Uses and applications of energy changes
Energy changes • Exothermic and endothermic reactions
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Key concepts
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Definitions and cause → effect
An exothermic reaction releases thermal energy because the total energy of the products is lower than that of the reactants; the excess energy transfers to the surroundings and raises temperature. An endothermic reaction absorbs thermal energy because the products have higher total energy than the reactants; heat flows from the surroundings into the reaction and lowers the temperature.
Common exothermic applications
Disposable hand warmers use an exothermic oxidation of iron. Exposure to air causes iron to oxidise; the reaction releases heat and increases the temperature of the pack. Combustion of fuels in heaters and engines releases large amounts of energy rapidly; useful heat arises from bond formation in product molecules. Neutralisation reactions (acid + alkali) release heat and find use in some industrial heating processes where moderate, controllable heat is required.
Common endothermic applications
Instant cold packs use an endothermic dissolution, commonly of ammonium nitrate in water. The salt absorbs energy from the water and surroundings during dissolution and reduces temperature. Photosynthesis is an endothermic process that stores solar energy in chemical bonds; it serves as the basis for biomass production and some industrial photochemical processes. Thermal decomposition and calcination in industry absorb heat to break bonds and transform materials.
Evaluation criteria for uses
Energy change magnitude determines whether a reaction supplies sufficient heating or cooling for the intended use; larger enthalpy changes yield greater temperature effects per mole. Reaction rate determines how quickly the desired temperature change appears; a fast exothermic reaction gives quick heating but may be hard to control. Controllability and reversibility affect reuse and safety: reversible systems allow energy storage and release cycles, while irreversible systems often suit single-use applications.
Limiting factors and safety
Activation energy and reaction kinetics limit practical use: high activation energy may require a catalyst or ignition energy and may reduce convenience. By-products, toxicity and corrosiveness limit applications where exposure risks or material compatibility pose problems. Heat transfer constraints limit effectiveness; poor thermal conduction reduces useful temperature change. Cost and availability of reactants and the feasibility of managing waste products also limit application.
Interpreting data when evaluating uses
Enthalpy change (ΔH) data provide the energy per mole; conversion to energy per unit mass or per device allows direct comparison between options. Temperature change measurements combined with specific heat capacities allow estimation of energy transferred. Reaction stoichiometry and yield determine the actual energy available from a given amount of reactant. Safety data and material compatibility information inform practical feasibility alongside numerical energy data.
Key notes
Important points to keep in mind