The meals-ready-to-eat (MREs) in the military can be heated on a flameless heater. You can purchase a similar product called “Heater Meals”. Just pour water into the heater unit, wait a few minutes, and you have a hot meal. The source of energy in the heater is the exothermic reaction of magnesium with water: Mg(s) + 2 H2O(ℓ) → Mg(OH)2(s) + H2(g) ΔHf° (kJ/mol) Mg(s) 0 Mg(OH)2(s) –924.5 H2O(ℓ) –285.8 H2(g) 0 Calculate the mass, in g, of magnesium needed to supply the energy required to warm 25.0 mL of water from 25.0 °C to 85.0 °C. The density of water is 1.00 g/mL, and its specific heat capacity is 4.184 J/g∙°C. 0.432 g 0.126 g 1.244 g 2.740 g 0.745 g
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.
The meals-ready-to-eat (MREs) in the military can be heated on a flameless heater. You can purchase a similar product called “Heater Meals”. Just pour water into the heater unit, wait a few minutes, and you have a hot meal. The source of energy in the heater is the exothermic reaction of magnesium with water:
Mg(s) + 2 H2O(ℓ) → Mg(OH)2(s) + H2(g)
ΔHf° (kJ/mol)
Mg(s) 0
Mg(OH)2(s) –924.5
H2O(ℓ) –285.8
H2(g) 0
Calculate the mass, in g, of magnesium needed to supply the energy required to warm 25.0 mL of water from 25.0 °C to 85.0 °C. The density of water is 1.00 g/mL, and its specific heat capacity is 4.184 J/g∙°C.
0.432 g
0.126 g
1.244 g
2.740 g
0.745 g
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