Le Chateliers Principle
Endothermic and Exothermic Reactions
Definition:
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Endothermic Reactions: Reactions that absorb heat from their surroundings. The system’s energy increases, and the surroundings’ energy decreases. $ \Delta H > 0 $ (positive enthalpy change).
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Exothermic Reactions: Reactions that release heat to their surroundings. The system’s energy decreases, and the surroundings’ energy increases. $ \Delta H < 0 $ (negative enthalpy change).
Energy Diagrams:
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Endothermic: Reactants are at a lower energy level than products. The activation energy ( $ E_a $ ) is the energy required to reach the transition state. ## $$ \text{Reactants} \longrightarrow \text{Transition State} \longrightarrow \text{Products} $$
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Exothermic: Products are at a lower energy level than reactants. The activation energy ( $ E_a $ ) is still the energy required to reach the transition state. ## $$ \text{Reactants} \longrightarrow \text{Transition State} \longrightarrow \text{Products} $$
Examples: -
Endothermic: Melting ice ( $ H_2O(s) \rightarrow H_2O(l) $ ), photosynthesis.
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Exothermic: Combustion of methane ( $ CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l) $ ), neutralization reactions.
Relationship to Enthalpy:
[[Enthalpy ( $ H $ ) is a state function representing the heat content of a system at constant pressure. The change in enthalpy ( $ \Delta H $ ) is the difference between the enthalpy of the products and the enthalpy of the reactants: $ \Delta H = H_{products} - H_{reactants} $ . Enthalpy]]
**Relationship to Entropy:** Entropy
Factors Affecting Reaction Rate: Reaction Kinetics These factors (temperature, concentration, surface area, catalysts) affect how quickly a reaction reaches equilibrium, but do not change whether the reaction is endothermic or exothermic.
**Calorimetry:** Calorimetry This is a technique used to measure the heat changes in reactions, allowing us to determine whether a reaction is endothermic or exothermic and to quantify the amount of heat transferred.
Spontaneity: Gibbs Free Energy Whether a reaction will occur spontaneously depends not only on enthalpy but also on entropy and temperature, as described by the Gibbs Free Energy equation.