2.3 Energy Changes During Chemical Reactions
Book for Endothermic and Exothermic Reactions.
Heat of Reaction
| Exothermic | Endothermic | |
| 1) Products have less potential energy than reactants. The potential energy lost during the reaction is changed into heat. | 1) Products have more potential energy than reactants. The potential energy gained during the reaction is absorbed from the surroundings. | |
| 2) Surroundings will feel hot. Be aware! If your hand feels hot when you are holding a test tube containing a reaction, then heat is leaving the reaction in the test tube and is entering your hand. Understand the difference between your personal point of view as the observer and the point of view of the particles participating in the reaction in the test tube. | 2) Surroundings will feel cold. | |
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3) There are two ways to show the heat effect. a) The heat term is included in the reaction on the right side. Eg. H2 + Cl2 b) Use ?H notation: The reaction is written, then followed by a negative value for ?H that shows the "change in heat". Eg. H2 + Cl2 |
3) There are two ways to show the heat effect a) The heat term is included in the reaction on the left side. Eg. 2HCl + 184 kJ  H2 + Cl2b) Use ?H notation: The reaction is written, then followed by a positive value for ?H that shows the "change in heat". Eg. 2HCl H2 + Cl2, ?H= +184 kJ |
Examples
| Example: Solve the question in your notebook then check your solution. |
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Label each of these reactions as endo or exothermic.
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H= +83 kJ