Hess’ Law
Peter Jeschofnig, Ph.D.
Version 42-0158-00-01
Review the safety materials and wear goggles when
working with chemicals. Read the entire exercise
before you begin. Take time to organize the materials
you will need and set aside a safe work space in
which to complete the exercise.
Experiment Summary:
Students will have the opportunity to measure
temperature changes taking place in a calorimeter
during neutralization reactions and use the
measurements to calculate enthalpy of reaction.
They will illustrate the validity of Hazy’ Law by
comparing the values of enthalpy of two chemical
reactions.
Objectives
●● To measure temperature changes taking place in a calorimeter during neutralization reactions
and use the measurements to calculate enthalpy of reaction.
●● To compare the enthalpy of two chemical reactions and use these measured values to illustrate
the validity of Hess’ Law.
Materials
Materials From: Label or
Box/Bag: Qty Item Description:
Student Provides Distilled water
Watch
Coffee cups
Paper towels
From LabPaq 1 Thermometer - Digital
1 Goggles-Safety
4 Cup, Styrofoam, 8 oz
1 Cylinder-25-mL
From Experiment Bag
Hess' Law 2 Ammonia , NH3 (comes as aqueous
ammonia, NH4OH), - 2 M - 10 mL
2 Ammonium chloride, NH4Cl - 2M - 10mL
2 Hydrochloric acid, HCl - 2 M - 20 mL
2 Pipet, Long Thin Stem
2 Sodium hydroxide, NaOH - 2M - 20 mL
Note: The packaging and/or materials in this LabPaq may differ slightly from that which is listed
above. For an exact listing of materials, refer to the Contents List form included in the LabPaq.
Discussion and Review
Thermochemistry is the study of the heat energy involved in chemical reactions and changes of physical state. Nearly all chemical reactions involve the release or absorption of heat, a form of energy. The burning of any fuel such as gasoline, coal, or wood is an example of a heat-releasing reaction. Heat energy is called thermal energy, and it is always spontaneously transferred from hotter to colder matter.
The First Law of Thermodynamics is the Law of Energy Conservation. It states that the total energy of the universe must remain constant. Therefore, all energy transferred between a system and its surroundings must be accounted for as heat or work.
The standard S.I. unit for heat energy is the joule, J. It takes 4.184 joules, the equivalent of 1
calorie, to raise the temperature of one gram of water by 1° C. The kilojoule, kJ, is commonly used in many applications: 1000 joule = 1 kilojoule.
When a chemical reaction takes place in a stable environment where the temperature and
pressure remain constant, the system defined by the reactants and products either produces or
releases heat energy.
●● If the reacting system releases heat energy to its surroundings, a concurrent increase in
surroundings temperature is observed, and the reaction is exothermic
●● If the system absorbs heat energy from its surroundings, a decrease in the surroundings
temperature is observed, and the reaction is endothermic.
●● A measure of the amount of heat given off or absorbed in any chemical reaction is called the
enthalpy change or heat of reaction, and is given the symbol H.
When thermodynamic measurements are carried out at standard-state conditions where the
pressure is constant at 1 atm and the temperature is constant at 25oC, the reaction enthalpy is
designated as the standard enthalpy change or ΔH°. It is important to have standardized values because the enthalpy of a reaction can vary with different reaction conditions.
The following reaction for the formation of water from its constituents is exothermic:
H2(g) + ½ O2(g) à H2O(l); ΔH °f = -286 kJ
For every mole of H2O (l) formed at standard-state conditions, 286 kilojoules of heat energy are
released. When the standard enthalpy change of reaction describes the formation of 1 mol of
compound directly from its elements in their standard states as in this example, the value of ΔH of is called the standard heat of formation.
To determine the enthalpy change for a given reaction (ΔH°rxn), the summation of the heats of
formation (ΔH° f ) for the reactants are subtracted from the summation of the heats of formation ( ΔH ° f ) for the products.
ΔH° rxn = [n ΔH°f (products)] - [n ΔH°f (reactants)]
Tables containing the standard heats of formation for a number of compounds are available in the appendices of any general chemistry textbook.
Hess's Law states that if a reaction is the sum of two or more other reactions, the ΔH for the
overall process must be the sum of the ΔH values of the constituent reactions.
Enthalpy change (ΔH) is independent of the path that a reaction follows to move from reactants
to products. It only depends on the relative energy difference between the reactant and product
molecules at constant pressure. Enthalpy change is referred to as a state function due to its
independent of pathway. Since the enthalpy of a substance is not commonly determined, the
change in enthalpy when reactants are converted to products is often used to describe a chemical
or physical process.
The thermal energy absorbed or produced by a chemical process reflects a difference between
the enthalpy between the reactants and products (ΔH). For example, in the decomposition of
liquid water into its component elements, H2 (g) and O2 (g), there are two successive changes.
First, the liquid water is vaporized. Second, the water vapor decomposes into its constituent
elements shown below. The ΔH value for this overall process can be determined by adding the
ΔH values from the equations for each step as shown below.
(1) H2O (l) àH2O (g); ΔH 1 = +44 kJ
(2) H2O (g) àH2 (g) + ½ O2 (g); ΔH 2 = +242 kJ
_______________________________________________________________
(1) + (2) H2O (l) àH2 (g) + ½ O2 (g); ΔHnet = +286 kJ
In order to determine ΔH for the reaction NH3 + HCl àNH4Cl in this experiment, ΔH rxn for the
following two reactions will be measured:
1. NaOH (aq) + HCl (aq) àH2O (l) + NaCl (aq)
2. NaOH (aq) + NH4Cl (aq) àNH3 + NaCl + H2O (l)
Comparison of the calculated results for different parts of the experiment will verify the
generalization known as Hess's Law of Constant Heat Summation. In this case the target reaction NH3 + HCl àNH4Cl can also be performed directly and the results compared to reactions 1 and 2.
A Styrofoam coffee cup calorimeter will be used to measure the amount of heat energy evolved
or absorbed during the chemical reactions of this experiment. A digital thermometer is used to
measure the change in temperature between the final and initial temperatures of the solutions.
Unfortunately, it is impossible to have perfect insulation and some of the heat energy will be lost to the surroundings, including to the material from which the calorimeter is constructed.
Calibrating the calorimeter before using it to make measurements on an unknown system usually solves the problem of heat losses. A known amount of heat energy from a known process is released into the calorimeter system, and the temperature change is measured. A simple calculation is done to determine the amount of heat energy loss, called the heat capacity of the calorimeter or calorimeter constant. For this experiment it assumed that the heat capacity of the calorimeter is insignificant and it is ignored.
Another practical problem is that heat energy exchanges do not occur instantaneously; i.e., it takes time for energy to move from a hot object to a cold one. An acceptable solution to this problem is to obtain a cooling curve for the heat energy exchange in question and then extrapolate the data back to the exact time that the exchange began.