susb053 - SUSB-053 Calorimetric Determination of Enthalpies...

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SUSB-053 Calorimetric Determination of Enthalpies of Chemical Reactions Prepared by M. J. Akhtar and Robert F. Schneider (Rev 04/04) OBJECTIVE: 1) To determine heat released (the enthalpy change) in the neutralization of hydrochloric acid (HCl) and acetic acid (CH 3 COOH) with sodium hydroxide (NaOH). 2) To apply Hess’s Law to calculate the enthalpy of dissociation of acetic acid in aqueous solution, from the determined enthalpies of neutralization of hydrochloric and acetic acid. INTRODUCTION: Chemical reactions are usually accompanied by a change in energy due to the breaking and forming of bonds. To keep track of energy changes during chemical reactions that occur under constant atmospheric pressure, we use a thermodynamic quantity called the enthalpy, H, which is defined in terms of energy, E, pressure, P, and volume, V, as H = E + PV (Definition of Enthalpy) At constant pressure, the enthalpy change H is then H = E + P V (Enthalpy change at const P) (1) The first law of thermodynamics tells us that the change in energy is the sum of heat absorbed by the system (q) and the work done on the system (w). If heat flows out of the system or the system does work on the surroundings, the signs of q or w will be correspondingly negative. E = q abs + w (First Law of Thermodynamics) (2) In the absence of other forms of work (e.g., electrical, such as in a battery), the first law becomes E = q abs - P V (Assuming only PV work) (2a) Substituting equation (2a) for E in equation (1), the enthalpy change becomes H = (q - P V) + P V = q abs (Assuming only PV work) (3) Thus for a chemical reaction at constant pressure, the enthalpy change is equal to the heat change. When a chemical reaction occurs, the associated enthalpy change can be shown to be the difference between the enthalpy of the products and the reactants H = H product - H reactant
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When the enthalpy of the products of a reaction is greater than the enthalpy of the reactants, H is positive, heat is absorbed and the reaction is called endothermic . On the other hand, if the enthalpy of the products is less than that of the reactants, H is negative, and the reaction produces heat. Such reactions are called exothermic . While heat cannot be measured directly, we can measure its effect on a system – namely the increase or decrease in temperature of the system when the system absorbs or liberates heat. We measure the heat associated with a chemical reaction by measuring the temperature change ( T = T final - T initial ) of the reaction mixture. To do so, requires knowledge of how the temperature of the reaction mixture changes when a given amount of heat is absorbed or liberated by the mixture. This quantity is generally called the heat capacity (or specific heat) of a system, C sys – usually expressed as the quantity of heat in Joules (J) or Calories (cal), required to raise the temperature of one gram of the system by one Kelvin degree (which is the same as one Centigrade degree). Thus the heat (
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This note was uploaded on 11/12/2011 for the course CHEM CHEM 2B taught by Professor Cabrera during the Spring '09 term at San Jose City College.

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susb053 - SUSB-053 Calorimetric Determination of Enthalpies...

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