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1. Hess Law Report
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2. Hess Law
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
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 orwork.
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

3. - If the system absorbs heat energy from its surroundings, a decrease in the
surroundings
temperature is observed, and the reaction is endothermic...