Heat of Combustion

10 October 2016

Mapua Institute of Technology School of Chemical Engineering and Chemistry Physical Chemistry I Experiment No. 6 Heat of Combustion Alain John Cantaa, Jefunnie Matahumb, Marife Comprac, Nino Adrian Rodelasa, a b Bachelor of Science in Chemical Engineering, Mapua Institute of Technology, School of Chemistry and Chemical Engineering Bachelor of Science in Biological Engineering and Chemistry, Mapua Institute of Technology, School of Chemistry and Chemical Engineering c Bachelor of Science in Chemistry and Chemical Engineering, Mapua Institute of Technology, School of Chemistry and Chemical Engineering

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Heat of combustion Calorimeter Aromaticity Relative energies Heat of formation Date submitted: March 02, 2013 ABSTRACT The heat of combustion, ? H, can be measured through a bomb calorimeter. It is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat. It may be expressed with the quantities: energy/mole of fuel (kJ/mol), energy/mass of fuel, energy/volume of fuel.

The heat of combustion is conventionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation of the products and reactants. The experiment aimed to determine the heat of combustion of some hydrocarbons namely, benzoic acid, naphthalene, and sucrose and to be familiarized with calibration of bomb calorimeter. Parr 6200 Bomb Calorimeter, a device that measures heat of combustions of different compounds, and the Parr 6510 water handling system were used. The samples used were benzoic acid, naphthalene and sucrose.

The results of the experiment showed that naphthalene has the highest heat of combustion followed by benzoic acid and sucrose, respectively. The analysis of the molecular structure of the samples found that increasing aromaticity of compounds yield to a higher enthalpy of combustion. Discussion of the relative energies of the bonds broken in combustion had contributed to the overall concept of the experiment. In general, the heat of combustion of a certain compound can be measured in a typical bomb calorimeter and aromaticity is directly proportional to the heat of combustion of a particular compound. . Introduction Combustion is the chemical term for a process known more commonly as burning. It is one of the earliest chemical changes noted by humans, due at least in part to the dramatic effects it has on materials. 1The mechanism by which combustion takes place is well understood and is more correctly defined as a form of oxidation. This oxidation occurs so rapidly that noticeable 1 Charles E. Smith heat and light are produced. In general, the term “oxidation” refers to any chemical reaction in which a substance reacts with oxygen.

For example, when iron is exposed to air, it combines with oxygen in the air. That form of oxidation is known as rust. Combustion differs from rust in that the oxidation occurs much more rapidly, giving off heat in the process. The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic process where the change in internal energy is equal to the difference in heat added and work done by the system. When a chemical reaction occurs in an open container most of the energy gained or lost is in the form of heat. Calorimetry is the study of the measure of heat of both physical and chemical reaction. In a constant-volume calorimeter, the heat measured, qv is simply the internal energy of the system for no expansion work is done at constant volume. A calorimeter is the device used for the process of calorimetry. 3 There are many kinds of calorimeters namely the bomb calorimeter, the calvet-type calorimeter, the isothermal calorimeter, the differential scanning calorimeter and the constant-pressure calorimter. Calorimeters can measure heat changes and as well as heat constants such as heat of formation and heat of combustion.

Heat of combustion or Hc is the energy released when one mole of a compound is reacted with oxygen, forming carbon dioxide and water as end products. Theoretically, the heat of combustion can be calculated using the heat of formation of the products and reactants or using the bond enthalpies bonds formed and broken. One of the most widely used calorimeters today, is the bomb calorimeter. 4 A bomb calorimeter is a type of constant volume device which makes the system isolated from the surroundings. It is an excellent device in measuring heat of chemical processes including the heat of combustion.

The bomb calorimeter is used to completely burn substances to produce water and CO2 in an excess of oxygen. The heat of combustion released is absorbed by the calorimeter vessel in which the bomb is immersed, and results in a temperature increase ? T. The temperature measuring device in the bomb calorimeter, measures the heat change in the water. The bomb calorimeter uses the following equation to measure heat changes. Q? ?t * e m [1] where Q is the energy involved, ? t is the change ?? in temperature of the system, e is the energy equivalent of the calorimeter, and m is the mass of the sample.

In this type, the heat measured for the sample burned at constant volume, qv is the change in internal energy of the substance from its initial to final states. 1The enthalpy change, (the heat of combustion for this process) is related to the internal energy, by the equation below. 2 3 Physical Chemistry 4th edition Atkin’s Physical Chemistry 8th edition 4 Principles of Chemical Engineering. 7th Edition Page 2 of 11 The enthalpy of combustion can be calculated from the temperature rise, which results when the combustion reaction occurs under adiabatic conditions in a calorimeter.

It is important that the reaction in the calorimeter take place rapidly and completely. The objectives of the experiment were to setup and use a bomb calorimeter as well as to obtain the value of the heat of combustion of some hydrocarbons namely, benzoic acid, naphthalene, and sucrose using the bomb calorimeter. and electric heater and has a precision electronic thermometer. Temperatures are measured with a high precision electronic thermometer using a specially designed thermistor sensor sealed in a stainless steel probe which is fixed in the calorimeter cover.

Measurements are taken with 0. 0001o resolution over a 20oC to 40oC working range, with all readings shown in Celsius. It has a semi-automatic system for charging the bomb with oxygen. Oxygen from a commercial cylinder is connected to a microprocessor controlled solenoid installed in the calorimeter. In filling the bomb, the filling hose connector slips onto the bomb inlet valve. The filling proceeds automatically at a controlled rate to a pre-set pressure. 5 2. Materials and Methods 2. 1 Equipments used The equipments used were Parr 6200 Bomb Calorimeter and Parr 6510 Water Handling System.

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