Knowledge of reaction energy is closely connected with fuels, heating and cooling, food, and medicine. Let us deepen our understanding of its practical applications.
Basic Knowledge
① Heat of combustion of fuels: compare the heat of combustion per gram. Hydrogen 142 kJ/g, methane 55 kJ/g, gasoline 48 kJ/g, coal 24 kJ/g.
② Hand warmers and cooling packs: oxidation of iron powder (exothermic), dissolution of ammonium nitrate NH4NO3 (endothermic)
③ Food calories: protein 4 kcal/g, carbohydrates 4 kcal/g, lipids 9 kcal/g
④ Explosive reactions: TNT, nitroglycerin, and gunpowder undergo rapid heat release and produce large amounts of gas
⑤ Hydrogen energy: only H2O remains after combustion, so hydrogen is attracting attention as a decarbonization technology
Heat of combustion (energy density)（heat of combustion per unit mass）
Higher heating value HHV（energy when the water produced is liquid）
Lower heating value LHV（energy when the water produced is gas）
Food calories（metabolizable energy in food）
Catalytic combustion（low-temperature combustion using a platinum catalyst. Water heater）
Green hydrogen（hydrogen produced by water electrolysis using renewable energy）
Further Study (Principles and Applications)
Hydrogen has about three times the heat of combustion per unit mass of hydrocarbons, but less per unit volume, making storage a challenge. Methods under study include liquefaction (20 K), high-pressure tanks (70 MPa), organic hydrides (such as MCH), and conversion to ammonia.
Food metabolism is a process that slowly extracts bond energy and converts it into biological work through ATP. Lipids have an energy density of 9 kcal/g, more than twice that of carbohydrates and proteins, because they contain a high proportion of carbon and hydrogen and are close to a fully reduced state.
- Compare fuel heat of combustion in g, mol, and L units
- Hydrogen is greatest per gram; storage is a challenge
- Hand warmer = Fe oxidation; cooling pack = NH4NO3 dissolution
- Food: lipids 9, carbohydrates 4, proteins 4 kcal/g
- HHV (liquid) > LHV (gas) by the enthalpy of vaporization
- Hydrogen energy = decarbonization
- Catalytic combustion = low-temperature and safe
Points of Caution (Common Confusions and Frequent Mistakes)
① Use consistent units when comparing fuels (kJ/g, kJ/mol, MJ/L). ② A calorie (cal) and a kilocalorie (kcal) differ by a factor of 1000. Food labels use kcal. ③ Hydrogen is high per unit mass, but storage by volume is difficult. ④ The explosive power of gunpowder depends not only on heat but also on the amount of gas produced.
Practice
- When the molar heats of combustion of hydrogen H2 and methane CH4 are H2: -286 kJ and CH4: -891 kJ, which is greater per gram?
- Write the chemical equation for the reaction that produces heat in a disposable hand warmer.
- Explain why hydrogen energy is called “decarbonized energy,” showing the chemical equation for combustion.