Focal Length of Lenses

Experiment 3: Heats of Transition, Heats of Reaction, Specific Heats, and Hess's Law

GOAL AND OVERVIEW

A simple calorimeter will be made and calibrated. It will be used to determine the heat of fusion of ice, the specific heat of metals, and the heat of several chemical reactions. These heats of reaction will be used with Hess's law to determine another desired heat of reaction.

Objectives of the data analysis

 understand calorimetry and the concepts and uses of heat transfer

 understand heat transfer processes

 understand a variety of heat and heat-related topics, including specific heat and heat capacity, heat of fusion, and heat of a chemical reaction

 use Hess's Law to deduce an unknown heat of reaction

SUGGESTED REVIEW AND EXTERNAL READING

 See reference section 3; textbook information on calorimetry and thermochemistry

BACKGROUND

The system is the part of the physical world that is the focus of your attention. Usually, it is the contents of a reaction vessel. The internal energy, E, of the system which can be increased by:

i) the surroundings doing work (w) on the system; or.

ii) by the system absorbing heat (q) from the surroundings.

The change in energy, Efinal - Einitial, is the sum of heat and work:

(1)

This generalization is known as the first law of thermodynamics. It can also be stated as: the energy of the universe is constant.

Internal energy is not a convenient property to work with when studying processes occurring at constant pressure (e.g., biological processes or many chemical reactions done in lab). A more convenient property is the sum of the internal energy and pressure-volume work (wsys = PV at constant P). This is called the enthalpy of the system and is given the symbol H:

(2)

Enthalpy is simply a corrected energy. The correction, p×V, does not change the energy very much.

Enthalpy is such a useful property because, for any process occurring at constant pressure, the change in the enthalpy, H, is the heat absorbed or released by the system:

(3)

Chemists, biologists, and engineers are often interested in the amount of heat absorbed or given up by processes and the resulting effect on the temperature of the system. If the heat capacity of a material is known, the heat that flows into or out of a material can be determined by measuring the mass and temperature change of the material:

heat (q) = mass × heat capacity × temperature change

You can think of a system as "containing" enthalpy, just as you did with energy. Here are some of the contributions to the enthalpy of a system:

(1) Thermal Enthalpy. The higher the system's temperature, the greater its enthalpy. This makes sense because it takes heat to raise the temperature of an object.

(2) Phase Enthalpy. The liquid phase has higher enthalpy than the solid, and the vapor phase has higher enthalpy than the liquid. This also makes sense because it takes heat to melt a solid or to vaporize a liquid.

(3) Chemical Enthalpy. Chemical bonds store energy. When a chemical reaction occurs, the chemical enthalpy of the reactants changes when they form products.

i) If the reaction is exothermic, the chemical enthalpy decreases as the reaction proceeds.

ii) If the reaction is endothermic, the chemical enthalpy increases as the reaction proceeds.

For example, combustion is an exothermic reaction, which means that the chemical enthalpy decreases as the reactants change to products. The reason the products get hot is that enthalpy is conserved during the short time of the reaction. As the chemical enthalpy goes down, the thermal enthalpy must go up by a corresponding amount.

Enthalpy changes are determined by measuring the amount of heat involved in the process (Eq. 3), so numerical values can be found for each kind of enthalpy change. For the quantitative treatment of the above three kinds of enthalpy, all based on use of Eq. 3:

(1) Temperature. The enthalpy change for a process that takes a system from the initial temperature Ti to the final temperature Tf is proportional to the temperature change, Tf− Ti. For example, it takes as much heat to raise the system's temperature from 17oC to 18oC as it does from 18oC to 19oC. The proportionality constant is called the heat capacity, and is given the symbol C:

(4)

Because it is the difference in temperature that determines H, either oC or K temperatures will give the identical result.

The system's heat capacity is proportional to the amount of material. It takes 10 times as much heat to raise 10 g of material from 5oC to 6oC as it does to raise 1 g of the material. The heat energy required to raise the temperature of one gram of material its specific heat capacity, c. If amount is expressed in moles, then the molar heat capacity C is used.

Ctotal  m cper gram = n Cper mole (5a)

The system's enthalpy change (heat energy change) is amount times heat capacity (specific or molar):

H = Ctotal T = m cper gram T = n Cper mold T (5b)

If T of the system is positive, temperature increases, the system absorbs heat, and q (or H) is positive. If T of the system is negative, temperature decreases, the system gives off heat to its surroundings, and q (or H) is negative.

Unit analysis will help you determine how to use the above equation. H (or q) must have an energy unit. Units for heat capacity are:

c: energy per gram per degree e.g., J/oC.g

C: energy per mole per