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Chemical Energetics 1:

Chemical Energetics 2:. Instead of writing ?energy change' all the time chemists use the symbols:. ?H. Pronounced ?delta aitch'.?=greek letter ?delta' meaning change.H=heat.So, ?H means ?heat change'.. Temperature Changes 1. Some reactions cause the temperature of the reaction mixture to increase.This type of reaction is called exothermic.Heat energy is given out by the reaction mixture hence the surroundings increase in temperature..

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Chemical Energetics 1:

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    1. Chemical Energetics 1: All chemical reactions involve an energy change.

    2. Chemical Energetics 2: Instead of writing ‘energy change’ all the time chemists use the symbols:

    3. Temperature Changes 1 Some reactions cause the temperature of the reaction mixture to increase. This type of reaction is called exothermic. Heat energy is given out by the reaction mixture hence the surroundings increase in temperature.

    4. Temperature Changes 2 Some reactions cause the temperature of the reaction mixture to decrease. This type of reaction is called endothermic. Heat energy is taken in by the reaction mixture hence the surroundings decrease in temperature.

    5. Representing Energy Changes: The energy changes in a chemical reaction can be conveniently represented using energy level diagrams Energy level diagrams make it easier to decide whether a reaction is exothermic (gives out heat and gets hotter) or endothermic (takes in heat and gets cooler).

    6. Energy Level Diagrams This energy level diagram shows a reaction where ?H is negative (an exothermic reaction).

    7. Making and Breaking Bonds: Making chemical bonds is always exothermic.

    8. Reaction Profiles: These are a more detailed version of the energy level diagrams previously looked at. They show the endothermic bond breaking and the exothermic bond making stages of a reaction.

    9. Reaction Profile for: H2 + Cl2 ? 2HCl

    10. Calculating Energy Changes: The energy change for a reaction can be calculated using ‘bond energies’.

    11. Reaction Profile for: H2 + Cl2 ? 2HCl

    12. An Example Calculation: Consider the reaction between hydrogen and chlorine:

    13. Reaction Profile for: H2 + Cl2 ? 2HCl

    14. Chemical reactions take place in two stages: Endothermic reactions get cold, they take in heat. Exothermic reactions get hot, they give out heat.

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