Heat of formation values for four compounds are given in the table above. Compound ΔHf (kJ/mol) NH3(g) -46.0 CH4(g) -75.0 HCN(g) 135.1 H2O(g) -242.0 Calculate ΔHrxn for the reaction 2NH3(g) + 3O2(g) + 2CH4(g) → 2HCN(g) + 6H2O(g) Sketch an energy diagram for this change. Clearly label the reactants, products and ΔHrxn. As the reaction takes place, would the reaction container increase or decrease in temperature? Explain in terms of system and surroundings.
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.
Heat of formation values for four compounds are given in the table above.
|
Compound |
ΔHf (kJ/mol) |
|
NH3(g) |
-46.0 |
|
CH4(g) |
-75.0 |
|
HCN(g) |
135.1 |
|
H2O(g) |
-242.0 |
- Calculate ΔHrxn for the reaction 2NH3(g) + 3O2(g) + 2CH4(g) → 2HCN(g) + 6H2O(g)
- Sketch an energy diagram for this change. Clearly label the reactants, products and ΔHrxn.
- As the reaction takes place, would the reaction container increase or decrease in temperature? Explain in terms of system and surroundings.
The reaction given is
The value of change in enthalpy can be given as
The value of change in enthalpy will be
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