1.6.5Physical Chemistry
Hess’ Law — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 37 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5The Law
Statement
- Statement The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
Why it works
- Why it works Enthalpy is a state function, so it depends only on the starting and finishing states and not on how the change occurs.
Use
- Use Hess’ law lets us find enthalpy changes that cannot be measured directly, such as formation of compounds from their elements.
Cycle
- Cycle A cycle shows the target reaction along one path and an alternative route through other reactions with known enthalpy values.
Conservation of energy
- Conservation of energy The law is a consequence of the conservation of energy, as otherwise energy could be gained by going around a cycle.
Section 2 of 5Using Formation Data
Using Formation Data
Formula
- Formula ΔrH = ΣΔfH(products) − ΣΔfH(reactants), with each term multiplied by its coefficient from the balanced equation.
Elements
- Elements Elements in their standard states have ΔfH of zero, so they are left out of the sum.
Cycle direction
- Cycle direction Reactants break down to elements, then elements form the products, so the arrows go down to the elements and back up.
Sign care
- Sign care Reversing an arrow reverses the sign of the enthalpy value, and multiplying the equation multiplies the value.
| Substance | ΔfH° / kJ mol⁻¹ |
|---|---|
| CH₄(g) | −75 |
| CO₂(g) | −394 |
| H₂O(l) | −286 |
Section 3 of 5Method
1
Write the balanced equation for the reaction whose enthalpy change is needed, including state symbols.
2
Draw the cycle with the elements at the bottom, and arrows from reactants and products to the elements.
3
Set the reaction enthalpy equal to the route via elements: ΔrH = Σ(products) − Σ(reactants).
4
Multiply each value by its number of moles and watch the signs carefully.
5
Check the final answer has the correct sign and units, and compare with similar reactions for sense.
Section 4 of 5Worked Example
Question Calculate ΔH for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) using ΔfH°: CH₄ = −75, CO₂ = −394, H₂O(l) = −286 kJ mol⁻¹.
- ΔrH = Σ ΔfH(products) − Σ ΔfH(reactants).
- Products(−394) + 2 × (−286) = −394 − 572 = −966 kJ.
- Reactants(−75) + 2 × 0 = −75 kJ, because oxygen is an element.
- ΔrH = −966 − (−75) = −891 kJ mol⁻¹.
Answer ΔH = −891 kJ mol⁻¹ for the combustion of methane.
Section 5 of 5Exam Checklist & Tips
1.6.5 – Key Points for the Exam
- Hess' law The enthalpy change of a reaction is the same whichever route is taken.
- State function A property that depends only on the initial and final state of the system.
- Enthalpy cycle A diagram linking a reaction to alternative routes using known enthalpy values.
- ΔfH° The standard enthalpy change when one mole of a compound is formed from its elements.
Common Mistakes
- Forgetting to multiply the enthalpy of formation by the coefficient in the equation.
- Subtracting products from reactants instead of reactants from products.
- Including a non-zero value for an element in its standard state.
- Not reversing the sign when an arrow in the cycle is followed backwards.
Last updated:
\n