Chemistry
Thermochemistry
Thermochemistry tracks heat in chemical reactions. The key quantity is enthalpy change ΔH — negative for exothermic (releases heat), positive for endothermic (absorbs heat). Hess's law lets you sum step ΔH's to get the overall ΔH for any reaction.
How to Approach Thermochemistry
Identify the type of calculation
Calorimetry: q = m·c·ΔT (heat from temperature change). Standard enthalpy: ΔH°_rxn = ΣΔH°_f(products) - ΣΔH°_f(reactants). Hess's law: combine equations whose ΔH you know.
Apply the right formula
For calorimetry, mass × specific heat × temperature change. For standard enthalpy from formation values, look up ΔH°_f for each species and sum (with stoichiometric coefficients).
Watch signs and stoichiometry
Exothermic ΔH is negative. If you double a reaction, double its ΔH. If you reverse a reaction, negate its ΔH. Hess's law manipulations require careful sign-tracking.
Frequently Asked Questions
What's the difference between heat (q) and enthalpy (H)?+
Heat is energy transferred due to temperature difference. Enthalpy is a thermodynamic property: H = U + PV. At constant pressure, ΔH = q — the heat equals the enthalpy change exactly.
Why is Hess's law true?+
Because enthalpy is a state function — it depends only on initial and final states, not the path. So the ΔH for any reaction equals the sum of ΔH's for any sequence of steps connecting reactants to products.
What's a calorimeter constant?+
The heat capacity of the calorimeter itself (the apparatus, not the contents). You determine it by adding a known quantity of heat and measuring the temperature rise. Used to correct for heat absorbed by the device.
Related Topics
More step-by-step guides in Chemistry and adjacent subjects.
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