Enthalpy and Hess's Law
Total enthalpy change is independent of the path.
About this topic
Enthalpy change (ΔH) is the heat absorbed or released at constant pressure. Hess's law states that the total enthalpy change of a reaction is the same regardless of the route taken, so enthalpy changes of steps can be added. Standard enthalpies of formation allow ΔH = Σ ΔHf(products) − Σ ΔHf(reactants).
Enthalpy is a state function, which means it depends only on the initial and final states. This lets chemists calculate energies for reactions that are difficult to measure directly by combining known reactions. When reversing a reaction, change the sign of ΔH; when multiplying the equation by a number, multiply ΔH too. Standard enthalpy of formation is the energy change when one mole of a compound forms from its elements in their standard states, and is zero for elements.
Worked example: C(s) + O2 gives CO2 with ΔH = minus 394 kJ, and CO + 1/2 O2 gives CO2 with ΔH = minus 283 kJ. To find C + 1/2 O2 gives CO, subtract the second from the first: ΔH = minus 394 minus (minus 283) = minus 111 kJ. Applications include calculating energy content of fuels and foods.
Typical errors include forgetting to flip signs, not scaling by coefficients, and mixing up products minus reactants with reactants minus products. Check that equations add up to the target.
Key ideas
- ΔH = Σ products − Σ reactants
- Add steps to find unknown ΔH
- Standard enthalpies of formation
Quick check
Use cases: where Enthalpy and Hess's Law helps
- Homework: review the key idea behind Enthalpy and Hess's Law before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Enthalpy and Hess's Law without notes.
- Visual learning: play with the interactive lab to see how Enthalpy and Hess's Law changes when you adjust its inputs.
- Teaching a friend: explain Enthalpy and Hess's Law in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Enthalpy and Hess's Law?
Total enthalpy change is independent of the path.
What will I learn in Enthalpy and Hess's Law?
ΔH = Σ products − Σ reactants. Add steps to find unknown ΔH. Standard enthalpies of formation.
Is Enthalpy and Hess's Law part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Enthalpy and Hess's Law lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Enthalpy and Hess's Law?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
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