• Many students confuse the direction of energy flow, thinking that an exothermic reaction absorbs energy rather than releases it to the surroundings, leading to incorrect identification of reaction types.
• Students often forget that bond breaking requires energy and bond forming releases energy, and incorrectly state the opposite when explaining why a reaction is exothermic or endothermic.
• A common error is stating that a catalyst changes the overall energy change of a reaction, when in fact it only lowers the activation energy by providing an alternative reaction pathway.

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