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.
![**Question:**
How many moles of H₂ are required to produce -3751 kJ of heat in the following reaction?
N₂ (g) + 3 H₂ (g) → 2 NH₃ (g) ΔH° = -91.8 kJ/mol
**Explanation:**
The reaction described is the synthesis of ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂) gas. The enthalpy change (ΔH°) for this reaction is -91.8 kJ/mol, indicating that the reaction releases 91.8 kJ of heat for every mole of N₂ that reacts (or equivalently for every 2 moles of NH₃ produced).
To determine how many moles of H₂ are required to produce -3751 kJ of heat, one must set up a proportion based on the enthalpy change:
- From the balanced equation: 3 moles of H₂ produce -91.8 kJ.
- Therefore, to produce -3751 kJ, calculate the moles of H₂ needed using the ratio:
\[
\frac{-3751 \, \text{kJ}}{-91.8 \, \text{kJ/mol}} = \text{moles of N₂ reacted}
\]
- Since each mole of N₂ requires 3 moles of H₂:
\[
\text{Moles of H₂ needed} = 3 \times \text{moles of N₂ reacted}
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