INSTANT COLD PACK ammonium nitrate Inner bag of water SINGLE USE ÖNLY (a) (b) Figure 5.7 (a) An oxyacetylene torch produces heat by the combustion of acetylene in oxygen. The energy released by this exothermic reaction heats and then melts the metal being cut. The sparks are tiny bits of the molten metal flying away. (b) A cold pack uses an endothermic process to create the sensation of cold. (credit a: modification of work by "Skatebiker"/Wikimedia commons) Standard Molar Enthalpies of Combustion Substance Combustion Reaction Enthalpy of Combustion, AH: kJ at 25 °C) mol carbon C(s) + O2(8) → CO2(g) -393.5 hydrogen H,(g) + 0,(g) → H, O() -285.8 magnesium Mg(s) + 0,(3) → MgO(s) -601.6 sulfur S(s) + O2(g) → SO2(g) -296.8 carbon CO(g) + 0,(g) → CO,(g) -283.0 monoxide methane CH4(8) + 202(8) → CO2(8) + 2H2O(1) -890.8 acetylene C2H2(g)+ 02(8) –→ 2C0,(g) + H2O(1) -1301.1 C,H5OH(1) +30,(8) → 2CO,(g)+ 3H,O(I) ethanol -1366.8 methanol CH3 OH(1) + CO2(g) + 2H2O(1) -726.1 C3H18(1) + 02(3) –→ 8CO,(g) + 9H2O(!) isooctane -5461 Table 5.2
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Although the gas used in an oxyacetylene torch (see figure) is essentially pure acetylene, the heat produced by combustion of one mole of acetylene in such a torch is likely not equal to the enthalpy of combustion of acetylene listed in the given table. Considering the conditions for which the tabulated data are reported, suggest an explanation.
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