Assignment: Hess Law
Assignment: Hess’ Law
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Assignment: 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.
