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UW-Madison CHEM 104 - Spontaneous Reactions
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CHEM 104 1st Edition Lecture 24- Components of thermochemistry: o Will the reaction happen? (Are reactants converting to products?) o If so, how fast will it go? (i.e. kinetics)- Product favored reactions are spontaneous, reactant favored are nonspontaneous o Ex. of spontaneous: Glycerol(l) + KMnO4(s)  K2CO3(s) + Mn2O3(s) + CO2(g) + H2O(g)o Ex. of nonspontaneous: H2O(l)  2H2(g) + O2(g) - ∆E = q + wo E = internal energy, q = energy transferred as heat, w = energy transferred as work o Q and w are always positive if heat/work are being transferred into the system, vice versao ∆E = Efinal – Einitial  ∆E is negative if energy is lost to the surroundings, positive if energy is gained from the surroundings- Heat transfer occurs when: o There is temperature difference between system/surroundings Q = mc∆T o System undergoes phase changeo Chemical reaction occurs - Ex. of heat/energy conversion: Cheeto + O2 CO2 + H2O 9.2 Cal/cheeto * 1000 cal/1 Cal * 4.185 J/1 cal = 38000 J/cheeto - Enthalpy change, ∆H = qp (heat transferred into system at constant pressure) o ∆E = ∆H + w (w is small for chemical equations, so ∆E ~ ∆H) - How to obtain ∆H for a system:o Calorimetry o Calculate from known thermochemical equations (Hess’s law) o Calculate from standard molar enthalpies of formation o Estimate from bond enthalpies - Processes can be spontaneous at some temperatures but not at others o Ex: melting of water is temperature dependent - In a reversible process, the system changes so that the system and surroundings can be put back in their original states by the same pathwayso Irreversible processes cannot be undone by exactly reversing the change to the system  Spontaneous reactions are


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