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Chemistry 1301A/B Lecture Notes - Lecture 18: Bond-Dissociation Energy, Intermolecular Force, Joule

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31 Aug 2017

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Western University

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Chemistry 1301A/B

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The enthalpy change of a physical or chemical process depends only on the beginning conditions (reactants) and the end conditions (products), and is independent of the pathway of the process. In other words, the enthalpy change for a given reaction under a given set of conditions (generally satp) is always a constant value. Therefore, h can be determined experimentally using hess" law, or can be determined theoretically by applying the principles of the law to a series of pre- determined reactions that will sum to give the target reaction (no experiment required). The following principles must be observed: when an equation is reversed, the sign of h is reversed, when an equation is multiplied by a coefficient, the h is multiplied by that same factor. The individual reactions are manipulated in such a way that their sum will give the overall target reaction, and therefore the corresponding h value.

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Related Questions

Find enthalpy of MgO given this information

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)