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Thermochemistry Lab

Investigate enthalpy changes in chemical reactions using three methods: Hess's Law path addition, bond energy calculations, and enthalpy level diagrams. Discover why energy is released or absorbed and verify that enthalpy is a state function.

Guided Experiment: Hess's Law Path Independence

If enthalpy is a state function, does the total ΔH depend on the reaction path or only the initial and final states?

Write your hypothesis in the Lab Report panel, then click Next.

Controls

Reaction Steps
kJ/mol
kJ/mol

Results

ΔHtotal=−110.5+−283.0=−393.5 kJ/mol\Delta H_{\text{total}} = -110.5 + -283.0 = -393.5\ \text{kJ/mol}
C + O₂ → CO₂
-393.5 kJ/mol
Exothermic
Step-by-step:
Step 1: C + ½O₂ → CO-110.5
Step 2: CO + ½O₂ → CO₂-283.0
Total ΔH-393.5 kJ/mol

Hess's Law Energy Level Diagram

Data Table

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#TrialReactionMethodΔH(kJ/mol)Type
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Reference Guide

Enthalpy Changes

Enthalpy (H) measures the heat content of a system at constant pressure. The enthalpy change ΔH tells you whether a reaction releases or absorbs heat.

ΔH=Hproducts−Hreactants\Delta H = H_{\text{products}} - H_{\text{reactants}}
  • Exothermic (ΔH < 0): heat is released to surroundings
  • Endothermic (ΔH > 0): heat is absorbed from surroundings

Hess's Law

Hess's Law states that the total enthalpy change is independent of the reaction path. You can add or subtract known reactions to find an unknown ΔH.

ΔHtotal=ΔH1+ΔH2+⋯\Delta H_{\text{total}} = \Delta H_1 + \Delta H_2 + \cdots

Reversing a reaction changes the sign of ΔH. Multiplying a reaction by a factor multiplies ΔH by the same factor.

Bond Energies

Breaking bonds requires energy input; forming bonds releases energy. The net enthalpy change equals the energy to break reactant bonds minus the energy released forming product bonds.

ΔH≈ΣEbroken−ΣEformed\Delta H \approx \Sigma E_{\text{broken}} - \Sigma E_{\text{formed}}

Example: C-H bond energy = 413 kJ/mol, O=O = 498 kJ/mol, C=O = 799 kJ/mol, O-H = 463 kJ/mol.

Enthalpy Diagrams

Enthalpy diagrams show the relative energy levels of reactants and products. The vertical gap between levels equals ΔH. A higher reactant bar means exothermic; a higher product bar means endothermic.

ΔHrxn∘=ΣΔHf∘(products)−ΣΔHf∘(reactants)\Delta H^{\circ}_{rxn} = \Sigma \Delta H^{\circ}_{f}(\text{products}) - \Sigma \Delta H^{\circ}_{f}(\text{reactants})

Standard enthalpies of formation (ΔH°f) for elements in their standard states equal zero.