What is Thermochemistry?
Thermochemistry is the branch of chemistry that studies the heat energy absorbed or released during chemical reactions and physical changes. It centers on enthalpy (ΔH), a measure of a system's total heat content at constant pressure.
Thermochemistry studies heat flow in reactions; the enthalpy change ΔH = H(products) − H(reactants) tells you whether a reaction releases heat (exothermic, ΔH < 0) or absorbs heat (endothermic, ΔH > 0).
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Step-by-step worked examples
How much heat is released when 50 g of water cools from 80°C to 20°C? (c_water = 4.18 J/g°C)
ΔT = 20 − 80 = −60°C q = m·c·ΔT = 50 × 4.18 × (−60) = −12,540 J The negative sign shows heat is released (exothermic).
Calculate the heat absorbed when 25 g of iron is heated from 20°C to 100°C. (c_iron = 0.45 J/g°C)
ΔT = 100 − 20 = 80°C q = m·c·ΔT = 25 × 0.45 × 80 = 900 J Iron absorbs 900 J of heat.
A reaction releases 250 kJ of heat while producing 2 mol of product. What is ΔH per mole?
Total heat released = −250 kJ (exothermic) ΔH per mole = −250 kJ ÷ 2 mol = −125 kJ/mol
Flashcards
Quick quiz
Q1.A reaction has ΔH = −92 kJ/mol. What type of reaction is it?
Q2.50 g of water is heated by 10°C. How much heat is absorbed? (c=4.18 J/g°C)
Q3.What is the SI unit of enthalpy change?
Q4.Which best describes Hess's Law?
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Common mistakes
Assuming all reactions release heat. — Correct: Some reactions absorb heat (endothermic, ΔH > 0), like photosynthesis or melting ice.
Forgetting the sign of ΔH. — Correct: The sign tells direction of heat flow: negative = released, positive = absorbed.
Confusing heat (q) with temperature (T). — Correct: Heat is energy transferred; temperature is a measure of average kinetic energy — they aren't the same thing.
Ignoring units when using q = m·c·ΔT. — Correct: Keep mass in grams and specific heat in J/(g·°C) so the units cancel correctly.
FAQ
What is thermochemistry?
Thermochemistry is the study of heat energy changes that accompany chemical reactions and phase changes.
What is the formula for enthalpy change?
ΔH = H(products) − H(reactants); heat transferred can be calculated with q = m·c·ΔT.
How do you calculate thermochemistry heat problems?
Use q = m·c·ΔT with mass, specific heat capacity, and temperature change to find heat absorbed or released.
What are examples of thermochemistry in real life?
Burning fuel (exothermic), melting ice (endothermic), and cooking food all involve enthalpy changes studied in thermochemistry.