CHEM-UA 126 Chapter Notes -Spontaneous Process, Equilibrium Constant, Jmol

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21 Oct 2014

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1st law of thermodynamics - the law of conservation of energy. Enthalpy change ( h ) - the amount of heat (q) absorbed by a system for a reaction carried out at a constant pressure. Hess"s law of heat summation - enthalpy change for a process is the same whether it occurs in a single step or as a series of steps. Spontaneous reaction - a net change in amount(s) is observed to take place without intervention from outside of the system. If under a particular set of conditions a given process is not spontaneous, then its reverse is not spontaneous. Entropy (s) - randomness; s=k lnw k=1. 3806488 10 23j / k (boltzmann"s constant) W= # of microstates consistent with the macrostate of the system. Macroscopic state is specified by variables (such as p, v, n, and t for an ideal gas, found in the equation p= nrt. Microstate - a distribution of particles and energy within a system.

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Experiment 11 ENTHALPY OF REACTION (HESS'S LAW) IDEAS Both the physical and biological worlds are characterized by a complex interplay of chemical reactions. In all chemical reactions, heat is released or absorbed. Combustion is one of the commonly observed chemical reactions accompanied by the evolution of heat energy, which may be harnessed of for such as the heating of homes, or which may be converted to other forms energy, such as mechanical energy in the steam engine or the internal combustion engine. One of the pioneering studies on the heat of chemical reactions was made by Lavoisier and ace in 1 Investigating the heat associated with various reactions, including combustion, they showed that each reaction was associated with a specific amount of heat and that the heat evolved in chemical reaction is equal to the heat absorbed in the reverse reaction. Lavoisier had proposed the modern theory of combustion as the combination of a substance with aygen. He also suggested the hypothesis that respiration is merely slow combustion. With Laplace, Lavoisier attempted to compare the quantity of heat given off by a burning candle with that given off by a live chicken. The quantity of heat was determined by measuring the quantity of ice that melted, in an insulated container, in the presence of the candle or the chicken. Little progress was made in thermochemistry until about fifty years later, when G. Hess (1802- 18500 studied heats of reaction in greater detail and stated the law a840) that bears his name: The heat evolved or absorbed in a chemical reaction is the same whether the process is accomplished in one step or in many. The law suggested, therefore, that both the reaction and the thermodynamic data could be treated algebraically. Although it was an empirical law when Hess proposed it, it was theoretically justified a few years later when the principle of conservation of energy was proposed. Hess's law can be illustrated by the following problem. Calculate the change in heat (AH) for the following reaction: 1. Mgo(s) 2H+(aq) Use the following information to solve the problem: AH- -35 kl/mole AH -56 kJ/mole 3, H+(aq) oHraq)

1(a). Is the process of KNO₃ dissolving in water spontaneous at all temperatures studied? Briefly explain.

(b). Is the reaction in (a) one that gives off heat or requires heat? Briefly explain.

(c). Is yoru value of deltaS (80.70) consistent with the expected change in disorder for the reaction KNO_{3 (s)}<-->K^__(aq) + NO^-_3(aq)? Briefly explain.

2. A few compounds exist whose solubility decreases as the tmerpature increases. How would the values for deltaG, deltaH, and deltaS for these reactions be different from those values observed for the solubility of KNO₃? Briefly explain. [value of Ksp for 6 determinations: 59.75, 39.94, 28.63, 24.21, 23.04, and 19.52].

3(a). What assumption si made about the reaction at temperature at which crystals become visible? (Temperatures in K include: 341.7, 324.6, 318.9, 308.5, 307.3, 306.2)

(b). If this assumption were not true, how would the results for delta G change? (delta G values for each determination respectively: -11619.3, -9958.3, -8881.9, -8181.9, -8022.4, -7560.9)

CHEM-UA 126 Chapter Notes -Spontaneous Process, Equilibrium Constant, Jmol

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