Calorimetría

EXPERIMENT 21 Calorimetría Parte 1:

Determinación de la constante del calorímetro

[pic 1]

Objectivos

*Encontrar la constante calorimétrica del calorímetro.

Introduction

Thermodynamics is the study of heat in motion. Whenever chemicals are mixed, react, or undergo a physical change, the chemical's heat content (enthalpy) also changes. This is easily detected by measuring the change in temperature of the system.

Suppose one liter of water is heated from room temperature to its boiling point at one atmosphere pressure. The product (water at 1000C) has a higher energy content than the reactant (water at 250C). The first law of thermodynamics is the equation that deals with energy changes. It states that energy must be conserved, that is, that any difference in energy must be accounted for. In this process, there is a difference in enthalpy between the products and reactants,

AH = Hproduct - Hreactant

Here, Hproduct would be the heat content of the hot water and Hreactant would be the heat content of the room temperature water. AH, the difference in their heats, would have to be accounted for, that is, would have to come from (or go to) somewhere. In this case, AH would represent the amount of heat that was needed to accomplish the heating.

Most of the time, we are not interested in any particular value of enthalpy. Instead, we are interested in the change in the enthalpy, AH, in going from reactants to products. There is no way of directly measuring an enthalpy under a given set of conditions—we can only measure how much it changes.

[pic 2]Since we are interested in measuring the change in the amount of heat, it is important to do so in a container that doesn't lose any heat to the surroundings (or absorb any heat from them either). That way, all of the heat change that we measure will he due to the reaction itself The device most commonly used to measure enthalpy changes is called a calorimeter. The most familiar type is a ThermosTM bottle. Due to an insulating vacuum jacket, the ThermosTM bottle does not exchange heat with its surroundings, and any liquid in it retains its temperature (hot or cold). Since a calorimeter is not perfectly insulated, in order for a calorimetric measurement to be accurate, the reaction taking place must be fast, complete, and lead to only one set of products.

Most often, the calorimeter is filled with a known quantity of water. The reaction then takes place in the water, which heats up or cools off, depending on whether the reaction releases heat (is exothermic) or absorbs heat (is endothermic). By measuring the temperature change, the heat of reaction may be calculated using the equation

AH = m cp AT

Here, m is the number of grams of water, Cp is the gram heat capacity of the water (4.184 J g-l K-l ) and AT is the temperature change (in either oc or K). The heat capacity is defined as the amount of heat necessary to raise the temperature of a quantity of material by one degree. The most familiar unit of heat capacity in the United States is the BTU (British Thermal Unit), defined as the amount of heat necessary to raise the temperature of one pound of water by IOF. Another familiar unit, the calorie, is the amount of heat necessary to raise the temperature of one gram of water by IOC. The metric unit for energy is the Joule (equal to a kg m2/sec2, there are 4.184 Joules in I calorie).

In a perfectly insulated calorimeter, all the heat produced by the reaction would be taken up by the solution. In reality, some of the heat produced is lost, heating up the apparatus (the thermometer and calorimeter). This needs to be taken into account in the calculations.

[pic 3][pic 4]We need to know how much energy is required to raise the temperature of the apparatus by IOC. This can be found by performing a reaction which releases a known amount of heat into the solution (and thereby, the apparatus). By measuring the AT of the solution, the amount of heat taken up by the solution can be calculated. The remainder must have been absorbed by the apparatus or lost to the surroundings.

The simplest "reaction" is to mix equal quantities of hot and room temperature water. If the system absorbs no heat, the final temperature of the mixture would be halfwav between the hot and room temperatures. In reality, the calorimeter apparatus absorbs some heat. It is initially at room temperature, and absorbs some heat from the hot water when it is added. Therefore, the final temperature will be somewhat less than halfway between the hot and cold water temperatures.

As the calorimeter reaction occurs, the temperature is read every few seconds. A graph of temperature (y axis) vs. time (x axis) is made, shown in Figure 1. The temperature points rise sharply at first, and then begin to drop.

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