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The profile for same reaction but with a catalyst is also shown. Figure 13: An energy profile diagram demonstrating the effect of a catalyst for the generic exothermic reaction of X + Y →Z. The catalyst offers an alternate reaction pathway (shown in red) where the rate determining step has a smaller ΔG≠.
An energy profile of an exothermic reaction. In an exothermic reaction, by definition, the enthalpy change has a negative value: ΔH = H products - H reactants < 0. where a larger value (the higher energy of the reactants) is subtracted from a smaller value (the lower energy of the products). For example, when hydrogen burns: 2H 2 (g) + O 2 (g ...
According to the IUPAC, an exothermic reaction is "a reaction for which the overall standard enthalpy change ΔH⚬ is negative". [4] Some examples of exothermic process are fuel combustion, condensation and nuclear fission, [5] which is used in nuclear power plants to release large amounts of energy. [6]
Generic potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction X + Y to give Z. The presence of the catalyst opens a different reaction pathway (shown in red) with lower activation energy. The final result and the overall thermodynamics are the same.
where ln denotes the natural logarithm, is the thermodynamic equilibrium constant, and R is the ideal gas constant.This equation is exact at any one temperature and all pressures, derived from the requirement that the Gibbs free energy of reaction be stationary in a state of chemical equilibrium.
Endothermic reactions absorb energy from the surroundings, while exothermic reactions release energy. Some reactions occur spontaneously, while others necessitate an external energy input. The reaction can be visualized using a reaction coordinate diagram to show the activation energy and potential energy throughout the reaction.
Schematic potential energy diagram showing the effect of a catalyst in an endothermic chemical reaction. The presence of a catalyst opens a different reaction pathway (in red) with lower activation energy. The final result and the overall thermodynamics are the same.
With the catalyst, the energy required to enter transition state decreases, thereby decreasing the energy required to initiate the reaction. A substance that modifies the transition state to lower the activation energy is termed a catalyst; a catalyst composed only of protein and (if applicable) small molecule cofactors is termed an enzyme.