What is Thermochemistry and Enthalpy?
Thermochemistry is the branch of chemistry studying heat changes during chemical reactions. Enthalpy (H) is the total heat content of a system. When a reaction occurs, the change in enthalpy (ΔH) tells us whether heat is released (exothermic, ΔH < 0) or absorbed (endothermic, ΔH > 0).
Enthalpy change is calculated as ΔH = H(products) − H(reactants). If ΔH is negative, the reaction releases heat and is exothermic. If ΔH is positive, the reaction absorbs heat and is endothermic. The magnitude of ΔH is called the heat of reaction.
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Step-by-step worked examples
Combustion of methane releases 890 kJ/mol. Write the enthalpy change and classify the reaction.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH = −890 kJ/mol Negative ΔH indicates exothermic reaction (heat released) Energy is released, so this is an exothermic combustion
Melting ice absorbs 6.01 kJ/mol. Is this endothermic or exothermic?
H2O(s) → H2O(l) ΔH = +6.01 kJ/mol Positive ΔH indicates endothermic process Energy must be absorbed to break ice crystal structure
If H(reactants) = −250 kJ/mol and H(products) = −100 kJ/mol, calculate ΔH.
ΔH = H(products) − H(reactants) ΔH = −100 − (−250) ΔH = −100 + 250 = +150 kJ/mol Endothermic: energy is absorbed
Flashcards
Quick quiz
Q1.Exothermic reaction means:
Q2.Which has ΔH > 0?
Q3.ΔH formula is:
Q4.If burning 1 mol of ethane releases 1560 kJ, ΔH =
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Common mistakes
Exothermic and endothermic are the same thing. — Correct: Exothermic releases heat (ΔH < 0); endothermic absorbs heat (ΔH > 0).
All combustion reactions are endothermic. — Correct: All combustion reactions are exothermic; they release energy.
ΔH = H(reactants) − H(products). — Correct: ΔH = H(products) − H(reactants).
A reaction with large ΔH is always fast. — Correct: ΔH (energy released/absorbed) and reaction rate are independent.
FAQ
What is thermochemistry?
The branch of chemistry that studies heat changes during chemical reactions and the enthalpy of substances.
What does ΔH represent?
The change in enthalpy: the difference between the enthalpy of products and reactants. Negative means exothermic (heat released).
Difference between exothermic and endothermic?
Exothermic: ΔH < 0, heat released, surroundings warm up. Endothermic: ΔH > 0, heat absorbed, surroundings cool down.
Why is enthalpy important in chemistry?
It tells us how much heat a reaction releases or absorbs, which is critical for safety, energy production, and industrial processes.