Page 127 - Physical chemistry understanding our chemical world
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94    ENERGY AND THE FIRST LAW OF THERMODYNAMICS


                       Why does a match emit heat when lit?

                      Reintroducing calorimetry

                      ‘Lighting’ a match means initiating a simple combustion reaction. Carbohydrates
                      in the wood combine chemically with oxygen in the air to form water and carbon
                      dioxide. The amount of heat liberated is so great that it catches fire (causing the water
                      to form as steam rather than liquid water).
                        Heat is evolved because the internal energy of the system changes during the
                      combustion reaction. Previously, the oxygen was a gaseous element characterized by
                      O=O bonds, and the wood was a solid characterized by C–C, C–H and C–O bonds.
                      The burning reaction completely changes the number and type of bonds, so the internal
                      energies of the oxygen and the wood alter. This explains the change in  U.
                        We know from Equation (3.5) that  U = q + w. Because  U changes, one or
                      both of q and w must change. It is certain that much energy is liberated because we
                      feel the heat, so the value of q is negative. Perhaps work w is also performed because
                      gases are produced by the combustion reaction, causing movement of the atmosphere
                      around the match (i.e. w is positive).
                        The simplest way to measure the change in internal energy  U is to perform a
                      reaction in a vessel of constant volume and to look at the amount of heat evolved.
                      We perform a reaction in a sealed vessel of constant volume called a calorimeter.In
                      practice, we perform the reaction and look at the rise in temperature. The calorimeter is
                      completely immersed in a large reservoir of water (see Figure 3.6) and its temperature
                      is monitored closely before, during, and after the reaction. If we know the heat


                                Water stirrer  Oxygen  Electrical   Resistance
                                              inlet   contacts     thermometer













                                                                                   Water





                                             Bomb     Sample

                      Figure 3.6 Schematic representation of the bomb calorimeter for measuring the changes in internal
                      energy that occur during combustion. The whole apparatus approximates to an adiabatic chamber,
                      so we enclose it within a vacuum jacket (like a Dewar flask)
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