Hydrolysis is exothermic when the products of the reaction are more stable, and thus have lower energy, than the reactants.
To understand why hydrolysis can be exothermic, let's break down the concepts:
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Hydrolysis Defined: Hydrolysis is a chemical reaction where a molecule is cleaved into two parts by the addition of a water molecule. One fragment gains a hydroxyl group (-OH) from the water molecule, and the other fragment gains a hydrogen atom (H).
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Energy and Stability: Molecules with lower energy are generally more stable. A system will tend towards lower energy states. If the hydrolysis reaction results in products with a lower overall energy level than the original reactants, energy is released in the form of heat, making the reaction exothermic.
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Bond Energies and Formation: In a chemical reaction, bonds are broken and new bonds are formed. Bond breaking requires energy (endothermic), and bond formation releases energy (exothermic). Whether a reaction is overall endothermic or exothermic depends on the balance between the energy required to break bonds in the reactants and the energy released when new bonds form in the products.
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Stability Determines the Outcome: As the provided reference correctly indicates, the relative stability of reactants and products dictates whether a specific hydrolysis reaction is exothermic or endothermic.
Here's how it applies to exothermic hydrolysis:
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Reactants (Higher Energy): The molecule undergoing hydrolysis, along with the water molecule, possess a certain amount of stored chemical energy. These molecules are relatively less stable.
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Hydrolysis Reaction: The water molecule breaks apart, and its components are added to the original molecule, cleaving it.
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Products (Lower Energy): The resulting product molecules are more stable, meaning they have lower energy than the original reactants. The bonds formed in the products release more energy than was required to break the bonds in the reactants and the water molecule.
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Exothermic Release: The excess energy is released as heat, making the reaction exothermic.
Example:
Consider the hydrolysis of ATP (adenosine triphosphate) to ADP (adenosine diphosphate) and inorganic phosphate (Pi). This is a highly exothermic reaction, releasing a significant amount of energy. This is because the products (ADP and Pi) are more stable and have lower energy than the reactant (ATP).
In summary, hydrolysis is exothermic when the products formed are more stable (lower energy) than the reactants, leading to a net release of energy in the form of heat.
