Thermochemistry
Track heat between system and surroundings, read reaction-energy diagrams, and calculate calorimetry, bond-energy, and Hess’s-law changes.
Thermochemistry
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Track where energy goes and keep the sign consistent
The system is the reaction being studied; everything else is the surroundings.
In an exothermic reaction, the system releases heat and ΔH is negative. In an endothermic reaction, the system absorbs heat and ΔH is positive.
ΔH = Hproducts−Hreactantsq = mcΔTIn an ideal calorimeter, heat lost by one part equals heat gained by the other.
Exothermic: ΔH<0
Products lie below reactants on an energy diagram.
Endothermic: ΔH>0
Products lie above reactants.
Breaking bonds absorbs energy
Forming bonds releases energy.
Catalysts change activation energy
They do not change ΔH or the initial and final states.
Hess’s law adds reaction steps
Reverse a reaction to reverse ΔH; multiply the equation to multiply ΔH.
Use SIGN → SCALE → SURROUNDINGS
SIGN: Did the system release heat (negative) or absorb heat (positive)?
SCALE: If the chemical equation is multiplied or reversed, change ΔH the same way.
SURROUNDINGS: In calorimetry, qreaction and qsolution have opposite signs.
For bond energies:
ΔH≈bonds broken−bonds formedWhy it works
Most thermochemistry errors are sign errors, scaling errors, or confusion between heat gained by the solution and heat lost by the reaction.
Five forms you should recognize
Problem: A reaction warms its container.
Reasoning: Heat moved from the reaction system to the surroundings, so the process is exothermic and ΔH is negative.
Problem: How much heat does 100 g water absorb when it warms by 5°C? Use c = 4.18 J per gram per °C.
q = 100(4.18)(5) = 2090 JProblem: Reactants are at 100 kJ, the peak at 160 kJ, and products at 40 kJ.
activation energy = 160−100 = 60 kJΔH = 40−100 = −60 kJProblem: For H₂ + Cl₂→2HCl, breaking bonds requires 679 kJ and forming bonds releases 862 kJ.
ΔH≈679−862 = −183 kJProblem: A→B is + 40 kJ and B→C is −70 kJ.
A→C: +40 + (−70) = −30 kJThe intermediate B cancels when the steps are added.
Check before you commit
- Giving exothermic reactions a positive ΔH
- Forgetting qreaction and qsurroundings have opposite signs
- Using final temperature instead of ΔT
- Saying a catalyst changes ΔH
- Reversing an equation without reversing ΔH
- Treating bond breaking as an energy-releasing process
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Thermochemistry FAQ
How is this different from Physics Heat and Thermodynamics?
This page centers on energy changes in chemical reactions. The Physics reviewer treats heat transfer, temperature, thermal expansion, and thermodynamic systems more broadly.
Can an exothermic reaction still need activation energy?
Yes. It may release energy overall but still require an initial barrier to be overcome.
Why can temperature remain constant during melting?
Absorbed energy changes intermolecular arrangement rather than average kinetic energy during the phase change.
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