We expect the forward reaction to proceed spontaneously on the basis that the entropy of the system has increased due to the formation of mobile ions in solution (ΔS is positive).īut, the forward reaction is not favoured energetically, that is, the enthalpy of the products will be greater than the enthalpy of the reactant molecules because energy has been absorbed (ΔH is also positive).īecause we know the reaction occurs spontaneously, we can watch the sodium nitrate crystals dissolve in the liquid water, this tells us that the main driving force for this reaction is the tendency to maximise entropy.Ĭan a reaction be spontaneous if there is a decrease in entropy (ΔS is negative)? Yes, IF the driving force towards increased entropy is great enough!įor example, sodium nitrate dissolves SPONTANEOUSLY in water to form an aqueous solution, even though the reaction is endothermic, that is, the vessel holding the solution gets cooler as the dissolution reaction absorbs heat energy from its surroundings. (ii) A reaction will always be nonspontaneous if:Ĭan an endothermic reaction (ΔH positive) ever be spontaneous? (i) A reaction will always be spontaneous if: It's probably just as well that this reaction is nonspontaneous, if lightning during a thunderstorm could supply enough energy to start the reaction then the carbon dioxide gas and water vapor in the atmosphere would begin to form ethanol, and it would rain alcohol! (5 gaseous molecules on the left side but only 3 on the right hand side). entropy of the system has decreased NOT increased.(enthalpy of product molecules H reactants, reaction is endothermic, ΔH is positive) enthalpy is minimised because heat is a product of the reaction.The combustion of ethanol is a spontaneous chemical reaction because, as the reaction proceeds from left to right: We can represent the combustion of ethanol in a balanced chemical reaction as shown below:Ĭ 2H 5OH (l) + 3O 2(g) → 2CO 2(g) + 3H 2O (g) ΔH = -1368 kJ mol -1 This combustion reaction releases 1368 kJ of heat energy per mole of ethanol consumed in the reaction. The products of the combustion reaction are carbon dioxide gas (CO 2(g)) and water (H 2O (g) at temperatures above 100☌). Play the game now! Spontaneous and Nonspontaneous ReactionsĪ reaction which is exothermic (ΔH negative) and results in an increase in the entropy of the system (ΔS positive) will always be spontaneous.Ĭonsider the combustion of a fuel like ethanol (C 2H 5OH (l)) in oxygen gas (O 2(g)) in air. Put another way, a reaction is most likely to be spontaneous if ΔH is negative AND ΔS is positive. ΔS = S products - S reactants = a positive number = +Ī reaction is most likely to be spontaneous if it is exothermic AND there is an increase in entropy of the system. Reactions are driven towards maximum entropy, that is, S products > S reactants, then The enthalpy of a chemical system will be minimised when the enthalpy of the products (H products) is less than the enthalpy of the reactants (H reactants), that is:Įntropy change for the system (ΔS) is defined as the entropy of the products minus the entropy of the reactants:
INTRIGUE AND ENTROPY FREE
No ads = no money for us = no free stuff for you! Enthalpy as a Driving Force of Chemical ReactionsĬhemical reactions tend to proceed spontaneously in the direction that allows for Some spontaneous reactions are fast, others are slow. We cannot predict the rate of a chemical reaction by determining whether the reaction is spontaneous or nonspontaneous.
0 kommentar(er)
