Thermochemistry and Enthalpy Changes
Introduction:
Thermochemistry is the branch of chemistry that deals with the study of energy changes associated with chemical reactions. Enthalpy change is a measure of heat absorbed or released during a reaction.
Key Concepts:
• Thermochemistry involves the study of energy changes in chemical systems.
• Enthalpy change is a measure of heat absorbed or released during a reaction.
• Enthalpy change is represented by ΔH, where ΔH is negative for exothermic reactions and positive for endothermic reactions.
• The enthalpy change of a chemical reaction can be measured using calorimetry.
• The enthalpy change of a reaction can be calculated using Hess’s law.
• Standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its elements in their standard states.
Equations and Formulas:
• ΔH = H products – H reactants
• ΔH is negative for exothermic reactions and positive for endothermic reactions.
• q = mCΔT, where q is the heat absorbed or released, m is the mass, C is the specific heat capacity, and ΔT is the temperature change.
Examples:
-
Combustion of methane: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
The enthalpy change of the reaction is -890 kJ/mol, indicating that the reaction is exothermic. -
Dissolution of ammonium nitrate: NH4NO3(s) → NH4+(aq) + NO3-(aq)
The enthalpy change of the reaction is +25.7 kJ/mol, indicating that the reaction is endothermic. -
Formation of water from its elements:
2H2(g) + O2(g) → 2H2O(l)
The enthalpy change of the reaction is -286 kJ/mol, indicating that the reaction is exothermic.
References for Further Learning:
- Chang, R. (2010). Chemistry, 10th edition. New York: McGraw-Hill.
- Atkins, P. and de Paula, J. (2010). Atkins’ Physical Chemistry, 9th edition. Oxford: Oxford University Press.
- Rosenfeld, J. (2005). Thermochemistry and thermodynamics. Boca Raton: CRC Press.
