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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) → 2H 2 O (g) ΔH⚬ = −483.6 kJ/mol [3]
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]
Van 't Hoff plot for an exothermic reaction. For an exothermic reaction, heat is released, making the net enthalpy change negative. Thus, according to the definition of the slope: =, For an exothermic reaction Δ r H < 0, so
As discussed earlier, can have a positive or negative sign. If Δ H {\displaystyle \Delta H} has a positive sign, the system uses heat and is endothermic ; if Δ H {\displaystyle \Delta H} is negative, then heat is produced and the system is exothermic .
An ideal mixture is any in which the arithmetic mean (with respect to mole fraction) of the two pure substances is the same as that of the final mixture. Among other important thermodynamic simplifications, this means that enthalpy of mixing is zero: Δ H m i x , i d e a l = 0 {\displaystyle \Delta H_{mix,ideal}=0} .
Δ latt H corresponds to U L in the text. The downward arrow "electron affinity" shows the negative quantity –EA F, since EA F is usually defined as positive. For ionic compounds, the standard enthalpy of formation is equivalent to the sum of several terms included in the Born–Haber cycle. For example, the formation of lithium fluoride,
The energy released by the solvation of the ammonium ions and nitrate ions is less than the energy absorbed in breaking up the ammonium nitrate ionic lattice and the attractions between water molecules. Dissolving potassium hydroxide is exothermic, as more energy is released during solvation than is used in breaking up the solute and solvent.
In thermodynamics, a spontaneous process is a process which occurs without any external input to the system. A more technical definition is the time-evolution of a system in which it releases free energy and it moves to a lower, more thermodynamically stable energy state (closer to thermodynamic equilibrium).