# Hess's Law - Hess's Law October 3 2005 Introduction The...

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Hess’s Law – October 3, 2005 Introduction The purpose of this experiment was to find the enthalpy change, ΔH, for the formation of NH 4 Cl (g) from HCl (aq) and NH 3(g) . This was to be done using Hess’s Law and heats of formation from the reactions of HCl (aq) and NaOH (aq) to form H 2 O (l) and NaCl (aq) and from NH 4 Cl and NaOH (aq) to form NH 3 (g) , NaCl (aq) , and H 2 O (l) . The chemical equations are as follows: 1) NaOH (aq) + HCl (aq) H 2 O (l) + NaCl (aq) 2) NH 4 Cl (aq) + NaOH (aq) NH 3 (g) + NaCl (aq) , + H 2 O (l) 3) HCl (aq) + NH 3(g) NH 4 Cl (g) Hess’s Law states that in going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. That means that if it takes two or tree reactions to produce a single product the enthalpy change will be the same as if the product was formed by only one reaction. The first two equations can be combined algebraically to form the third equation. To get the final reaction, the previous equation may need to be reversed or they may need to be multiplied by some number to cancel out unwanted products and reactants. If an equation is reversed or multiplied then the heat of formation has to be revered or multiplied. To get the enthalpy change for the third equation, the ΔH for equation 2 has to be added to equation 1. Another way that was used to find the heats of formations, was to use the standard enthalpies of formation equation, . ) (reactants ΔH n (products) ΔH n ΔH f p f p reaction ° - ° = ° The enthalpy change for a given reaction can be calculated by subtracting the heats of

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formation of the reactants from the heats of formations of the products. Enthalpy is merely the internal energy of the system or reaction. If the enthalpy change is positive, then the reaction is endothermic and if the change is negative, the reaction is exothermic. Since there is no instrument that can directly measure heat, the heat gain or heat loss must be calculated. The theory used in this lab was that the heat of formation or the gain of or lose of heat will be equal to the products of the mass, heat capacity and the change in temperature or T C M Q × × = . Q is equal to the heat energy gained or lost and the delta T is the change in temperature, final temperature minus the original temperature. Q is also equal to ΔH , because pressure and volume were held constant throughout the experiment. C is equal to the specific heat capacity of the substance, which is the value in J/°C ◦ g. This is the amount of energy it takes for a substance to get hot. Calorimetry was also used in the experiment. To help make the results more
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Hess's Law - Hess's Law October 3 2005 Introduction The...

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